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Global Biogeochemical Cycles 
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ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
Published by American Geophysical Union (AGU)
[17 journals]
[4 followers] Follow Subscription journalISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
Published by American Geophysical Union (AGU)
[17 journals]- Patterns and trends in nitrogen use and nitrogen recovery efficiency in world agriculture
- Authors: Richard T. Conant; Aaron B. Berdanier, Peter R. Grace
Pages: n/a - n/a
Abstract: Worldwide increases in nitrogen (N) inputs to croplands have been and will continue to be an important contributor to growing more food. But a substantial portion of N inputs to croplands are not captured in harvested products and leave the field, contributing to air and water pollution. Whether the proportion of N inputs captured in harvest grows, shrinks, or remains unchanged will have important impacts on both food production and N pollution. We created a new global N input database (fertilizer, manure, fixation, deposition, and residues) that enables evaluation of trends in nitrogen use and recovery by country and by crop from the 1960s through 2007. These data show that despite growth in yields, increased N fertilization, differences in efficiency of N use between OECD 1 and other countries have persisted over nearly 50 years and exhibit no sign of convergence. The high yield, high nitrogen input systems characteristic of rich countries have released large amounts of reactive N to the environment, but have operated with greater efficiency ‐ recovering a greater portion of added N in crops. Aggregate yields in OECD countries are 70% greater than in non–OECD countries on N input rates just 54% greater. Variation in recovery efficiency between countries suggests that there is scope for improvements through enhanced N delivery and capture in the world's low–yielding croplands and that increasing efficiency of N use is an important component of meeting food demand in the future.
PubDate: 2013-06-04T04:30:02.996629-05:
DOI: 10.1002/gbc.20053
- Authors: Richard T. Conant; Aaron B. Berdanier, Peter R. Grace
- Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean
- Abstract: Dissolved iron (dFe) distributions, atmospheric and vertical subduction fluxes of dFe were determined in the upper water column for two meridional transects of the Atlantic Ocean. The data demonstrate the disparity between the iron biogeochemistry of the North and South Atlantic Ocean and show well defined gradients of size fractionated iron species in surface waters between geographic provinces. The highest dFe and lowest mixed layer residence times (0.4‐2.7 y) were found in the northern tropical and subtropical regions. In contrast, the South Atlantic Gyre had lower dFe concentrations ( 5 y), presumably due to lower atmospheric inputs and more efficient biological recycling of iron in this region. Vertical input fluxes of dFe to surface waters ranged from 20 ‐ 170 nmol m‐2 d‐1 in the North Atlantic and tropical provinces, whereas average fluxes of 6 ‐ 13 nmol m‐2 d‐1 were estimated for the South Atlantic. Our estimates showed that the variable dFe distribution over the surface Atlantic (50% of total vertical Fe flux to surface waters) rather than upwelling or vertical mixing. This demonstrates the strength of the connection between land‐derived atmospheric Fe fluxes and the biological cycling of carbon and nitrogen in the Atlantic Ocean.
- Abstract: Dissolved iron (dFe) distributions, atmospheric and vertical subduction fluxes of dFe were determined in the upper water column for two meridional transects of the Atlantic Ocean. The data demonstrate the disparity between the iron biogeochemistry of the North and South Atlantic Ocean and show well defined gradients of size fractionated iron species in surface waters between geographic provinces. The highest dFe and lowest mixed layer residence times (0.4‐2.7 y) were found in the northern tropical and subtropical regions. In contrast, the South Atlantic Gyre had lower dFe concentrations ( 5 y), presumably due to lower atmospheric inputs and more efficient biological recycling of iron in this region. Vertical input fluxes of dFe to surface waters ranged from 20 ‐ 170 nmol m‐2 d‐1 in the North Atlantic and tropical provinces, whereas average fluxes of 6 ‐ 13 nmol m‐2 d‐1 were estimated for the South Atlantic. Our estimates showed that the variable dFe distribution over the surface Atlantic (50% of total vertical Fe flux to surface waters) rather than upwelling or vertical mixing. This demonstrates the strength of the connection between land‐derived atmospheric Fe fluxes and the biological cycling of carbon and nitrogen in the Atlantic Ocean.
- Response of global soil consumption of atmospheric methane to changes in atmospheric climate and nitrogen deposition
- Abstract: Soil consumption of atmospheric methane plays an important secondary role in regulating the atmospheric CH4 budget, next to the dominant loss mechanism involving reaction with the hydroxyl radical (OH). Here we used a process‐based biogeochemistry model to quantify soil consumption during the 20th and 21st centuries. We estimated that global soils consumed 32–36 Tg CH4 yr‐1 during the 1990s. Natural ecosystems accounted for 84% of the total consumption, and agricultural ecosystems only consumed 5 Tg CH4 yr‐1 in our estimations. During the 20th century, the consumption rates increased at 0.03‐0.20 Tg CH4 yr‐2 with seasonal amplitudes increasing from 1.44 to 3.13 Tg CH4 month‐1. Deserts, shrub‐lands, and xeric woodlands were the largest sinks. Atmospheric CH4 concentrations and soil moisture exerted significant effects on the soil consumption while nitrogen deposition had a moderate effect. During the 21st century, the consumption is predicted to increase at 0.05‐1.0 Tg CH4 yr‐2, and total consumption will reach 45–140 Tg CH4 yr‐1 at the end of the 2090s, varying under different future climate scenarios. Dry areas will persist as sinks, boreal ecosystems will become stronger sinks, mainly due to increasing soil temperatures. Nitrogen deposition will modestly reduce the future sink strength at the global scale. When we incorporated the estimated global soil consumption into our chemical transport model simulations, we found that, nitrogen deposition suppressed the total methane sink by 26 Tg during the period 1998–2004; resulting in 6.6 ppb higher atmospheric CH4 mixing ratios compared to without considering nitrogen deposition effects. On average, a cumulative increase of every one Teragram soil CH4 consumption decreased atmospheric CH4 mixing ratios by 0.26 ppb during the period 1998–2004.
- Abstract: Soil consumption of atmospheric methane plays an important secondary role in regulating the atmospheric CH4 budget, next to the dominant loss mechanism involving reaction with the hydroxyl radical (OH). Here we used a process‐based biogeochemistry model to quantify soil consumption during the 20th and 21st centuries. We estimated that global soils consumed 32–36 Tg CH4 yr‐1 during the 1990s. Natural ecosystems accounted for 84% of the total consumption, and agricultural ecosystems only consumed 5 Tg CH4 yr‐1 in our estimations. During the 20th century, the consumption rates increased at 0.03‐0.20 Tg CH4 yr‐2 with seasonal amplitudes increasing from 1.44 to 3.13 Tg CH4 month‐1. Deserts, shrub‐lands, and xeric woodlands were the largest sinks. Atmospheric CH4 concentrations and soil moisture exerted significant effects on the soil consumption while nitrogen deposition had a moderate effect. During the 21st century, the consumption is predicted to increase at 0.05‐1.0 Tg CH4 yr‐2, and total consumption will reach 45–140 Tg CH4 yr‐1 at the end of the 2090s, varying under different future climate scenarios. Dry areas will persist as sinks, boreal ecosystems will become stronger sinks, mainly due to increasing soil temperatures. Nitrogen deposition will modestly reduce the future sink strength at the global scale. When we incorporated the estimated global soil consumption into our chemical transport model simulations, we found that, nitrogen deposition suppressed the total methane sink by 26 Tg during the period 1998–2004; resulting in 6.6 ppb higher atmospheric CH4 mixing ratios compared to without considering nitrogen deposition effects. On average, a cumulative increase of every one Teragram soil CH4 consumption decreased atmospheric CH4 mixing ratios by 0.26 ppb during the period 1998–2004.
- Future arctic ocean primary productivity from CMIP5 simulations: Uncertain outcome, but consistent mechanisms
- Abstract: Net Arctic Ocean primary production (PP) is expected to increase over this century, due to less perennial sea ice and more available light, but could decrease depending on changes in nitrate (NO3) supply. Here, CMIP5 simulations performed with 11 Earth System Models are analyzed in terms of PP, surface NO3 and sea ice coverage over 1900‐2100. Whereas the mean model simulates reasonably well Arctic‐integrated PP (511 TgC/yr, 1998‐2005) and projects a mild 58 TgC/yr increase by 2080‐2099 for the strongest climate change scenario, models do not agree on the sign of future PP change. However, similar mechanisms operate in all models. The perennial ice loss‐driven increase in PP is in most models NO3‐limited. The Arctic surface NO3 is decreasing over the 21st century (‐2.3 ± 1 mmol/m3), associated with shoaling mixed layer and with decreasing NO3 in the nearby North Atlantic and Pacific waters. However, the inter‐model spread in the degree of NO3 limitation is initially high, resulting from >1000 yr spin‐up simulations. This initial NO3 spread, combined with the trend, causes a large variation in the timing of oligotrophy onset – which directly controls the sign of future PP change. Virtually all models agree in the open ocean zones on more spatially‐integrated PP and less PP per unit area. The source of model uncertainty is located in the sea ice zone, where a subtle balance between light and nutrient limitations determines the PP change. Hence, it is argued that reducing uncertainty on present Arctic NO3 in the sea ice zone would render Arctic PP projections much more consistent.
- Abstract: Net Arctic Ocean primary production (PP) is expected to increase over this century, due to less perennial sea ice and more available light, but could decrease depending on changes in nitrate (NO3) supply. Here, CMIP5 simulations performed with 11 Earth System Models are analyzed in terms of PP, surface NO3 and sea ice coverage over 1900‐2100. Whereas the mean model simulates reasonably well Arctic‐integrated PP (511 TgC/yr, 1998‐2005) and projects a mild 58 TgC/yr increase by 2080‐2099 for the strongest climate change scenario, models do not agree on the sign of future PP change. However, similar mechanisms operate in all models. The perennial ice loss‐driven increase in PP is in most models NO3‐limited. The Arctic surface NO3 is decreasing over the 21st century (‐2.3 ± 1 mmol/m3), associated with shoaling mixed layer and with decreasing NO3 in the nearby North Atlantic and Pacific waters. However, the inter‐model spread in the degree of NO3 limitation is initially high, resulting from >1000 yr spin‐up simulations. This initial NO3 spread, combined with the trend, causes a large variation in the timing of oligotrophy onset – which directly controls the sign of future PP change. Virtually all models agree in the open ocean zones on more spatially‐integrated PP and less PP per unit area. The source of model uncertainty is located in the sea ice zone, where a subtle balance between light and nutrient limitations determines the PP change. Hence, it is argued that reducing uncertainty on present Arctic NO3 in the sea ice zone would render Arctic PP projections much more consistent.
- A leaky model of long‐term soil phosphorus dynamics
- Abstract: Soil phosphorus (P) leaks rapidly from newly formed land surfaces to upland rivers and lakes, surface water P concentrations peaking early before declining as soil apatite (Ca5(PO4)3(OH)) becomes depleted. We present lake sediment P profiles that record this leakage through the early Holocene. The results are entirely consistent with our re‐analysis of published soil chronosequence data, but conflict with more recent quantitative interpretations of global soil P dynamics that identify far slower loss rates [Porder and Hilley, 2011; Porder et al., 2007]. P inherited from the bedrock on soil formation, long regarded as the major source for terrestrial ecosystems, only lasts ~104 years rather than the previously suggested 106 years, and thus is, globally, much less important in the long‐term than atmospheric supply. This changes the conceptualization of terrestrial P dynamics, with the “terminal steady state” of Walker and Syers [1976] being the norm not the exception, and with soil P export being little if at all controlled by biotic retention mechanisms. High early export of P from newly‐formed soil causes a peak in the productivity of terrestrial surface waters, before a decline as the soil P pool depletes. Globally, the 18 × 106 km2 of terrain exposed since the Last Glacial Maximum potentially produced a substantial surge in runoff P, with greatest impacts likely in high latitude restricted basin seas and maximal area of deglaciated terrain.
- Abstract: Soil phosphorus (P) leaks rapidly from newly formed land surfaces to upland rivers and lakes, surface water P concentrations peaking early before declining as soil apatite (Ca5(PO4)3(OH)) becomes depleted. We present lake sediment P profiles that record this leakage through the early Holocene. The results are entirely consistent with our re‐analysis of published soil chronosequence data, but conflict with more recent quantitative interpretations of global soil P dynamics that identify far slower loss rates [Porder and Hilley, 2011; Porder et al., 2007]. P inherited from the bedrock on soil formation, long regarded as the major source for terrestrial ecosystems, only lasts ~104 years rather than the previously suggested 106 years, and thus is, globally, much less important in the long‐term than atmospheric supply. This changes the conceptualization of terrestrial P dynamics, with the “terminal steady state” of Walker and Syers [1976] being the norm not the exception, and with soil P export being little if at all controlled by biotic retention mechanisms. High early export of P from newly‐formed soil causes a peak in the productivity of terrestrial surface waters, before a decline as the soil P pool depletes. Globally, the 18 × 106 km2 of terrain exposed since the Last Glacial Maximum potentially produced a substantial surge in runoff P, with greatest impacts likely in high latitude restricted basin seas and maximal area of deglaciated terrain.
- Estimating wetland methane emissions from the Northern High Latitudes from 1990 to 2009 using artificial Neural Networks
- Abstract: Methane (CH4) emissions from wetland ecosystems in nothern high latitudes provide a potentially positive feedback to global climate warming. Large uncertainties still remain in estimating wetland CH4 emisions at regional scales. Here, we develop a statistical model of CH4 emissions using an artificial neural network (ANN) approach and field observations of CH4 fluxes. Six explanatory variables (air temperature, precipitation, water table depth, soil organic carbon, soil total porosity, and soil pH) are included in the development of ANN models, which are then extrapolated to the northern high latitudes to estimate monthly CH4 emissions from 1990 to 2009. We estimate that the annual wetland CH4 source from the northern high latitudes (north of 45°N) is 48.7 Tg CH4 yr‐1 (1 Tg = 1012 g) with an uncertainty range of 44.0 ~ 53.7 Tg CH4 yr‐1. The estimated wetland CH4 emissions show a large spatial variability over the northern high latitudes, due to variations in hydrology, climate, and soil conditions. Significant inter‐annual and seasonal variations of wetland CH4 emissions exist in the past two decades, and the emissions in this period are most sensitive to variations in water table position. To improve future assessment of wetland CH4 dynamics in this region, research priorities should be directed to better characterizing hydrological processes of wetlands, including temporal dynamics of water table position and spatial dynamics of wetland areas.
- Abstract: Methane (CH4) emissions from wetland ecosystems in nothern high latitudes provide a potentially positive feedback to global climate warming. Large uncertainties still remain in estimating wetland CH4 emisions at regional scales. Here, we develop a statistical model of CH4 emissions using an artificial neural network (ANN) approach and field observations of CH4 fluxes. Six explanatory variables (air temperature, precipitation, water table depth, soil organic carbon, soil total porosity, and soil pH) are included in the development of ANN models, which are then extrapolated to the northern high latitudes to estimate monthly CH4 emissions from 1990 to 2009. We estimate that the annual wetland CH4 source from the northern high latitudes (north of 45°N) is 48.7 Tg CH4 yr‐1 (1 Tg = 1012 g) with an uncertainty range of 44.0 ~ 53.7 Tg CH4 yr‐1. The estimated wetland CH4 emissions show a large spatial variability over the northern high latitudes, due to variations in hydrology, climate, and soil conditions. Significant inter‐annual and seasonal variations of wetland CH4 emissions exist in the past two decades, and the emissions in this period are most sensitive to variations in water table position. To improve future assessment of wetland CH4 dynamics in this region, research priorities should be directed to better characterizing hydrological processes of wetlands, including temporal dynamics of water table position and spatial dynamics of wetland areas.
- Global trends in surface ocean pCO2 from in situ data
- Abstract: Ocean carbon uptake substantially reduces the rate of anthropogenic carbon accumulation in the atmosphere, and thus slows global climate change. In the interest of understanding how this ocean carbon sink has responded to climate variability and climate change in recent decades, trends in globally observed surface ocean partial pressure of CO2(pCO2s.ocean) are evaluated over sixteen gyre‐scale biomes covering the globe. Trends from decadal to multidecadal timescales between 1981–2010 are considered. On decadal timescales, pCO2s.ocean trends have been of variable magnitude and sensitive to the chosen start and end years. On longer timeframes, several regions of the tropics and subtropics display pCO2s.ocean trends that are parallel to or shallower than trends in atmospheric pCO2, consistent with the ocean's long‐term response to carbon accumulation in the atmosphere, and with the supply of waters with low anthropogenic carbon from the deep ocean. Data is too sparse in the high latitudes to determine this long‐term response. In many biomes, pCO2s.ocean trends steeper than atmospheric do occur on shorter timescales, which is consistent with forcing by climactic variability. In the Southern Ocean, the influence of a positive trend in the Southern Annular Mode has waned and the carbon sink has strengthened since the early 2000s. In North Atlantic subtropical and equatorial biomes, warming has become a significant and persistent contributor to the observed increase in pCO2s.ocean since the mid‐2000s. This long‐term warming, previously attributed to both multi‐decadal climate variability and anthropogenic forcing, is beginning to reduce ocean carbon uptake.
- Abstract: Ocean carbon uptake substantially reduces the rate of anthropogenic carbon accumulation in the atmosphere, and thus slows global climate change. In the interest of understanding how this ocean carbon sink has responded to climate variability and climate change in recent decades, trends in globally observed surface ocean partial pressure of CO2(pCO2s.ocean) are evaluated over sixteen gyre‐scale biomes covering the globe. Trends from decadal to multidecadal timescales between 1981–2010 are considered. On decadal timescales, pCO2s.ocean trends have been of variable magnitude and sensitive to the chosen start and end years. On longer timeframes, several regions of the tropics and subtropics display pCO2s.ocean trends that are parallel to or shallower than trends in atmospheric pCO2, consistent with the ocean's long‐term response to carbon accumulation in the atmosphere, and with the supply of waters with low anthropogenic carbon from the deep ocean. Data is too sparse in the high latitudes to determine this long‐term response. In many biomes, pCO2s.ocean trends steeper than atmospheric do occur on shorter timescales, which is consistent with forcing by climactic variability. In the Southern Ocean, the influence of a positive trend in the Southern Annular Mode has waned and the carbon sink has strengthened since the early 2000s. In North Atlantic subtropical and equatorial biomes, warming has become a significant and persistent contributor to the observed increase in pCO2s.ocean since the mid‐2000s. This long‐term warming, previously attributed to both multi‐decadal climate variability and anthropogenic forcing, is beginning to reduce ocean carbon uptake.
- Processes affecting greenhouse gas production in experimental boreal reservoirs
- Abstract: Flooding land for water reservoir creation has many environmental impacts including the production of the greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4). To assess processes governing GHG emissions from the flooding of terrestrial carbon, three experimental reservoirs were constructed in upland boreal forest areas of differing carbon stores as part of the Flooded Upland Dynamics Experiment (FLUDEX). We calculated process‐based GHG budgets for these reservoirs over five years following the onset of flooding. Stable isotopic budgets of carbon were necessary to separate community respiration (CR), which produces CO2, from net primary production (NPP), which consumes CO2, and to separate CH4 production from CH4 consumption via oxidation. NPP removed up to 44% of the CO2 produced from CR. CR and NPP exhibited different year‐after‐year trends. CH4 flux to the atmosphere increased about 2‐fold over three years, yet isotopic budgets showed CH4 production in flooded soils increased nearly 10‐fold. CH4 oxidation near the flooded soil–water interface greatly decreased the CH4 flux from the water column to the atmosphere. Ebullition was the most important conduit of CH4 to the atmosphere after three years. Although CH4 production increased with time, the total GHG flux, in CO2 equivalents, declined. Contrary to expectations, neither CR nor total GHG fluxes were directly related to the quantity of organic carbon flooded. Instead, these reservoirs produced a strikingly similar amount of CO2 equivalents over five years.
- Abstract: Flooding land for water reservoir creation has many environmental impacts including the production of the greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4). To assess processes governing GHG emissions from the flooding of terrestrial carbon, three experimental reservoirs were constructed in upland boreal forest areas of differing carbon stores as part of the Flooded Upland Dynamics Experiment (FLUDEX). We calculated process‐based GHG budgets for these reservoirs over five years following the onset of flooding. Stable isotopic budgets of carbon were necessary to separate community respiration (CR), which produces CO2, from net primary production (NPP), which consumes CO2, and to separate CH4 production from CH4 consumption via oxidation. NPP removed up to 44% of the CO2 produced from CR. CR and NPP exhibited different year‐after‐year trends. CH4 flux to the atmosphere increased about 2‐fold over three years, yet isotopic budgets showed CH4 production in flooded soils increased nearly 10‐fold. CH4 oxidation near the flooded soil–water interface greatly decreased the CH4 flux from the water column to the atmosphere. Ebullition was the most important conduit of CH4 to the atmosphere after three years. Although CH4 production increased with time, the total GHG flux, in CO2 equivalents, declined. Contrary to expectations, neither CR nor total GHG fluxes were directly related to the quantity of organic carbon flooded. Instead, these reservoirs produced a strikingly similar amount of CO2 equivalents over five years.
- Diel patterns of oceanic dimethylsufide (DMS) cycling: Microbial and physical drivers
- Abstract: Dimethylsulfide (DMS) is a biogenic gas with potential climatic effects, and its marine emission depends on the interplay between microbial activity and physical forcing in the oceanic upper mixed layer (UML). We investigated the diel cycling patterns of DMS and its precursor dimethylsulfoniopropionate (DMSP) in four experiments (28 to 48 h long) performed in meso‐ to ultraoligotrophic Mediterranean and Sargasso Sea waters. Samples taken every 4 or 6 h were analyzed for dimethylated sulfur pools and incubated to measure DMS and DMSP cycling rates, as well as primary and bacterial production. In all four experiments, DMS budgets showed pronounced day vs. night variability. In the three summer experiments, gross community DMS production (GPDMS) increased by 2 to 3‐fold from night‐ to daytime, peaking 0–4 h after solar noon. This excess GPDMS was balanced by higher photochemical and microbial sinks during the day, effectively buffering DMS concentrations. In the only winter experiment, GPDMS exhibited opposed temporal dynamics and peaked at nighttime in parallel to total DMSP consumption. Community DMSP to DMS conversion yields were generally 15% (even > 50%) during the day in summer, presumably due to phytoplankton radiative stress. Our data suggest that (1) diel variability should be taken into account in process studies, diagnostic, and prognostic models of DMS cycling, and (2) the community DMS yield is a key variable that defines characteristic DMS cycling regimes.
- Abstract: Dimethylsulfide (DMS) is a biogenic gas with potential climatic effects, and its marine emission depends on the interplay between microbial activity and physical forcing in the oceanic upper mixed layer (UML). We investigated the diel cycling patterns of DMS and its precursor dimethylsulfoniopropionate (DMSP) in four experiments (28 to 48 h long) performed in meso‐ to ultraoligotrophic Mediterranean and Sargasso Sea waters. Samples taken every 4 or 6 h were analyzed for dimethylated sulfur pools and incubated to measure DMS and DMSP cycling rates, as well as primary and bacterial production. In all four experiments, DMS budgets showed pronounced day vs. night variability. In the three summer experiments, gross community DMS production (GPDMS) increased by 2 to 3‐fold from night‐ to daytime, peaking 0–4 h after solar noon. This excess GPDMS was balanced by higher photochemical and microbial sinks during the day, effectively buffering DMS concentrations. In the only winter experiment, GPDMS exhibited opposed temporal dynamics and peaked at nighttime in parallel to total DMSP consumption. Community DMSP to DMS conversion yields were generally 15% (even > 50%) during the day in summer, presumably due to phytoplankton radiative stress. Our data suggest that (1) diel variability should be taken into account in process studies, diagnostic, and prognostic models of DMS cycling, and (2) the community DMS yield is a key variable that defines characteristic DMS cycling regimes.
- History of nutrient inputs to the Northeastern United States, 1930–2000
- Abstract: Humans have dramatically altered nutrient cycles at local to global scales. We examined changes in anthropogenic nutrient inputs to the northeastern United States (NE) from 1930 to 2000. We created a comprehensive time series of anthropogenic N and P inputs to 437 counties in the NE at five‐year intervals. Inputs included atmospheric N deposition, biological N2 fixation, fertilizer, detergent P, livestock feed, and human food. Exports included exports of feed and food and volatilization of ammonia. N inputs to the NE increased throughout the study period, primarily due to increases in atmospheric deposition and fertilizer. P inputs increased until 1970 and then declined due to decreased fertilizer and detergent inputs. Livestock consistently consumed the majority of nutrient inputs over time and space. The area of crop agriculture declined during the study period but consumed more nutrients as fertilizer. We found that stoichiometry (N:P) of inputs and absolute amounts of N matched nutritional needs (livestock, humans, crops) when atmospheric components (N deposition, N2‐fixation) were not included. Differences between N and P led to major changes in N:P stoichiometry over time, consistent with global trends. N:P decreased from 1930 to 1970 due to increased inputs of P, and increased from 1970 to 2000 due to increased N deposition and fertilizer and decreases in P fertilizer and detergent use. We found that nutrient use is a dynamic product of social, economic, political, and environmental interactions. Therefore, future nutrient management must take into account these factors to design successful and effective nutrient reduction measures.
- Abstract: Humans have dramatically altered nutrient cycles at local to global scales. We examined changes in anthropogenic nutrient inputs to the northeastern United States (NE) from 1930 to 2000. We created a comprehensive time series of anthropogenic N and P inputs to 437 counties in the NE at five‐year intervals. Inputs included atmospheric N deposition, biological N2 fixation, fertilizer, detergent P, livestock feed, and human food. Exports included exports of feed and food and volatilization of ammonia. N inputs to the NE increased throughout the study period, primarily due to increases in atmospheric deposition and fertilizer. P inputs increased until 1970 and then declined due to decreased fertilizer and detergent inputs. Livestock consistently consumed the majority of nutrient inputs over time and space. The area of crop agriculture declined during the study period but consumed more nutrients as fertilizer. We found that stoichiometry (N:P) of inputs and absolute amounts of N matched nutritional needs (livestock, humans, crops) when atmospheric components (N deposition, N2‐fixation) were not included. Differences between N and P led to major changes in N:P stoichiometry over time, consistent with global trends. N:P decreased from 1930 to 1970 due to increased inputs of P, and increased from 1970 to 2000 due to increased N deposition and fertilizer and decreases in P fertilizer and detergent use. We found that nutrient use is a dynamic product of social, economic, political, and environmental interactions. Therefore, future nutrient management must take into account these factors to design successful and effective nutrient reduction measures.
- Annual cycles of ecological disturbance and recovery underlying the subarctic Atlantic spring plankton bloom
- Abstract: Satellite measurements allow global assessments of phytoplankton concentrations and, from observed temporal changes in biomass, direct access to net biomass accumulation rates (r). For the subarctic Atlantic basin, analysis of annual cycles in r reveal that initiation of the annual blooming‐phase does not occur in spring after stratification surpasses a critical threshold, but rather in early winter when growth conditions for phytoplankton are deteriorating. This finding has been confirmed with in situ profiling float data. The objective of the current study was to test whether satellite‐based annual cycles in r are reproduced by the Biogeochemical Element Cycling ‐ Community Climate System Model and, if so, to use the additional ecosystem properties resolved by the model to better understand factors controlling phytoplankton blooms. We find that the model gives a similar early onset time for the blooming phase, that this initiation is largely due to the physical disruption of phytoplankton‐grazer interactions during mixed layer deepening, and that parallel increases in phytoplankton specific division and loss rates during spring maintain the subtle disruption in food web equilibrium that ultimately yields the spring bloom climax. The link between winter mixing and bloom dynamics is illustrated by contrasting annual plankton cycles between regions with deeper and shallower mixing. We show that maximum water column inventories of phytoplankton vary in proportion to maximum winter mixing depth, implying that future reductions in winter mixing may dampen plankton cycles in the subarctic Atlantic. We propose that ecosystem disturbance‐recovery sequences are a unifying property of global ocean plankton blooms.
- Abstract: Satellite measurements allow global assessments of phytoplankton concentrations and, from observed temporal changes in biomass, direct access to net biomass accumulation rates (r). For the subarctic Atlantic basin, analysis of annual cycles in r reveal that initiation of the annual blooming‐phase does not occur in spring after stratification surpasses a critical threshold, but rather in early winter when growth conditions for phytoplankton are deteriorating. This finding has been confirmed with in situ profiling float data. The objective of the current study was to test whether satellite‐based annual cycles in r are reproduced by the Biogeochemical Element Cycling ‐ Community Climate System Model and, if so, to use the additional ecosystem properties resolved by the model to better understand factors controlling phytoplankton blooms. We find that the model gives a similar early onset time for the blooming phase, that this initiation is largely due to the physical disruption of phytoplankton‐grazer interactions during mixed layer deepening, and that parallel increases in phytoplankton specific division and loss rates during spring maintain the subtle disruption in food web equilibrium that ultimately yields the spring bloom climax. The link between winter mixing and bloom dynamics is illustrated by contrasting annual plankton cycles between regions with deeper and shallower mixing. We show that maximum water column inventories of phytoplankton vary in proportion to maximum winter mixing depth, implying that future reductions in winter mixing may dampen plankton cycles in the subarctic Atlantic. We propose that ecosystem disturbance‐recovery sequences are a unifying property of global ocean plankton blooms.
- Biosphere model simulations of interannual variability in terrestrial 13C/12C exchange
- Abstract: Previous studies suggest that a large part of the variability in the atmospheric ratio of 13CO2/12Co2 originates from carbon exchange with the terrestrial biosphere rather than with the oceans. Since this variability is used to quantitatively partition the total carbon sink, we here investigate the contribution of interannual variability (IAV) in biospheric exchange to the observed atmospheric 13C variations. We use the SiBCASA biogeochemical model, including a detailed isotopic fractionation scheme, separate 12C and 13C biogeochemical pools, and satellite‐observed fire disturbances. This model of 12CO2 and 13CO2 thus also produces return fluxes of 13CO2 from its differently aged pools, contributing to the so‐called disequilibrium flux. Our simulated terrestrial 13C budget closely resembles previously published model results for plant discrimination and disequilibrium fluxes, and similarly suggests that variations in C3 discrimination and year‐to‐year variations in C3 and C4 productivity are the main drivers of their IAV. But the year‐to‐year variability in the isotopic disequilibrium flux is much lower (1σ = ± 1.5 PgC ‰ yr–1) than required (± 12.5 PgC ‰ yr–1) to match atmospheric observations, under the common assumption of low variability in net ocean CO2 fluxes. This contrasts with earlier published results. It is currently unclear how to increase IAV in these drivers suggesting that SIBCASA still misses processes that enhance variability in plant discrimination and relative C3/C4 productivity. Alternatively, 13C budget terms other than terrestrial disequilibrium fluxes, including possibly the atmospheric growth rate, must have significantly more IAV in order to close the atmospheric 13C budget on a year‐to‐year basis.
- Abstract: Previous studies suggest that a large part of the variability in the atmospheric ratio of 13CO2/12Co2 originates from carbon exchange with the terrestrial biosphere rather than with the oceans. Since this variability is used to quantitatively partition the total carbon sink, we here investigate the contribution of interannual variability (IAV) in biospheric exchange to the observed atmospheric 13C variations. We use the SiBCASA biogeochemical model, including a detailed isotopic fractionation scheme, separate 12C and 13C biogeochemical pools, and satellite‐observed fire disturbances. This model of 12CO2 and 13CO2 thus also produces return fluxes of 13CO2 from its differently aged pools, contributing to the so‐called disequilibrium flux. Our simulated terrestrial 13C budget closely resembles previously published model results for plant discrimination and disequilibrium fluxes, and similarly suggests that variations in C3 discrimination and year‐to‐year variations in C3 and C4 productivity are the main drivers of their IAV. But the year‐to‐year variability in the isotopic disequilibrium flux is much lower (1σ = ± 1.5 PgC ‰ yr–1) than required (± 12.5 PgC ‰ yr–1) to match atmospheric observations, under the common assumption of low variability in net ocean CO2 fluxes. This contrasts with earlier published results. It is currently unclear how to increase IAV in these drivers suggesting that SIBCASA still misses processes that enhance variability in plant discrimination and relative C3/C4 productivity. Alternatively, 13C budget terms other than terrestrial disequilibrium fluxes, including possibly the atmospheric growth rate, must have significantly more IAV in order to close the atmospheric 13C budget on a year‐to‐year basis.
- Evidence for changes in carbon isotopic fractionation by phytoplankton between 1960 and 2010
- Abstract: Rising CO2 is expected to drive a myriad of environmental changes in the surface ocean. Deciphering the phytoplankton response to this complex change is difficult. Here we determine whether a trend in the biological fractionation of stable carbon isotopes (εp) has occurred over the past 50 years. εp is primarily controlled by the acquistion and intracellular transport of inorganic carbon and the rate of carbon fixation. In turn, these processes are sensitive to phytoplankton physiology, community composition, and, notably, inorganic carbon availability. εp may therefore carry a signal of biological response to climate change. Temporal and spatial records of εp can be deciphered from the difference between the stable carbon isotopic composition of particulate organic matter (δ13CPOC) and that of the ambient inorganic carbon pool (δ13CCO2). Here we establish a global record of εp extending from the 1960s to today, extracted from a newly compiled dataset of global measured δ13CPOC and part measured/part climatology δ13CCO2. We find that εp has changed significantly since the 1960s in the low‐ to mid‐latitude surface ocean. The increase is most pronounced in the subtropics, where it is is estimated at > 0.015 ‰ per year. Our findings of such rates of change are further supported by a high resolution temporal record from a single sediment trap near Bermuda. Our results are consistent with the idea that εp is affected by increased inorganic carbon availability driven by the rise in atmospheric CO2.
- Abstract: Rising CO2 is expected to drive a myriad of environmental changes in the surface ocean. Deciphering the phytoplankton response to this complex change is difficult. Here we determine whether a trend in the biological fractionation of stable carbon isotopes (εp) has occurred over the past 50 years. εp is primarily controlled by the acquistion and intracellular transport of inorganic carbon and the rate of carbon fixation. In turn, these processes are sensitive to phytoplankton physiology, community composition, and, notably, inorganic carbon availability. εp may therefore carry a signal of biological response to climate change. Temporal and spatial records of εp can be deciphered from the difference between the stable carbon isotopic composition of particulate organic matter (δ13CPOC) and that of the ambient inorganic carbon pool (δ13CCO2). Here we establish a global record of εp extending from the 1960s to today, extracted from a newly compiled dataset of global measured δ13CPOC and part measured/part climatology δ13CCO2. We find that εp has changed significantly since the 1960s in the low‐ to mid‐latitude surface ocean. The increase is most pronounced in the subtropics, where it is is estimated at > 0.015 ‰ per year. Our findings of such rates of change are further supported by a high resolution temporal record from a single sediment trap near Bermuda. Our results are consistent with the idea that εp is affected by increased inorganic carbon availability driven by the rise in atmospheric CO2.
- Controls on dissolved organic carbon quantity and chemical character in temperate rivers of North America
- Abstract: Understanding the processes controlling the transfer and chemical composition of dissolved organic carbon (DOC) in freshwater systems is crucial to understanding the carbon cycle and the effects of DOC on water quality. Previous studies have identified watershed‐scale controls on bulk DOC flux and concentration among small basins but fewer studies have explored controls among large basins or simultaneously considered the chemical composition of DOC. Because the chemical character of DOC drives riverine biogeochemical processes such as metabolism and photodegradation, accounting for chemical character in watershed‐scale studies will improve the way bulk DOC variability in rivers is interpreted. We analyzed DOC quantity and chemical character near the mouths of 17 large North American rivers, primarily between 2008 and 2010, and identified watershed characteristics that controlled variability. We quantified DOC chemical character using both specific ultraviolet absorbance at 254 nm (SUVA254) and XAD‐resin fractionation. Mean DOC concentration ranged from 2.1 to 47 mgC L‐1 and mean SUVA254 ranged from 1.3 to 4.7 L mgC‐1 m‐1. We found a significant positive correlation between basin wetland‐cover and both bulk DOC concentration (R2 = 0.78; p
- Abstract: Understanding the processes controlling the transfer and chemical composition of dissolved organic carbon (DOC) in freshwater systems is crucial to understanding the carbon cycle and the effects of DOC on water quality. Previous studies have identified watershed‐scale controls on bulk DOC flux and concentration among small basins but fewer studies have explored controls among large basins or simultaneously considered the chemical composition of DOC. Because the chemical character of DOC drives riverine biogeochemical processes such as metabolism and photodegradation, accounting for chemical character in watershed‐scale studies will improve the way bulk DOC variability in rivers is interpreted. We analyzed DOC quantity and chemical character near the mouths of 17 large North American rivers, primarily between 2008 and 2010, and identified watershed characteristics that controlled variability. We quantified DOC chemical character using both specific ultraviolet absorbance at 254 nm (SUVA254) and XAD‐resin fractionation. Mean DOC concentration ranged from 2.1 to 47 mgC L‐1 and mean SUVA254 ranged from 1.3 to 4.7 L mgC‐1 m‐1. We found a significant positive correlation between basin wetland‐cover and both bulk DOC concentration (R2 = 0.78; p
- Humic substances may control dissolved iron distributions in the global ocean: Implications from numerical simulations
- Abstract: This study used an ocean general circulation model to simulate the marine iron cycle in an investigation of how simulated distributions of weak iron‐binding ligands would be expected to control dissolved iron concentrations in the ocean, with a particular focus on deep ocean waters. The distribution of apparent oxygen utilization was used as a proxy for humic substances that have recently been hypothesized to account for the bulk of weak iron‐binding ligands in seawater. Compared to simulations using a conventional approach with homogeneous ligand distributions, the simulations that incorporated spatially variable ligand concentrations exhibited substantial improvement in the simulation of global dissolved iron distributions as revealed by comparisons with available field data. The improved skill of the simulations resulted largely because the spatially variable ligand distributions led to a more reasonable basin‐scale variation of the residence time of iron when present at high concentrations. The model results, in conjunction with evidence from recent field studies, suggest that humic substances play an important role in the iron cycle in the ocean.
- Abstract: This study used an ocean general circulation model to simulate the marine iron cycle in an investigation of how simulated distributions of weak iron‐binding ligands would be expected to control dissolved iron concentrations in the ocean, with a particular focus on deep ocean waters. The distribution of apparent oxygen utilization was used as a proxy for humic substances that have recently been hypothesized to account for the bulk of weak iron‐binding ligands in seawater. Compared to simulations using a conventional approach with homogeneous ligand distributions, the simulations that incorporated spatially variable ligand concentrations exhibited substantial improvement in the simulation of global dissolved iron distributions as revealed by comparisons with available field data. The improved skill of the simulations resulted largely because the spatially variable ligand distributions led to a more reasonable basin‐scale variation of the residence time of iron when present at high concentrations. The model results, in conjunction with evidence from recent field studies, suggest that humic substances play an important role in the iron cycle in the ocean.
- Atmospheric deposition fluxes of 26 elements over the Southern Indian Ocean: time series on Kerguelen and Crozet Islands
- Abstract: Atmospheric deposition of dust is suspected to have a significant impact on biogeochemical processes in High‐Nutrient‐Low‐Chlorophyll waters of the open ocean. In this study, we report time series of atmospheric deposition samples collected over two years at three different sites on Kerguelen and Crozet Islands in the Southern Indian Ocean. Total atmospheric deposition fluxes were measured for a large suite of elements (Al, As, Ba, Ca, Ce, Co, Cr, Cu, Fe, K, La, Li, Mg, Mn, Na, Nd, Ni, Pb, Rb, S, Si, Sr, Ti, U, V, Zn). Most of them are identified as coming from sea‐salt or crustal sources, but enrichment factor variabilities of Pb, As, Cr, Cu and V highlight an anthropogenic contribution during the austral winter for these five elements. For Al, Fe, Mn and Si, deposition fluxes are similar for both Kerguelen and Crozet Islands. Fluxes for the other non‐sea‐salt elements exhibit differences below a factor of five with a decreasing gradient from Crozet to Kerguelen.
- Abstract: Atmospheric deposition of dust is suspected to have a significant impact on biogeochemical processes in High‐Nutrient‐Low‐Chlorophyll waters of the open ocean. In this study, we report time series of atmospheric deposition samples collected over two years at three different sites on Kerguelen and Crozet Islands in the Southern Indian Ocean. Total atmospheric deposition fluxes were measured for a large suite of elements (Al, As, Ba, Ca, Ce, Co, Cr, Cu, Fe, K, La, Li, Mg, Mn, Na, Nd, Ni, Pb, Rb, S, Si, Sr, Ti, U, V, Zn). Most of them are identified as coming from sea‐salt or crustal sources, but enrichment factor variabilities of Pb, As, Cr, Cu and V highlight an anthropogenic contribution during the austral winter for these five elements. For Al, Fe, Mn and Si, deposition fluxes are similar for both Kerguelen and Crozet Islands. Fluxes for the other non‐sea‐salt elements exhibit differences below a factor of five with a decreasing gradient from Crozet to Kerguelen.
- Response of methanogenic archaea to Late Pleistocene and Holocene climate changes in the Siberian Arctic
- Abstract: In order to investigate the link between the methane dynamics in permafrost deposits and climate changes in the past, we studied the abundance, composition, and methane production of methanogenic communities in Late Pleistocene and Holocene sediments of the Siberian Arctic. We detected intervals of increased methane concentrations in Late Pleistocene and Holocene deposits along a 42 ka old permafrost sequence from Kurungnakh Island in the Lena Delta (northeast Siberia). Increased amounts of archaeal life markers (intact phospholipid ethers) and a high variety in genetic fingerprints detected by 16S ribosomal ribonucleic acid gene analyses of methanogenic archaea suggest presently living and presumably active methanogenic archaea in distinct layers predominantly in Holocene deposits, but also in deep frozen ground at 17 m depth. Potential methanogenic activity was confirmed by incubation experiments. By comparing methane concentrations, microbial incubation experiments, gene analysis of methanogens, and microbial life markers (intact phospholipid esters and ethers) to already partly degraded membrane lipids, such as archaeol and isoprenoid glycerol dialkyl glycerol tetraethers, we demonstrated that archaeol likely represents a signal of past methanogenic archaea. The archaeol signal was used to reconstruct the response of methanogenic communities to past temperature changes in the Siberian Arctic, and the data suggest higher methane emissions occurred during warm periods, particularly during an interval in the Late Pleistocene and during the Holocene. This new data on present and past methanogenic communities in the Siberian terrestrial permafrost imply that these microorganisms will respond to the predicted future temperature rise in the Arctic with increasing methane production, as demonstrated in previous warmer periods.
- Abstract: In order to investigate the link between the methane dynamics in permafrost deposits and climate changes in the past, we studied the abundance, composition, and methane production of methanogenic communities in Late Pleistocene and Holocene sediments of the Siberian Arctic. We detected intervals of increased methane concentrations in Late Pleistocene and Holocene deposits along a 42 ka old permafrost sequence from Kurungnakh Island in the Lena Delta (northeast Siberia). Increased amounts of archaeal life markers (intact phospholipid ethers) and a high variety in genetic fingerprints detected by 16S ribosomal ribonucleic acid gene analyses of methanogenic archaea suggest presently living and presumably active methanogenic archaea in distinct layers predominantly in Holocene deposits, but also in deep frozen ground at 17 m depth. Potential methanogenic activity was confirmed by incubation experiments. By comparing methane concentrations, microbial incubation experiments, gene analysis of methanogens, and microbial life markers (intact phospholipid esters and ethers) to already partly degraded membrane lipids, such as archaeol and isoprenoid glycerol dialkyl glycerol tetraethers, we demonstrated that archaeol likely represents a signal of past methanogenic archaea. The archaeol signal was used to reconstruct the response of methanogenic communities to past temperature changes in the Siberian Arctic, and the data suggest higher methane emissions occurred during warm periods, particularly during an interval in the Late Pleistocene and during the Holocene. This new data on present and past methanogenic communities in the Siberian terrestrial permafrost imply that these microorganisms will respond to the predicted future temperature rise in the Arctic with increasing methane production, as demonstrated in previous warmer periods.
- Winners and Losers: Ecological and Biogeochemical Changes in a Warming Ocean
- Abstract: We employ a marine ecosystem model, with diverse and flexible phytoplankton communities, coupled to an earth system model of intermediate complexity to explore mechanisms that will alter the biogeography and productivity of phytoplankton populations in a warming world. Simple theoretical frameworks and sensitivity experiments reveal that ecological and biogeochemical changes are driven by a balance between two impacts of a warming climate: higher metabolic rates (the “direct” effect), and changes in the supply of limiting nutrients, and altered light environments (the “indirect” effect). On globally integrated productivity the two effects compensate to a large degree. Regionally the competition between effects is more complicated; patterns of productivity changes are different between high and low latitudes and are also regulated by how the supply of the limiting nutrient changes. These complex regional patterns are also found in the changes to broad phytoplankton functional groups. On the finer ecological scale of diversity within functional groups, we find that ranges of some phytoplankton types are reduced while those of others (potentially minor players in the present ocean) expand. Combined change in areal extent of range and in regionally available nutrients leads to global “winners and losers”. The model suggests that the strongest and most robust signal of the warming ocean is likely to be the large turnover in local phytoplankton community composition.
- Abstract: We employ a marine ecosystem model, with diverse and flexible phytoplankton communities, coupled to an earth system model of intermediate complexity to explore mechanisms that will alter the biogeography and productivity of phytoplankton populations in a warming world. Simple theoretical frameworks and sensitivity experiments reveal that ecological and biogeochemical changes are driven by a balance between two impacts of a warming climate: higher metabolic rates (the “direct” effect), and changes in the supply of limiting nutrients, and altered light environments (the “indirect” effect). On globally integrated productivity the two effects compensate to a large degree. Regionally the competition between effects is more complicated; patterns of productivity changes are different between high and low latitudes and are also regulated by how the supply of the limiting nutrient changes. These complex regional patterns are also found in the changes to broad phytoplankton functional groups. On the finer ecological scale of diversity within functional groups, we find that ranges of some phytoplankton types are reduced while those of others (potentially minor players in the present ocean) expand. Combined change in areal extent of range and in regionally available nutrients leads to global “winners and losers”. The model suggests that the strongest and most robust signal of the warming ocean is likely to be the large turnover in local phytoplankton community composition.
- 210Pb and 137Cs in Margin Sediments of the Arctic Ocean: Controls on Boundary Scavenging
- Abstract: 210Pb and 137Cs were measured in 25 sediment cores collected during the International Polar Year (IPY) from a transect spanning the North American Arctic margin, from the North Bering Sea to Baffin Bay/Davis Strait. Profiles and inventories of the radioisotopes were used to determine sediment mixing and accumulation at each site and to assess the intensity of scavenging and burial. Sediment accumulation rates derived from 210Pb and validated using 137Cs, are between ≤0.04 and 0.23 g cm‐2 yr‐1. 210Pb cannot be used to derive sedimentation rates for vigorously biomixed sediments from the North Bering‐Chukchi shelf. Elevated 137Cs activities and inventories in recently deposited sediments imply delayed inputs of particle‐associated 137Cs to the sediments, likely transported from the watershed to the coast and subsequently redistributed to shelf/slope sediments. Inventories of 210Pbex in all cores meet or exceed the estimated supply of 210Pbex from atmospheric deposition and decay of 226Ra in the water column. This implies that, in contrast to the deep Arctic Ocean basin, there is a sufficient supply of suspended particulates along the North American arctic margin to scavenge the supply of 210Pbex. 210Pbex inventories in sediments are up to 21‐fold greater than the in situ supply at some sites. Large inventories of 210Pbex in sediments along the North Bering‐Chukchi shelf result primarily from focusing, while those along the north Chukchi slope (Barrow Canyon) and in Baffin Bay/Davis Strait reflect strong boundary scavenging, likely supported by lateral exchanges with deep/interior Atlantic‐origin waters.
- Abstract: 210Pb and 137Cs were measured in 25 sediment cores collected during the International Polar Year (IPY) from a transect spanning the North American Arctic margin, from the North Bering Sea to Baffin Bay/Davis Strait. Profiles and inventories of the radioisotopes were used to determine sediment mixing and accumulation at each site and to assess the intensity of scavenging and burial. Sediment accumulation rates derived from 210Pb and validated using 137Cs, are between ≤0.04 and 0.23 g cm‐2 yr‐1. 210Pb cannot be used to derive sedimentation rates for vigorously biomixed sediments from the North Bering‐Chukchi shelf. Elevated 137Cs activities and inventories in recently deposited sediments imply delayed inputs of particle‐associated 137Cs to the sediments, likely transported from the watershed to the coast and subsequently redistributed to shelf/slope sediments. Inventories of 210Pbex in all cores meet or exceed the estimated supply of 210Pbex from atmospheric deposition and decay of 226Ra in the water column. This implies that, in contrast to the deep Arctic Ocean basin, there is a sufficient supply of suspended particulates along the North American arctic margin to scavenge the supply of 210Pbex. 210Pbex inventories in sediments are up to 21‐fold greater than the in situ supply at some sites. Large inventories of 210Pbex in sediments along the North Bering‐Chukchi shelf result primarily from focusing, while those along the north Chukchi slope (Barrow Canyon) and in Baffin Bay/Davis Strait reflect strong boundary scavenging, likely supported by lateral exchanges with deep/interior Atlantic‐origin waters.
- Legacy impacts of all‐time anthropogenic emissions on the global mercury cycle
- Abstract: Elevated mercury (Hg) in marine and terrestrial ecosystems is a global health concern because of the formation of toxic methyl mercury. Humans have emitted Hg to the atmosphere for millennia and this Hg has deposited and accumulated into ecosystems globally. Here we present a global biogeochemical model with fully‐coupled atmospheric, terrestrial and oceanic Hg reservoirs to better understand human influence on Hg cycling and timescales for responses. We drive the model with a historical inventory of anthropogenic emissions from 2000 BC to present. Results show that anthropogenic perturbations introduced to surface reservoirs (atmosphere, ocean, or terrestrial) accumulate and persist in the subsurface ocean for decades to centuries. The simulated present‐day atmosphere is enriched by a factor of 2.6 relative to 1840 levels, consistent with sediment archives, and by a factor of 7.5 relative to natural levels (2000 BC). Legacy anthropogenic Hg re‐emitted from surface reservoirs accounts for 60% of present‐day atmospheric deposition, compared to 27% from primary anthropogenic emissions, and 13% from natural sources. We find that only 17% of the present‐day Hg in the surface ocean is natural, and that half of its anthropogenic enrichment originates from pre‐1950 emissions. Although Asia is presently the dominant contributor to primary anthropogenic emissions, only 17% of the surface ocean reservoir is of Asian anthropogenic origin, as compared to 30% of North American and European origin. The accumulated burden of legacy anthropogenic Hg means that future deposition will increase even if primary anthropogenic emissions are held constant. Aggressive global Hg emissions reductions will be necessary just to maintain oceanic Hg concentrations at present levels.
- Abstract: Elevated mercury (Hg) in marine and terrestrial ecosystems is a global health concern because of the formation of toxic methyl mercury. Humans have emitted Hg to the atmosphere for millennia and this Hg has deposited and accumulated into ecosystems globally. Here we present a global biogeochemical model with fully‐coupled atmospheric, terrestrial and oceanic Hg reservoirs to better understand human influence on Hg cycling and timescales for responses. We drive the model with a historical inventory of anthropogenic emissions from 2000 BC to present. Results show that anthropogenic perturbations introduced to surface reservoirs (atmosphere, ocean, or terrestrial) accumulate and persist in the subsurface ocean for decades to centuries. The simulated present‐day atmosphere is enriched by a factor of 2.6 relative to 1840 levels, consistent with sediment archives, and by a factor of 7.5 relative to natural levels (2000 BC). Legacy anthropogenic Hg re‐emitted from surface reservoirs accounts for 60% of present‐day atmospheric deposition, compared to 27% from primary anthropogenic emissions, and 13% from natural sources. We find that only 17% of the present‐day Hg in the surface ocean is natural, and that half of its anthropogenic enrichment originates from pre‐1950 emissions. Although Asia is presently the dominant contributor to primary anthropogenic emissions, only 17% of the surface ocean reservoir is of Asian anthropogenic origin, as compared to 30% of North American and European origin. The accumulated burden of legacy anthropogenic Hg means that future deposition will increase even if primary anthropogenic emissions are held constant. Aggressive global Hg emissions reductions will be necessary just to maintain oceanic Hg concentrations at present levels.
- Fine root dynamics along an elevational gradient in tropical Amazonian and Andean forests
- Abstract: The key role of tropical forest belowground carbon stocks and fluxes is well recognised as one of the main components of the terrestrial ecosystem carbon cycle. This study presents the first detailed investigation of spatial and temporal patterns of fine root stocks and fluxes in tropical forests along an elevational gradient, ranging from the Peruvian Andes (3020 m) to lowland Amazonia (194 m), with mean annual temperatures of 11.8°C to 26.4 °C and annual rainfall values of 1900 to 1560 mm yr‐1, respectively. Specifically, we analyse abiotic parameters controlling fine root dynamics, fine root growth characteristics, and seasonality of net primary productivity along the elevation gradient. Root and soil carbon stocks were measured by means of soil cores, and fine root productivity was recorded using rhizotron chambers and ingrowth cores. We find that mean annual fine root below ground net primary productivity in the montane forests (0–30 cm depth) ranged between 4.27±0.56 Mg C ha‐1 yr‐1 (1855 m) and 1.72±0.87 Mg C ha‐1 yr‐1 (3020 m). These values include a correction for finest roots (
- Abstract: The key role of tropical forest belowground carbon stocks and fluxes is well recognised as one of the main components of the terrestrial ecosystem carbon cycle. This study presents the first detailed investigation of spatial and temporal patterns of fine root stocks and fluxes in tropical forests along an elevational gradient, ranging from the Peruvian Andes (3020 m) to lowland Amazonia (194 m), with mean annual temperatures of 11.8°C to 26.4 °C and annual rainfall values of 1900 to 1560 mm yr‐1, respectively. Specifically, we analyse abiotic parameters controlling fine root dynamics, fine root growth characteristics, and seasonality of net primary productivity along the elevation gradient. Root and soil carbon stocks were measured by means of soil cores, and fine root productivity was recorded using rhizotron chambers and ingrowth cores. We find that mean annual fine root below ground net primary productivity in the montane forests (0–30 cm depth) ranged between 4.27±0.56 Mg C ha‐1 yr‐1 (1855 m) and 1.72±0.87 Mg C ha‐1 yr‐1 (3020 m). These values include a correction for finest roots (
- The contribution of Fe (III) and humic acid reduction to ecosystem respiration in drained thaw lake basins of the Arctic Coastal Plain
- Abstract: Previous research showed that anaerobic respiration using iron (Fe) oxides as terminal electron acceptor contributed substantially to ecosystem respiration (ER) in a drained thaw lake basin (DTLB) on the Arctic coastal plain. As DTLB age, the surface organic layer thickens, progressively burying the Fe‐rich mineral layers. We therefore hypothesized that Fe (III) availability and Fe reduction would decline with basin age. We studied four DTLB across an age gradient, comparing seasonal changes in the oxidation state of dissolved and extractable Fe pools and the estimated contribution of Fe reduction to ER. The organic layer thickness did not strictly increase with age for these four sites, though soil Fe levels decreased with increasing organic layer thickness. However, there were surprisingly high levels of Fe minerals in organic layers, especially in the ancient basin where cryoturbation may have transported Fe upward through the profile. Net reduction of Fe oxides occurred in the latter half of the summer, and contributed an estimated 40–45% to ecosystem respiration in the sites with the thickest organic layers and 61–63% in the sites with the thinnest organic layers. All sites had high concentrations of soluble Fe (II) and Fe (III), explained by the presence of siderophores, and this pool became progressively more reduced during the first half of the summer. Redox titrations with humic acid (HA) extracts and chelated Fe support our view that this pattern indicates the reduction of HA during this interval. We conclude that Fe (III) and HA reduction contribute broadly to ER in the Arctic coastal plain.
- Abstract: Previous research showed that anaerobic respiration using iron (Fe) oxides as terminal electron acceptor contributed substantially to ecosystem respiration (ER) in a drained thaw lake basin (DTLB) on the Arctic coastal plain. As DTLB age, the surface organic layer thickens, progressively burying the Fe‐rich mineral layers. We therefore hypothesized that Fe (III) availability and Fe reduction would decline with basin age. We studied four DTLB across an age gradient, comparing seasonal changes in the oxidation state of dissolved and extractable Fe pools and the estimated contribution of Fe reduction to ER. The organic layer thickness did not strictly increase with age for these four sites, though soil Fe levels decreased with increasing organic layer thickness. However, there were surprisingly high levels of Fe minerals in organic layers, especially in the ancient basin where cryoturbation may have transported Fe upward through the profile. Net reduction of Fe oxides occurred in the latter half of the summer, and contributed an estimated 40–45% to ecosystem respiration in the sites with the thickest organic layers and 61–63% in the sites with the thinnest organic layers. All sites had high concentrations of soluble Fe (II) and Fe (III), explained by the presence of siderophores, and this pool became progressively more reduced during the first half of the summer. Redox titrations with humic acid (HA) extracts and chelated Fe support our view that this pattern indicates the reduction of HA during this interval. We conclude that Fe (III) and HA reduction contribute broadly to ER in the Arctic coastal plain.
- Atmospheric Δ14C reduction in simulations of atlantic overturning circulation shutdown
- Abstract: A rapid reduction in the Atlantic meridional overturning circulation (AMOC) can significantly disrupt the global heat transport and likely triggered abrupt climate change during the last glacial cycle. A slowdown in AMOC has long been assumed to inhibit the exchange of carbon between the atmosphere and the deep ocean and thus cause radiocarbon (14C), which is produced in the atmosphere, to accumulate in the atmosphere. Indeed previous model studies have demonstrated that a reduction in AMOC leads to higher atmospheric 14C abundance (Δ14C). However, this seems inconsistent with the observed rise in atmospheric pCO2 during Heinrich 1 and the Younger Dryas stadial events and the emerging view that this CO2 rise resulted from the deep ocean venting “old” carbon. Using an earth system model, we offer an alternative scenario that AMOC slowdown and an accompanying dynamical response in the south (i.e., the bipolar seesaw) can in fact lead to a decline in atmospheric Δ14C. This decline is realized in the model when the bipolar seesaw and thus the flux of old carbon from the Southern Ocean are sufficiently large so as to overcome the accumulation of 14C in the atmosphere as AMOC is reduced. The bipolar seesaw we describe invokes an oceanic teleconnection, whereby a freshwater perturbation in the North Atlantic drives a southern Δ14C response, but this does not necessarily preclude an atmospheric teleconnection.
- Abstract: A rapid reduction in the Atlantic meridional overturning circulation (AMOC) can significantly disrupt the global heat transport and likely triggered abrupt climate change during the last glacial cycle. A slowdown in AMOC has long been assumed to inhibit the exchange of carbon between the atmosphere and the deep ocean and thus cause radiocarbon (14C), which is produced in the atmosphere, to accumulate in the atmosphere. Indeed previous model studies have demonstrated that a reduction in AMOC leads to higher atmospheric 14C abundance (Δ14C). However, this seems inconsistent with the observed rise in atmospheric pCO2 during Heinrich 1 and the Younger Dryas stadial events and the emerging view that this CO2 rise resulted from the deep ocean venting “old” carbon. Using an earth system model, we offer an alternative scenario that AMOC slowdown and an accompanying dynamical response in the south (i.e., the bipolar seesaw) can in fact lead to a decline in atmospheric Δ14C. This decline is realized in the model when the bipolar seesaw and thus the flux of old carbon from the Southern Ocean are sufficiently large so as to overcome the accumulation of 14C in the atmosphere as AMOC is reduced. The bipolar seesaw we describe invokes an oceanic teleconnection, whereby a freshwater perturbation in the North Atlantic drives a southern Δ14C response, but this does not necessarily preclude an atmospheric teleconnection.
- Phosphorus cycling in the Sargasso Sea: Investigation using the oxygen isotopic composition of phosphate, enzyme labeled fluorescence, and turnover times
- Abstract: Dissolved inorganic phosphorus (DIP) concentrations in surface water of vast areas of the ocean are extremely low (< 10 nM) and phosphorus (P) availability could limit primary productivity in these regions. We explore the use of oxygen isotopic signature of dissolved phosphate (δ18OPO4) to investigate biogeochemical cycling of P in the Sargasso Sea, Atlantic Ocean. Additional techniques for studying P dynamics including 33P based DIP turnover time estimates and percent of cells expressing alkaline phosphatase (AP) activity as measured by enzyme‐labeling fluorescence (ELF) are also used. In surface waters δ18OPO4 values were lower than equilibrium by 3‐6‰, indicative of dissolved organic phosphorous (DOP) remineralization by extracellular enzymes. An isotope mass balance model using a variety of possible combinations of enzymatic pathways and substrates indicates that DOP remineralization in the euphotic zone can account for a large proportion on P utilized by phytoplankton (as much as 82%). Relatively short DIP turnover times (4‐8 hours) and high expression of AP (38‐77% of the cells labeled) are consistent with extensive DOP utilization and low DIP availability in the euphotoc zone. In deep water where DOP utilization rates are lower δ18OPO4 values approach isotopic equilibrium and DIP turnover times are longer. Our data suggests that in the euphotic zone of the Sargasso Sea DOP may be appreciably remineralized and utilized by phytoplankton and bacteria to supplement cellular requirements. A substantial fraction of photosynthesis in this region is supported by DOP uptake.
- Abstract: Dissolved inorganic phosphorus (DIP) concentrations in surface water of vast areas of the ocean are extremely low (< 10 nM) and phosphorus (P) availability could limit primary productivity in these regions. We explore the use of oxygen isotopic signature of dissolved phosphate (δ18OPO4) to investigate biogeochemical cycling of P in the Sargasso Sea, Atlantic Ocean. Additional techniques for studying P dynamics including 33P based DIP turnover time estimates and percent of cells expressing alkaline phosphatase (AP) activity as measured by enzyme‐labeling fluorescence (ELF) are also used. In surface waters δ18OPO4 values were lower than equilibrium by 3‐6‰, indicative of dissolved organic phosphorous (DOP) remineralization by extracellular enzymes. An isotope mass balance model using a variety of possible combinations of enzymatic pathways and substrates indicates that DOP remineralization in the euphotic zone can account for a large proportion on P utilized by phytoplankton (as much as 82%). Relatively short DIP turnover times (4‐8 hours) and high expression of AP (38‐77% of the cells labeled) are consistent with extensive DOP utilization and low DIP availability in the euphotoc zone. In deep water where DOP utilization rates are lower δ18OPO4 values approach isotopic equilibrium and DIP turnover times are longer. Our data suggests that in the euphotic zone of the Sargasso Sea DOP may be appreciably remineralized and utilized by phytoplankton and bacteria to supplement cellular requirements. A substantial fraction of photosynthesis in this region is supported by DOP uptake.
- Carbon evasion/accumulation ratio in boreal lakes is linked to Nitrogen
- Abstract: The role of lakes in landscape carbon (C) cycling and primary drivers behind freshwater C balance have remained poorly known, although lakes are an important landscape component and cover 10% of Finland's total area. We studied CO2 evasion and average Holocene C accumulation in 82 boreal lakes (0.04 ‐ 1 540 km2; max depth 1 ‐ 93 m) located between the latitudes 60 °C and 69 °C. Both CO2 evasion and C accumulation correlated with numerous drivers and were closely linked to lake area and maximum depth. The total aquatic C retention (C evasion + C accumulation) was largely determined by lake area (r2 = 0.96, p
- Abstract: The role of lakes in landscape carbon (C) cycling and primary drivers behind freshwater C balance have remained poorly known, although lakes are an important landscape component and cover 10% of Finland's total area. We studied CO2 evasion and average Holocene C accumulation in 82 boreal lakes (0.04 ‐ 1 540 km2; max depth 1 ‐ 93 m) located between the latitudes 60 °C and 69 °C. Both CO2 evasion and C accumulation correlated with numerous drivers and were closely linked to lake area and maximum depth. The total aquatic C retention (C evasion + C accumulation) was largely determined by lake area (r2 = 0.96, p
- Amounts, isotopic character and ages of organic and inorganic carbon exported from rivers to ocean margins: 2. Assessment of natural and anthropogenic controls
- Abstract: Riverine exports of carbon (C) and organic matter (OM) are regulated by a variety of natural and anthropogenic factors. Understanding the relationships between these various factors and C and OM exports can help to constrain global C budgets, and allow assessment of current and future anthropogenic impacts on both riverine and global C cycles. We quantified the effects of multiple natural and anthropogenic controls on riverine export fluxes and compositions of particulate organic C, dissolved organic C, and dissolved inorganic C for a regional group of eight rivers in the northeastern U.S. Potential controls related to hydrogeomorphology and regional climate, soil order, soil texture, bedrock lithology, land use, and anthropogenic factors were analyzed individually, collectively, and at scales of both local and regional influence. Factors related to hydrogeomorphology and climate, followed in importance by land use and anthropogenic factors, exhibited the strongest impacts on riverine C exports and compositions, particularly at smaller localized scales. The effects of hydrogeomorphology and climate were primarily related to volumetric flow, which resulted in greater exports of terrestrial and total C. Principal anthropogenic factors included impacts of wastewater treatment plants (WWTPs) and river impoundments. The presence of WWTPs as well as anthropogenic use of carbonate‐based materials (e.g., limestone) may have substantially increased riverine C exports, particularly fossil C exports, in the study region. The presence of nuclear power plants in the associated watersheds is also discussed because of the potential for anthropogenic 14C inputs and subsequent biasing of aquatic C studies utilizing natural abundance 14C.
- Abstract: Riverine exports of carbon (C) and organic matter (OM) are regulated by a variety of natural and anthropogenic factors. Understanding the relationships between these various factors and C and OM exports can help to constrain global C budgets, and allow assessment of current and future anthropogenic impacts on both riverine and global C cycles. We quantified the effects of multiple natural and anthropogenic controls on riverine export fluxes and compositions of particulate organic C, dissolved organic C, and dissolved inorganic C for a regional group of eight rivers in the northeastern U.S. Potential controls related to hydrogeomorphology and regional climate, soil order, soil texture, bedrock lithology, land use, and anthropogenic factors were analyzed individually, collectively, and at scales of both local and regional influence. Factors related to hydrogeomorphology and climate, followed in importance by land use and anthropogenic factors, exhibited the strongest impacts on riverine C exports and compositions, particularly at smaller localized scales. The effects of hydrogeomorphology and climate were primarily related to volumetric flow, which resulted in greater exports of terrestrial and total C. Principal anthropogenic factors included impacts of wastewater treatment plants (WWTPs) and river impoundments. The presence of WWTPs as well as anthropogenic use of carbonate‐based materials (e.g., limestone) may have substantially increased riverine C exports, particularly fossil C exports, in the study region. The presence of nuclear power plants in the associated watersheds is also discussed because of the potential for anthropogenic 14C inputs and subsequent biasing of aquatic C studies utilizing natural abundance 14C.
- Dissolved organic nitrogen in the global surface ocean: Distribution and fate
- Abstract: The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean's oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface‐accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone.
- Abstract: The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean's oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface‐accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone.
- Contributions of long‐distance dust transport to atmospheric P inputs in the Yucatan Peninsula
- Abstract: Atmospheric deposition is not typically considered in conceptual models of P cycling in terrestrial ecosystems, but in P‐limited tropical forests that receive significant inputs of dust, it may play an important role in sustaining ecosystem productivity. We used models and observations to quantify total atmospheric P inputs and the contribution of long‐distance dust transport to tropical dry forests in the Yucatan peninsula over a 10 year period. Total atmospheric P input was estimated from atmospheric bulk deposition sampling in the southern Yucatan peninsula in 2007 and 2010–2011, and P input from dust deposition was estimated using two independent methods: zonal dust flux divergence based on MODIS AOD retrievals, and MATCH atmospheric transport modeling. Total atmospheric P input was 265 ± 80 g P ha‐1 yr‐1, and dust P input was 46 ± 12 g P ha‐1 yr‐1. There was significant seasonal and interannual variation, with local biomass burning accounting for high P inputs in April and May, and dust transport from June through August. We found that MATCH underestimates P deposition from dust to the Yucatan because of underestimation of dust transport relative to MODIS remote sensing. Dust accounted for 25% of total atmospheric P inputs in the Yucatan, indicating local sources dominate atmospheric inputs. However, dust is still an important source of P for tropical dry forests in the Yucatan since it is a new input of P that occurs during the early growing season of the forest, and is large enough to offset leaching and erosional losses from soils.
- Abstract: Atmospheric deposition is not typically considered in conceptual models of P cycling in terrestrial ecosystems, but in P‐limited tropical forests that receive significant inputs of dust, it may play an important role in sustaining ecosystem productivity. We used models and observations to quantify total atmospheric P inputs and the contribution of long‐distance dust transport to tropical dry forests in the Yucatan peninsula over a 10 year period. Total atmospheric P input was estimated from atmospheric bulk deposition sampling in the southern Yucatan peninsula in 2007 and 2010–2011, and P input from dust deposition was estimated using two independent methods: zonal dust flux divergence based on MODIS AOD retrievals, and MATCH atmospheric transport modeling. Total atmospheric P input was 265 ± 80 g P ha‐1 yr‐1, and dust P input was 46 ± 12 g P ha‐1 yr‐1. There was significant seasonal and interannual variation, with local biomass burning accounting for high P inputs in April and May, and dust transport from June through August. We found that MATCH underestimates P deposition from dust to the Yucatan because of underestimation of dust transport relative to MODIS remote sensing. Dust accounted for 25% of total atmospheric P inputs in the Yucatan, indicating local sources dominate atmospheric inputs. However, dust is still an important source of P for tropical dry forests in the Yucatan since it is a new input of P that occurs during the early growing season of the forest, and is large enough to offset leaching and erosional losses from soils.
- Inorganic carbon loading as a primary driver of dissolved carbon dioxideconcentrations in the lakes and reservoirs of the contiguous United States
- Abstract: AbstractAccurate quantification of CO2 flux across the air‐water interface and identification ofthe mechanisms driving CO2concentrations in lakes and reservoirs is critical to integrating aquatic systems into large‐scale carbon budgets, and to predicting the response of these systems to changes in climate or terrestrial carbon cycling. Large‐scale estimates of the role of lakes and reservoirs in the carbon cycle, however, typically must rely on aggregation of spatially and temporally inconsistent data from disparate sources.We performed a spatially comprehensive analysis of CO2concentration and air‐water fluxes in lakes and reservoirs of the contiguous United States using large, consistent data sets, and modeled the relative contribution of inorganic and organic carbon loading to vertical CO2 fluxes. Approximately 70% of lakes and reservoirs are supersaturated with respect to the atmosphere during the summer (June–September). Although there is considerable inter‐ and intraregional variability, lakes and reservoirs represent a net source of CO2 to the atmosphere of approximately 40Gg C d‐1 during the summer. While in‐lake CO2 concentrations correlate with indicators of in‐lake net ecosystem productivity (NEP), virtually no relationship exists between dissolved organic carbon (DOC) and pCO2,aq. Modeling suggests that hydrologic dissolved inorganic carbon (DIC) supports pCO2,aqin most supersaturated systems (to the extent that 12% of supersaturated systems simultaneously exhibit positive NEP), and also supports primary production in most CO2‐undersaturated systems. DIC loading appears to be an important determinant of CO2concentrations and fluxes across the air‐water interface in the majority of lakes and reservoirs in the contiguous United States.
- Abstract: AbstractAccurate quantification of CO2 flux across the air‐water interface and identification ofthe mechanisms driving CO2concentrations in lakes and reservoirs is critical to integrating aquatic systems into large‐scale carbon budgets, and to predicting the response of these systems to changes in climate or terrestrial carbon cycling. Large‐scale estimates of the role of lakes and reservoirs in the carbon cycle, however, typically must rely on aggregation of spatially and temporally inconsistent data from disparate sources.We performed a spatially comprehensive analysis of CO2concentration and air‐water fluxes in lakes and reservoirs of the contiguous United States using large, consistent data sets, and modeled the relative contribution of inorganic and organic carbon loading to vertical CO2 fluxes. Approximately 70% of lakes and reservoirs are supersaturated with respect to the atmosphere during the summer (June–September). Although there is considerable inter‐ and intraregional variability, lakes and reservoirs represent a net source of CO2 to the atmosphere of approximately 40Gg C d‐1 during the summer. While in‐lake CO2 concentrations correlate with indicators of in‐lake net ecosystem productivity (NEP), virtually no relationship exists between dissolved organic carbon (DOC) and pCO2,aq. Modeling suggests that hydrologic dissolved inorganic carbon (DIC) supports pCO2,aqin most supersaturated systems (to the extent that 12% of supersaturated systems simultaneously exhibit positive NEP), and also supports primary production in most CO2‐undersaturated systems. DIC loading appears to be an important determinant of CO2concentrations and fluxes across the air‐water interface in the majority of lakes and reservoirs in the contiguous United States.
- Amounts, isotopic character and ages of organic and inorganic carbon exported from rivers to ocean margins: 1. Estimates of terrestrial losses and inputs to the Middle Atlantic Bight
- Abstract: Rivers transport carbon (C) from terrestrial ecosystems to the coastal ocean, providing significant heterotrophic support within both rivers and receiving coastal waters. The amounts and ages of these terrestrial‐river‐coastal ocean C fluxes, however,are still poorly constrained. To address this uncertainty, a study of eight rivers discharging to the Middle Atlantic Bight (MAB) was undertaken. The rivers were sampled periodically over two years for concentrations and δ13C and Δ14C signatures of particulate organic C (POC), dissolved organic C (DOC) and dissolved inorganic C (DIC). For the watersheds draining to the MAB, it was estimated that ~ 3,800 Gg of terrestrial organic C (OC) and 700 Gg of terrestrial inorganic C was removed annually by fluvial transport. Of the terrestrial OC loss, ~ 64 % was contemporary C representing approximately 1 % of the annual terrestrial net primary productivity. Net fluvial C inputs to the MAB shelf were estimated to be ~ 70 Gg·yr−1 of POC, 280 Gg·yr−1 of DOC and 800 Gg·yr−1 of DIC. Terrestrial C, as opposed to in situ produced river C, comprised the majority of the riverine POC and DOC flux and aroundhalf of the total C flux. A smaller but significant fraction (< 25 %) of the river C flux was further composed of aged materials deriving from fossil C and aged soil OC. The timing of fluvial OC inputs to the MAB, which appear to be temporally offset from peak MAB primary production, could help support the net heterotrophy that has been observed there during periods of low productivity.
- Abstract: Rivers transport carbon (C) from terrestrial ecosystems to the coastal ocean, providing significant heterotrophic support within both rivers and receiving coastal waters. The amounts and ages of these terrestrial‐river‐coastal ocean C fluxes, however,are still poorly constrained. To address this uncertainty, a study of eight rivers discharging to the Middle Atlantic Bight (MAB) was undertaken. The rivers were sampled periodically over two years for concentrations and δ13C and Δ14C signatures of particulate organic C (POC), dissolved organic C (DOC) and dissolved inorganic C (DIC). For the watersheds draining to the MAB, it was estimated that ~ 3,800 Gg of terrestrial organic C (OC) and 700 Gg of terrestrial inorganic C was removed annually by fluvial transport. Of the terrestrial OC loss, ~ 64 % was contemporary C representing approximately 1 % of the annual terrestrial net primary productivity. Net fluvial C inputs to the MAB shelf were estimated to be ~ 70 Gg·yr−1 of POC, 280 Gg·yr−1 of DOC and 800 Gg·yr−1 of DIC. Terrestrial C, as opposed to in situ produced river C, comprised the majority of the riverine POC and DOC flux and aroundhalf of the total C flux. A smaller but significant fraction (< 25 %) of the river C flux was further composed of aged materials deriving from fossil C and aged soil OC. The timing of fluvial OC inputs to the MAB, which appear to be temporally offset from peak MAB primary production, could help support the net heterotrophy that has been observed there during periods of low productivity.
- Dark inorganic carbon fixation sustains the functioning of benthic deep‐sea ecosystems
- Abstract: Previous studies have provided evidence that dark inorganic carbon fixation is an important process for the functioning of the ocean interior. However, its quantitative relevance and ecological significance in benthic deep‐sea ecosystems remain unknown. We investigated the rates of inorganic carbon fixation together with prokaryotic abundance, biomass, assemblage composition, and heterotrophic carbon production in surface sediments of different benthic deep‐sea systems along the Iberian margin (northeastern Atlantic Ocean) and in the Mediterranean Sea. Inorganic carbon fixation rates in these surface deep‐sea sediments did not show clear depth‐related patterns, and on average, they accounted for 19% of the total heterotrophic biomass production. The incorporation rates of inorganic carbon were significantly related to the abundance of total Archaea (as determined by catalyzed reporter deposition fluorescence in‐situ hybridization), and were completely inhibited using an inhibitor of archaeal metabolism, N1‐guanyl‐1,7‐diaminoheptane. This suggests a major role of the archaeal assemblages in inorganic carbon fixation. We also show that benthic archaeal assemblages contribute ca. 25% of the total 3H‐leucine incorporation. Inorganic carbon fixation in surface deep‐sea sediments appears to be not only dependent upon chemosynthetic processes, but also on heterotrophic/ mixotrophic metabolism, as suggested by estimates of the chemolithotrophic energy requirements and the enhanced inorganic carbon fixation due to the increase in the availability of organic trophic resources. Overall, our data suggest that archaeal assemblages of surface deep‐sea sediments are responsible for the high rates of inorganic carbon incorporation, and thereby they sustain the functioning of the food webs and influence the carbon cycling of benthic deep‐sea ecosystems.
- Abstract: Previous studies have provided evidence that dark inorganic carbon fixation is an important process for the functioning of the ocean interior. However, its quantitative relevance and ecological significance in benthic deep‐sea ecosystems remain unknown. We investigated the rates of inorganic carbon fixation together with prokaryotic abundance, biomass, assemblage composition, and heterotrophic carbon production in surface sediments of different benthic deep‐sea systems along the Iberian margin (northeastern Atlantic Ocean) and in the Mediterranean Sea. Inorganic carbon fixation rates in these surface deep‐sea sediments did not show clear depth‐related patterns, and on average, they accounted for 19% of the total heterotrophic biomass production. The incorporation rates of inorganic carbon were significantly related to the abundance of total Archaea (as determined by catalyzed reporter deposition fluorescence in‐situ hybridization), and were completely inhibited using an inhibitor of archaeal metabolism, N1‐guanyl‐1,7‐diaminoheptane. This suggests a major role of the archaeal assemblages in inorganic carbon fixation. We also show that benthic archaeal assemblages contribute ca. 25% of the total 3H‐leucine incorporation. Inorganic carbon fixation in surface deep‐sea sediments appears to be not only dependent upon chemosynthetic processes, but also on heterotrophic/ mixotrophic metabolism, as suggested by estimates of the chemolithotrophic energy requirements and the enhanced inorganic carbon fixation due to the increase in the availability of organic trophic resources. Overall, our data suggest that archaeal assemblages of surface deep‐sea sediments are responsible for the high rates of inorganic carbon incorporation, and thereby they sustain the functioning of the food webs and influence the carbon cycling of benthic deep‐sea ecosystems.
- Diel vertical migration: ecological controls and impacts on the biological pump in a one‐dimensional ocean model
- Abstract: Diel vertical migration (DVM) of zooplankton and micronekton is widespread in the ocean and forms a fundamental component of the biological pump, but is generally overlooked in global models of the Earth System. We develop a parameterization of DVM in the ocean and integrate it with a size‐structured NPZD model. We assess the model's ability to recreate ecosystem and DVM patterns at three well observed Pacific sites, ALOHA, K2 and EQPAC, and use it to estimate the impact of DVM on marine ecosystems and biogeochemical dynamics. Our model includes: (1) a representation of migration dynamics in response to food availability and light intensity, (2) a representation of the digestive and metabolic processes that decouple zooplankton feeding from excretion, egestion and respiration, and (3) a light‐dependent parameterization of visual predation on zooplankton. The model captures the first order patterns in plankton biomass and productivity across the biomes, including the biomass of migrating organisms. We estimate that realistic migratory populations sustain active fluxes to the mesopelagic zone equivalent to between 15 and 40 % the particle export, and contribute up to half of the total respiration within the layers affected by migration. The localized active transport has important consequences for the cycling of oxygen, nutrients and carbon. We highlight the importance of decoupling zooplankton feeding and respiration and excretion with depth for capturing the impact of migration on the redistribution of carbon and nutrients in the upper ocean.
- Abstract: Diel vertical migration (DVM) of zooplankton and micronekton is widespread in the ocean and forms a fundamental component of the biological pump, but is generally overlooked in global models of the Earth System. We develop a parameterization of DVM in the ocean and integrate it with a size‐structured NPZD model. We assess the model's ability to recreate ecosystem and DVM patterns at three well observed Pacific sites, ALOHA, K2 and EQPAC, and use it to estimate the impact of DVM on marine ecosystems and biogeochemical dynamics. Our model includes: (1) a representation of migration dynamics in response to food availability and light intensity, (2) a representation of the digestive and metabolic processes that decouple zooplankton feeding from excretion, egestion and respiration, and (3) a light‐dependent parameterization of visual predation on zooplankton. The model captures the first order patterns in plankton biomass and productivity across the biomes, including the biomass of migrating organisms. We estimate that realistic migratory populations sustain active fluxes to the mesopelagic zone equivalent to between 15 and 40 % the particle export, and contribute up to half of the total respiration within the layers affected by migration. The localized active transport has important consequences for the cycling of oxygen, nutrients and carbon. We highlight the importance of decoupling zooplankton feeding and respiration and excretion with depth for capturing the impact of migration on the redistribution of carbon and nutrients in the upper ocean.
- Analysis of trends in fused AVHRR and MODIS NDVI data for 1982–2006: Indication for a CO2 fertilization effect in global vegetation
- Abstract: Recent studies report an increase in vegetation greenness in mid‐to‐high northern latitudes. This increase is observed in leaf‐out data in Europe and North America since the 1950s and in satellite data since the 1980s. Increased vegetation greenness is potentially a factor contributing to a land CO2 sink. Various causes for increased vegetation greenness are suggested, but their relative importance is uncertain. In the present study the effect of climate and CO2 fertilization on increased vegetation greenness and the land CO2 sink are investigated. The study is organized as follows: (1) A model is used to simulate monthly global normalized difference vegetation index (NDVI) fields for 1901–2006. The model is derived from NDVI, precipitation and temperature data for 1982–1999. The modeled fields, referred to as reconstructed vegetation index (RVI), are tested back in time on phenological data (1950s–1990s) and forward in time on MODIS data (2001–2006). The RVI represents the response of NDVI to variations in climate. (2) Residuals between RVI and NDVI are analyzed for associations with variations in down‐welling solar radiation, nitrogen deposition, satellite‐related artefacts, and CO2 fertilization. CO2 fertilization was the only factor that improved RVI modeling. (3) The effect of climate variations and CO2 fertilization on the land CO2 sink, as manifested in the RVI, is explored with the Carnegie Ames Stanford Assimilation (CASA) model. Climate (temperature and precipitation) and CO2 fertilization each explain ca. 40 % of the observed global trend in NDVI for 1982–2006. For 1901–2006, estimated trends in NDVI related to CO2 fertilization are four to five times larger than climate related trends. CASA simulations indicate that the CO2 fertilization effect on vegetation greenness contributes about 0.7 Pg C per year to the recent land CO2 sink. This is a conservative estimate and is likely larger. This effect of CO2 fertilization would be a large component of the land carbon sink. In the auxiliary material the RVI is used as a common standard to fuse MODIS and AVHRR NDVI data. This fusion compares well with SeaWiFS data.
- Abstract: Recent studies report an increase in vegetation greenness in mid‐to‐high northern latitudes. This increase is observed in leaf‐out data in Europe and North America since the 1950s and in satellite data since the 1980s. Increased vegetation greenness is potentially a factor contributing to a land CO2 sink. Various causes for increased vegetation greenness are suggested, but their relative importance is uncertain. In the present study the effect of climate and CO2 fertilization on increased vegetation greenness and the land CO2 sink are investigated. The study is organized as follows: (1) A model is used to simulate monthly global normalized difference vegetation index (NDVI) fields for 1901–2006. The model is derived from NDVI, precipitation and temperature data for 1982–1999. The modeled fields, referred to as reconstructed vegetation index (RVI), are tested back in time on phenological data (1950s–1990s) and forward in time on MODIS data (2001–2006). The RVI represents the response of NDVI to variations in climate. (2) Residuals between RVI and NDVI are analyzed for associations with variations in down‐welling solar radiation, nitrogen deposition, satellite‐related artefacts, and CO2 fertilization. CO2 fertilization was the only factor that improved RVI modeling. (3) The effect of climate variations and CO2 fertilization on the land CO2 sink, as manifested in the RVI, is explored with the Carnegie Ames Stanford Assimilation (CASA) model. Climate (temperature and precipitation) and CO2 fertilization each explain ca. 40 % of the observed global trend in NDVI for 1982–2006. For 1901–2006, estimated trends in NDVI related to CO2 fertilization are four to five times larger than climate related trends. CASA simulations indicate that the CO2 fertilization effect on vegetation greenness contributes about 0.7 Pg C per year to the recent land CO2 sink. This is a conservative estimate and is likely larger. This effect of CO2 fertilization would be a large component of the land carbon sink. In the auxiliary material the RVI is used as a common standard to fuse MODIS and AVHRR NDVI data. This fusion compares well with SeaWiFS data.
- Revisiting “Nutrient Trapping” in global coupled biogeochemical Ocean Circulation Models
- Abstract: We analyze an extensive set of global coupled biogeochemical ocean circulation models. The focus is on the equatorial Pacific. In all simulations, which are consistent with observed standing stocks of relevant biogeochemical species at the surface, we find spuriously enhanced (reduced) macronutrient (oxygen) concentrations in the deep eastern equatorial Pacific. This modeling problem, apparently endemic to global coupled biogeochemical ocean circulation models, was coined “nutrient trapping” by [39]. In contrast to [5] we argue that “nutrient trapping” is still a persistent problem, even in eddy‐permitting models and, further, that the scale of the problem retards model‐projections of nitrogen cycling. In line with previous work, our results indicate that a deficient circulation is at the core of the problem rather than an admittedly poor quantitative understanding of biogeochemical cycles. More specifically, we present indications that “nutrient trapping” in models is a result of a spuriously damped equatorial intermediate (zonal) current system – a phenomenon which awaits a comprehensive understanding and has, to‐date, not been successfully simulated.
- Abstract: We analyze an extensive set of global coupled biogeochemical ocean circulation models. The focus is on the equatorial Pacific. In all simulations, which are consistent with observed standing stocks of relevant biogeochemical species at the surface, we find spuriously enhanced (reduced) macronutrient (oxygen) concentrations in the deep eastern equatorial Pacific. This modeling problem, apparently endemic to global coupled biogeochemical ocean circulation models, was coined “nutrient trapping” by [39]. In contrast to [5] we argue that “nutrient trapping” is still a persistent problem, even in eddy‐permitting models and, further, that the scale of the problem retards model‐projections of nitrogen cycling. In line with previous work, our results indicate that a deficient circulation is at the core of the problem rather than an admittedly poor quantitative understanding of biogeochemical cycles. More specifically, we present indications that “nutrient trapping” in models is a result of a spuriously damped equatorial intermediate (zonal) current system – a phenomenon which awaits a comprehensive understanding and has, to‐date, not been successfully simulated.
- Basin scale survey of marine humic fluorescence in the Atlantic: Relationship to iron solubility and H2O2
- Abstract: Iron (Fe) is a limiting nutrient for phytoplankton productivity in many different oceanic regions. A critical aspect underlying iron limitation is its low solubility in seawater as this controls the distribution and transport of iron through the ocean. Processes which enhance the solubility of iron in seawater, either through redox reactions or organic complexation, are central to understanding the biogeochemical cycling of iron. In this work we combined iron solubility measurements with parallel factor (PARAFAC) data analysis of Coloured Dissolved Organic Matter (CDOM) fluorescence along a meridional transect through the Atlantic (PS ANT XXVI‐4) to examine the hypothesis that marine humic fluorescence is a potential proxy for iron solubility in the surface ocean. PARAFAC analysis revealed 4 components (C1‐4), two humic like substances (C2&4) and two protein‐like (C1&3). Overall none of the 4 components were significantly correlated with iron solubility, though humic‐like components were weakly correlated with iron solubility in iron replete waters. Our analysis suggests that the ligands responsible for maintaining iron in solution in the euphotic zone are sourced from both remineralisation processes and specific ligands produced in response to iron stress and are not easily related to bulk CDOM properties. The humic fluorescence signal was sharply attenuated in surface waters presumably most likely due to photo bleaching, though there was only a weak correlation with the transient photo product H2O2, suggesting longer lifetimes in the photic zone for the fluorescent components identified here.
- Abstract: Iron (Fe) is a limiting nutrient for phytoplankton productivity in many different oceanic regions. A critical aspect underlying iron limitation is its low solubility in seawater as this controls the distribution and transport of iron through the ocean. Processes which enhance the solubility of iron in seawater, either through redox reactions or organic complexation, are central to understanding the biogeochemical cycling of iron. In this work we combined iron solubility measurements with parallel factor (PARAFAC) data analysis of Coloured Dissolved Organic Matter (CDOM) fluorescence along a meridional transect through the Atlantic (PS ANT XXVI‐4) to examine the hypothesis that marine humic fluorescence is a potential proxy for iron solubility in the surface ocean. PARAFAC analysis revealed 4 components (C1‐4), two humic like substances (C2&4) and two protein‐like (C1&3). Overall none of the 4 components were significantly correlated with iron solubility, though humic‐like components were weakly correlated with iron solubility in iron replete waters. Our analysis suggests that the ligands responsible for maintaining iron in solution in the euphotic zone are sourced from both remineralisation processes and specific ligands produced in response to iron stress and are not easily related to bulk CDOM properties. The humic fluorescence signal was sharply attenuated in surface waters presumably most likely due to photo bleaching, though there was only a weak correlation with the transient photo product H2O2, suggesting longer lifetimes in the photic zone for the fluorescent components identified here.
- Climate warming shifts carbon allocation from stemwood to roots in calcium‐depleted spruce forests
- Abstract: Increased greening of northern forests, measured by the Normalized Difference Vegetation Index (NDVI), has been presented as evidence that a warmer climate has increased both net primary productivity (NPP) and the carbon sink in boreal forests. However, higher production and greener canopies may accompany changes in carbon allocation that favor foliage or fine roots over less decomposable woody biomass. Furthermore, tree core data throughout mid‐ and northern latitudes have revealed a divergence problem (DP), a weakening in tree ring responses to warming over the past half century that is receiving increasing attention, but remains poorly understood. Often, the same sites exhibit trend inconsistency phenomenon (TIP), namely positive, or no trends in growing season NDVI where negative trends in tree ring indexes are observed. Here we studied growth of two Norway spruce (Picea abies) stands in western Russia that exhibited both the DP and TIP but were subject to soil acidification and calcium depletion of differing timing and severity. Our results link the decline in radial growth starting in 1980 to a shift in carbon allocation from wood to roots driven by a combination of two factors: (a) soil acidification that depleted calcium and impaired root function and (b) earlier onset of the growing season that further taxed the root system. The latter change in phenology appears to act as a trigger at both sites to push trees into nutrient limitation as the demand for Ca increased with the longer growing season, thereby causing the shift in carbon allocation.
- Abstract: Increased greening of northern forests, measured by the Normalized Difference Vegetation Index (NDVI), has been presented as evidence that a warmer climate has increased both net primary productivity (NPP) and the carbon sink in boreal forests. However, higher production and greener canopies may accompany changes in carbon allocation that favor foliage or fine roots over less decomposable woody biomass. Furthermore, tree core data throughout mid‐ and northern latitudes have revealed a divergence problem (DP), a weakening in tree ring responses to warming over the past half century that is receiving increasing attention, but remains poorly understood. Often, the same sites exhibit trend inconsistency phenomenon (TIP), namely positive, or no trends in growing season NDVI where negative trends in tree ring indexes are observed. Here we studied growth of two Norway spruce (Picea abies) stands in western Russia that exhibited both the DP and TIP but were subject to soil acidification and calcium depletion of differing timing and severity. Our results link the decline in radial growth starting in 1980 to a shift in carbon allocation from wood to roots driven by a combination of two factors: (a) soil acidification that depleted calcium and impaired root function and (b) earlier onset of the growing season that further taxed the root system. The latter change in phenology appears to act as a trigger at both sites to push trees into nutrient limitation as the demand for Ca increased with the longer growing season, thereby causing the shift in carbon allocation.
- The contribution of nitrogen deposition to the photosynthetic capacity of forests
- Abstract: Global terrestrial carbon (C) sequestration has increased over the last few decades. The drivers of carbon sequestration, the geographical spread and magnitude of this sink are however hotly debated. Photosynthesis determines the total C uptake of terrestrial ecosystems and is a major flux of the global C balance. We contribute to the discussion on enhanced C sequestration by analyzing the influence of nitrogen (N) deposition on photosynthetic capacity (Amax) of forest canopies. Eddy covariance measurements of net exchange of carbon provide estimates of gross primary production, from which Amax is derived with a novel approach. Canopy Amax is combined with modeled N deposition, environmental variables and stand characteristics to study the relative effects on Amax for a unique global dataset of 80 forest FLUXNET sites. Canopy Amax relates positively to N deposition for evergreen needleleaf forests below an observed critical load of ~ 8 kg N ha‐1 yr‐1, with a slope of 2.0 ± 0.4 (S.E.) μmol CO2 m‐2 s‐1 per 1 kg N ha‐1 yr‐1. Above this threshold canopy Amax levels off, exhibiting a saturating response in line with the N saturation hypothesis. Climate effects on canopy Amax cannot be separated from the effect of N deposition due to considerable covariation. For deciduous broadleaf forests and forests in the temperate (‐continental) climate zones, the analysis shows the N deposition effect to be either small or absent. Leaf area index and foliar N concentration are positively but weakly related to Amax. We conclude that flux tower measurements of C fluxes provide valuable data to study physiological processes at the canopy scale. Future efforts need to be directed towards standardizing measures N cycling and pools within C monitoring networks to gain a better understanding of C and N interactions, and to disentangle the role of climate and N deposition in forest ecosystems.
- Abstract: Global terrestrial carbon (C) sequestration has increased over the last few decades. The drivers of carbon sequestration, the geographical spread and magnitude of this sink are however hotly debated. Photosynthesis determines the total C uptake of terrestrial ecosystems and is a major flux of the global C balance. We contribute to the discussion on enhanced C sequestration by analyzing the influence of nitrogen (N) deposition on photosynthetic capacity (Amax) of forest canopies. Eddy covariance measurements of net exchange of carbon provide estimates of gross primary production, from which Amax is derived with a novel approach. Canopy Amax is combined with modeled N deposition, environmental variables and stand characteristics to study the relative effects on Amax for a unique global dataset of 80 forest FLUXNET sites. Canopy Amax relates positively to N deposition for evergreen needleleaf forests below an observed critical load of ~ 8 kg N ha‐1 yr‐1, with a slope of 2.0 ± 0.4 (S.E.) μmol CO2 m‐2 s‐1 per 1 kg N ha‐1 yr‐1. Above this threshold canopy Amax levels off, exhibiting a saturating response in line with the N saturation hypothesis. Climate effects on canopy Amax cannot be separated from the effect of N deposition due to considerable covariation. For deciduous broadleaf forests and forests in the temperate (‐continental) climate zones, the analysis shows the N deposition effect to be either small or absent. Leaf area index and foliar N concentration are positively but weakly related to Amax. We conclude that flux tower measurements of C fluxes provide valuable data to study physiological processes at the canopy scale. Future efforts need to be directed towards standardizing measures N cycling and pools within C monitoring networks to gain a better understanding of C and N interactions, and to disentangle the role of climate and N deposition in forest ecosystems.
- Atmospheric CO2 response to volcanic eruptions: the role of ENSO, season, and variability
- Abstract: Tropical explosive volcanism is one of the most important natural factors that significantly impact the climate system and the carbon cycle on annual to multi‐decadal time scales. The three largest explosive eruptions in the last 50 years ‐ Agung, El Chichón, and Pinatubo ‐ occurred in spring/summer in conjunction with El Niño events and left distinct negative signals in the observational temperature and CO2 records. However, confounding factors such as seasonal variability and El Niño‐Southern Oscillation (ENSO) may obscure the forcing‐response relationship. We determine for the first time the extent to which initial conditions, i.e. season and phase of the ENSO, and internal variability influence the coupled climate and carbon cycle response to volcanic forcing and how this affects estimates of the terrestrial and oceanic carbon sinks. Ensemble simulations with the Earth System Model CSM1.4‐carbon predict that the atmospheric CO2 response is ~60% larger when a volcanic eruption occurs during El Niño and in winter than during La Niña conditions. Our simulations suggest that the Pinatubo eruption contributed 11 ± 6% to the 25 Pg terrestrial carbon sink inferred over the decade 1990‐1999 and ‐2 ± 1% to the 22 Pg oceanic carbon sink. In contrast to recent claims, trends in the airborne fraction of anthropogenic carbon cannot be detected when accounting for the decadal‐scale influence of explosive volcanism and related uncertainties. Our results highlight the importance of considering the role of natural variability in the carbon cycle for interpretation of observations and for data‐model intercomparison.
- Abstract: Tropical explosive volcanism is one of the most important natural factors that significantly impact the climate system and the carbon cycle on annual to multi‐decadal time scales. The three largest explosive eruptions in the last 50 years ‐ Agung, El Chichón, and Pinatubo ‐ occurred in spring/summer in conjunction with El Niño events and left distinct negative signals in the observational temperature and CO2 records. However, confounding factors such as seasonal variability and El Niño‐Southern Oscillation (ENSO) may obscure the forcing‐response relationship. We determine for the first time the extent to which initial conditions, i.e. season and phase of the ENSO, and internal variability influence the coupled climate and carbon cycle response to volcanic forcing and how this affects estimates of the terrestrial and oceanic carbon sinks. Ensemble simulations with the Earth System Model CSM1.4‐carbon predict that the atmospheric CO2 response is ~60% larger when a volcanic eruption occurs during El Niño and in winter than during La Niña conditions. Our simulations suggest that the Pinatubo eruption contributed 11 ± 6% to the 25 Pg terrestrial carbon sink inferred over the decade 1990‐1999 and ‐2 ± 1% to the 22 Pg oceanic carbon sink. In contrast to recent claims, trends in the airborne fraction of anthropogenic carbon cannot be detected when accounting for the decadal‐scale influence of explosive volcanism and related uncertainties. Our results highlight the importance of considering the role of natural variability in the carbon cycle for interpretation of observations and for data‐model intercomparison.
- Decadal changes in dissolved inorganic carbon in the Pacific Ocean
- Abstract: Using high‐quality data sets obtained about a decade apart, we examined the changes of dissolved inorganic carbon in the Pacific Ocean, separating anthropogenic and natural CO2. Observations along three transoceanic sections along 47°N, 179°E, and 17°S showed both decadal increases (>20 µmol kg–1) and decreases (10 µmol kg–1 in oceanic uptake of anthropogenic CO2, reflecting accumulation in mode waters. Along 17°S, increases of anthropogenic CO2 were >20 µmol kg–1, larger than expected from increases of anthropogenic CO2 in the atmosphere. The annual water‐column inventories of anthropogenic CO2 changes calculated in 20° longitudinal or 10° latitudinal bands throughout the Pacific Ocean revealed relatively high values (>0.7 mol m–2 a–1) in the subtropical regions of both hemispheres and low values in the tropical Pacific. This distribution pattern is similar to previous estimates for the Anthropocene, implying that the redistribution processes of anthropogenic CO2 have not changed on a basin scale over the last decade. We estimated the total anthropogenic and natural CO2 storage in the Pacific Ocean to be 8.4 ± 0.5 and 0.6 ± 0.4 Pg carbon decade–1, respectively.
- Abstract: Using high‐quality data sets obtained about a decade apart, we examined the changes of dissolved inorganic carbon in the Pacific Ocean, separating anthropogenic and natural CO2. Observations along three transoceanic sections along 47°N, 179°E, and 17°S showed both decadal increases (>20 µmol kg–1) and decreases (10 µmol kg–1 in oceanic uptake of anthropogenic CO2, reflecting accumulation in mode waters. Along 17°S, increases of anthropogenic CO2 were >20 µmol kg–1, larger than expected from increases of anthropogenic CO2 in the atmosphere. The annual water‐column inventories of anthropogenic CO2 changes calculated in 20° longitudinal or 10° latitudinal bands throughout the Pacific Ocean revealed relatively high values (>0.7 mol m–2 a–1) in the subtropical regions of both hemispheres and low values in the tropical Pacific. This distribution pattern is similar to previous estimates for the Anthropocene, implying that the redistribution processes of anthropogenic CO2 have not changed on a basin scale over the last decade. We estimated the total anthropogenic and natural CO2 storage in the Pacific Ocean to be 8.4 ± 0.5 and 0.6 ± 0.4 Pg carbon decade–1, respectively.
- Deposition fluxes and fate of polycyclic aromatic hydrocarbons in the Yangtze River estuarine‐inner shelf in the East China Sea
- Abstract: Surface sediments were obtained from a matrix of 76 sample sites in the inner shelf mud belts of the East China Sea (ECS) for a comprehensive study of the distribution, composition, deposition flux, and fate of polycyclic aromatic hydrocarbons (PAHs). The sampling sites covered an area of ~80,000 km2 extending ~1000 km from the mouth of the Yangtze River to the Min River in the inner shelf. The total deposition flux of the 16 USEPA priority PAHs (16 PAHs) of the Yangtze estuarine‐inner shelf was estimated to be 152 t/yr, accounting for ~38% of the total annual input of the 16 PAHs into the ECS. This indicates that the Yangtze estuarine‐inner shelf is one of the largest sinks of land‐based PAHs in the world. Principal component analysis indicated that the 16 PAHs in the northern Yangtze estuarine mud area were mostly phenanthrene while shifting to high‐molecular‐weight PAHs in the southern Min‐Zhe coastal mud area. The positive matrix factorization model revealed that the deposition flux of low‐molecular‐weight (LMW) PAHs decreased from north to south, most likely due to the mass transfer between the resuspended sediments triggered by the East Asian monsoon and the water columns, as the resuspended sediments are transported southward. This release of LMW PAHs from the sediments to the water columns could become an important secondary PAH source in ECS.
- Abstract: Surface sediments were obtained from a matrix of 76 sample sites in the inner shelf mud belts of the East China Sea (ECS) for a comprehensive study of the distribution, composition, deposition flux, and fate of polycyclic aromatic hydrocarbons (PAHs). The sampling sites covered an area of ~80,000 km2 extending ~1000 km from the mouth of the Yangtze River to the Min River in the inner shelf. The total deposition flux of the 16 USEPA priority PAHs (16 PAHs) of the Yangtze estuarine‐inner shelf was estimated to be 152 t/yr, accounting for ~38% of the total annual input of the 16 PAHs into the ECS. This indicates that the Yangtze estuarine‐inner shelf is one of the largest sinks of land‐based PAHs in the world. Principal component analysis indicated that the 16 PAHs in the northern Yangtze estuarine mud area were mostly phenanthrene while shifting to high‐molecular‐weight PAHs in the southern Min‐Zhe coastal mud area. The positive matrix factorization model revealed that the deposition flux of low‐molecular‐weight (LMW) PAHs decreased from north to south, most likely due to the mass transfer between the resuspended sediments triggered by the East Asian monsoon and the water columns, as the resuspended sediments are transported southward. This release of LMW PAHs from the sediments to the water columns could become an important secondary PAH source in ECS.
- Terrestrial dominance of organic matter in north temperate lakes
- Abstract: Aquatic ecosystems are hotspots of decomposition and sources of carbon dioxide to the atmosphere that are globally significant. Carbon exported from land (allochthonous) also supplements the carbon fixed by photosynthesis in aquatic ecosystems (autochthonous), contributing to the organic matter (OM) that supports aquatic consumers. Although the presence of terrestrial compounds in aquatic OM is well known, the contribution of terrestrial versus aquatic sources to the composition of OM has been quantified for only a handful of systems. Here we use stable isotopes of hydrogen and carbon to demonstrate that the terrestrial contribution (ΦTerr) to particulate organic matter (POM) is as large or larger (mean = 54.6% terrestrial) than the algal contribution in 39 lakes of the northern highlands region of Wisconsin and Michigan. Further, the largest carbon pool, dissolved organic matter (DOM), is strongly dominated by allochthonous material (mean for the same set of lakes approximately 100% terrestrial). Among lakes, increases in terrestrial contribution to POM are significantly correlated with more acidic pH. Extrapolating this relationship using a survey of pH in 1692 lakes in the region reveals that, with the exception of eutrophic lakes, most of the OM in lakes is of terrestrial origin. These results are consistent with the growing evidence that lakes are significant conduits for returning degraded terrestrial carbon to the atmosphere.
- Abstract: Aquatic ecosystems are hotspots of decomposition and sources of carbon dioxide to the atmosphere that are globally significant. Carbon exported from land (allochthonous) also supplements the carbon fixed by photosynthesis in aquatic ecosystems (autochthonous), contributing to the organic matter (OM) that supports aquatic consumers. Although the presence of terrestrial compounds in aquatic OM is well known, the contribution of terrestrial versus aquatic sources to the composition of OM has been quantified for only a handful of systems. Here we use stable isotopes of hydrogen and carbon to demonstrate that the terrestrial contribution (ΦTerr) to particulate organic matter (POM) is as large or larger (mean = 54.6% terrestrial) than the algal contribution in 39 lakes of the northern highlands region of Wisconsin and Michigan. Further, the largest carbon pool, dissolved organic matter (DOM), is strongly dominated by allochthonous material (mean for the same set of lakes approximately 100% terrestrial). Among lakes, increases in terrestrial contribution to POM are significantly correlated with more acidic pH. Extrapolating this relationship using a survey of pH in 1692 lakes in the region reveals that, with the exception of eutrophic lakes, most of the OM in lakes is of terrestrial origin. These results are consistent with the growing evidence that lakes are significant conduits for returning degraded terrestrial carbon to the atmosphere.
- Millennial scale impact on the marine biogeochemical cycle of mercury from early mining on the Iberian Peninsula
- Abstract: The high‐resolution mercury record of a Posidonia oceanica mat in the northwest Mediterranean provides an unprecedented testimony of changes in environmental mercury (Hg) loading to the coastal marine environment over the past 4315 yr BP. The period reconstructed made it possible to establish tentative preanthropogenic background Hg levels for the area (6.8 ± 1.5 ng g–1 in bulk sediments). A small, but significant, anthropogenic Hg increase was identifiable by ~2500 yr BP, in agreement with the beginning of intense mining in Spain. Changes in the record suggest four major periods of anthropogenic Hg pollution inputs to the Mediterranean: first, during the Roman Empire (2100–1800 yr BP); second, in the Late Middle Ages (970–650 yr BP); third, in the modern historical era (530–380 yr BP); and fourth, in the industrial period (last 250 years), with Hg concentrations two‐, four‐, five‐, and tenfold higher than background concentrations, respectively. Hg from anthropogenic sources has dominated during the last millennium (increase from ~12 to ~100 ng g–1), which can be related to the widespread historical exploitation of ore resources on the Iberian Peninsula. The chronology of Hg concentrations in the mat archive, together with other Hg pollution records from the Iberian Peninsula, suggests regional‐scale Hg transport and deposition and shows earlier marine Hg pollution than elsewhere in Europe. Moreover, the mat also records a higher number of historic contamination phases, in comparison with other natural archives, probably due to the fact that the bioaccumulating capacity of P. oceanica magnify environmental changes in Hg concentrations. In this study, we demonstrate the uniqueness of P. oceanica meadows as a long‐term archive recording trends in Hg abundance in the marine coastal environment, as well as its potential role in the Mediterranean as a long‐term Hg sink.
- Abstract: The high‐resolution mercury record of a Posidonia oceanica mat in the northwest Mediterranean provides an unprecedented testimony of changes in environmental mercury (Hg) loading to the coastal marine environment over the past 4315 yr BP. The period reconstructed made it possible to establish tentative preanthropogenic background Hg levels for the area (6.8 ± 1.5 ng g–1 in bulk sediments). A small, but significant, anthropogenic Hg increase was identifiable by ~2500 yr BP, in agreement with the beginning of intense mining in Spain. Changes in the record suggest four major periods of anthropogenic Hg pollution inputs to the Mediterranean: first, during the Roman Empire (2100–1800 yr BP); second, in the Late Middle Ages (970–650 yr BP); third, in the modern historical era (530–380 yr BP); and fourth, in the industrial period (last 250 years), with Hg concentrations two‐, four‐, five‐, and tenfold higher than background concentrations, respectively. Hg from anthropogenic sources has dominated during the last millennium (increase from ~12 to ~100 ng g–1), which can be related to the widespread historical exploitation of ore resources on the Iberian Peninsula. The chronology of Hg concentrations in the mat archive, together with other Hg pollution records from the Iberian Peninsula, suggests regional‐scale Hg transport and deposition and shows earlier marine Hg pollution than elsewhere in Europe. Moreover, the mat also records a higher number of historic contamination phases, in comparison with other natural archives, probably due to the fact that the bioaccumulating capacity of P. oceanica magnify environmental changes in Hg concentrations. In this study, we demonstrate the uniqueness of P. oceanica meadows as a long‐term archive recording trends in Hg abundance in the marine coastal environment, as well as its potential role in the Mediterranean as a long‐term Hg sink.
- Insignificant buffering capacity of Antarctic shelf carbonates
- Abstract: We combined data sets of measured sedimentary calcium carbonate (CaCO3) and satellite‐derived pelagic primary production to parameterize the relation between CaCO3 content on the Antarctic shelves and primary production in the overlying water column. CaCO3 content predicted in this way was in good agreement with the measured data. The parameterization was then used to chart CaCO3 content on the Antarctic shelves all around the Antarctic, using the satellite‐derived primary production. The total inventory of CaCO3 in the bioturbated layer of Antarctic shelf sediments was estimated to be 0.5 Pg C. This quantity is comparable to the total CO2 uptake by the Southern Ocean in only one to a few years (dependent on the uptake estimate and area considered), indicating that the dissolution of these carbonates will neither delay ocean acidification in this area nor augment the Southern Ocean CO2 uptake capacity.
- Abstract: We combined data sets of measured sedimentary calcium carbonate (CaCO3) and satellite‐derived pelagic primary production to parameterize the relation between CaCO3 content on the Antarctic shelves and primary production in the overlying water column. CaCO3 content predicted in this way was in good agreement with the measured data. The parameterization was then used to chart CaCO3 content on the Antarctic shelves all around the Antarctic, using the satellite‐derived primary production. The total inventory of CaCO3 in the bioturbated layer of Antarctic shelf sediments was estimated to be 0.5 Pg C. This quantity is comparable to the total CO2 uptake by the Southern Ocean in only one to a few years (dependent on the uptake estimate and area considered), indicating that the dissolution of these carbonates will neither delay ocean acidification in this area nor augment the Southern Ocean CO2 uptake capacity.
- Global modeling study of potentially bioavailable iron input from shipboard aerosol sources to the ocean
- Abstract: Iron (Fe) is an essential element for phytoplankton. The majority of iron is transported from arid and semiarid regions to the open ocean, but it is mainly in an insoluble form. Since most aquatic organisms can take up iron only in the dissolved form, aerosol iron solubility is a key factor that can influence the air‐sea CO2 fluxes and thus climate. Field observations have shown relatively high iron solubility in aerosols influenced by combustion sources, but specific emissions sources and their contributions to deposition fluxes largely remain uncertain. Here a global chemical transport model was used to investigate the effect of aerosol emissions from ship plumes on iron solubility in particles from the combustion and dust sources. The model results reveal that the oil combustion from shipping mainly contributes to high iron solubility (>10%) at low iron loading (1–110 ng m–3) observed over the high‐latitude North Atlantic Ocean, rather than the other combustion sources from continental industrialized regions. Due to continuing growth in global shipping and no regulations regarding particles emissions over the open ocean, the input of potentially bioavailable iron from ship plumes is likely to increase during the next century. The model results suggest that deposition of soluble iron from ships in 2100 contributes 30–60% of the soluble iron deposition over the high‐latitude North Atlantic and North Pacific.
- Abstract: Iron (Fe) is an essential element for phytoplankton. The majority of iron is transported from arid and semiarid regions to the open ocean, but it is mainly in an insoluble form. Since most aquatic organisms can take up iron only in the dissolved form, aerosol iron solubility is a key factor that can influence the air‐sea CO2 fluxes and thus climate. Field observations have shown relatively high iron solubility in aerosols influenced by combustion sources, but specific emissions sources and their contributions to deposition fluxes largely remain uncertain. Here a global chemical transport model was used to investigate the effect of aerosol emissions from ship plumes on iron solubility in particles from the combustion and dust sources. The model results reveal that the oil combustion from shipping mainly contributes to high iron solubility (>10%) at low iron loading (1–110 ng m–3) observed over the high‐latitude North Atlantic Ocean, rather than the other combustion sources from continental industrialized regions. Due to continuing growth in global shipping and no regulations regarding particles emissions over the open ocean, the input of potentially bioavailable iron from ship plumes is likely to increase during the next century. The model results suggest that deposition of soluble iron from ships in 2100 contributes 30–60% of the soluble iron deposition over the high‐latitude North Atlantic and North Pacific.
- Seasonal Variability of the Carbon Cycle in Sub‐Antarctic Surface Water in the South West Pacific
- Abstract: Few southern hemisphere time‐series measurements of biogeochemical tracers are available, and this scarcity is a major impediment in understanding the biological and physical processes underlying the oceanic carbon and nutrient cycles in vast parts of the global oceans. We make use of bi‐monthly measurements of carbonate parameters from 1998 through 2010 in upper Sub‐Antarctic Surface Water east of New Zealand's South Island at 45.85°S 171.50°E to investigate seasonal cycles and trends in these species and processes controlling their variability. This time‐series reveals positive trends in salinity‐normalized dissolved inorganic carbon (sDIC) and the partial pressure of carbon dioxide (pCO2) that are smaller than would be expected from the anthropogenic increase in atmospheric pCO2 alone, possibly due to a decrease of the average temperature over the observational period. The seasonal cycle of pCO2 is dominated by that of DIC, but is substantially modified by the influence of the annual cycle of sea surface temperature. Investigations with a δ13Coc‐constrained diagnostic box model suggest that net community production (NCP) is the dominant process controlling the observed seasonal variability in sDIC by removing 1.2 ± 0.7 mol C m−2 yr−1 from the mixed layer. This carbon drawdown, aided by an additional carbon removal due to horizontal transport, is balanced by vertical diffusion, entrainment, and air‐sea gas exchange of CO2. Oceanic pCO2 is below atmospheric pCO2 for nearly the entire year, leading to an annual mean surface ocean pCO2 undersaturation of about 12 μatm and an annual oceanic uptake of CO2 from the atmosphere of 0.9 ± 0.1 mol C m−2 yr−1.
- Abstract: Few southern hemisphere time‐series measurements of biogeochemical tracers are available, and this scarcity is a major impediment in understanding the biological and physical processes underlying the oceanic carbon and nutrient cycles in vast parts of the global oceans. We make use of bi‐monthly measurements of carbonate parameters from 1998 through 2010 in upper Sub‐Antarctic Surface Water east of New Zealand's South Island at 45.85°S 171.50°E to investigate seasonal cycles and trends in these species and processes controlling their variability. This time‐series reveals positive trends in salinity‐normalized dissolved inorganic carbon (sDIC) and the partial pressure of carbon dioxide (pCO2) that are smaller than would be expected from the anthropogenic increase in atmospheric pCO2 alone, possibly due to a decrease of the average temperature over the observational period. The seasonal cycle of pCO2 is dominated by that of DIC, but is substantially modified by the influence of the annual cycle of sea surface temperature. Investigations with a δ13Coc‐constrained diagnostic box model suggest that net community production (NCP) is the dominant process controlling the observed seasonal variability in sDIC by removing 1.2 ± 0.7 mol C m−2 yr−1 from the mixed layer. This carbon drawdown, aided by an additional carbon removal due to horizontal transport, is balanced by vertical diffusion, entrainment, and air‐sea gas exchange of CO2. Oceanic pCO2 is below atmospheric pCO2 for nearly the entire year, leading to an annual mean surface ocean pCO2 undersaturation of about 12 μatm and an annual oceanic uptake of CO2 from the atmosphere of 0.9 ± 0.1 mol C m−2 yr−1.
- Evidence for elevated emissions from high‐latitude wetlands contributing to high atmospheric CH4 concentration in the early Holocene
- Abstract: The major increase in atmospheric methane (CH4) concentration during the last glacial‐interglacial transition provides a useful example for understanding the interactions and feedbacks among Earth's climate, biosphere carbon cycling, and atmospheric chemistry. However, the causes of CH4 doubling during the last deglaciation are still uncertain and debated. Although the ice‐core data consistently suggest a dominant contribution from northern high‐latitude wetlands in the early Holocene, identifying the actual sources from the ground‐based data has been elusive. Here we present data syntheses and a case study from Alaska to demonstrate the importance of northern wetlands in contributing to high atmospheric CH4 concentration in the early Holocene. Our data indicate that new peatland formation as well as peat accumulation in northern high‐latitude regions increased more than threefold in the early Holocene in response to climate warming and the availability of new habitat as a result of deglaciation. Furthermore, we show that marshes and wet fens that represent early stages of wetland succession were likely more widespread in the early Holocene. These wetlands are associated with high CH4 emissions due to high primary productivity and the presence of emergent plant species that facilitate CH4 transport to the atmosphere. We argue that early wetland succession and rapid peat accumulation and expansion (not simply initiation) contributed to high CH4 emissions from northern regions, potentially contributing to the sharp rise in atmospheric CH4 at the onset of the Holocene.
- Abstract: The major increase in atmospheric methane (CH4) concentration during the last glacial‐interglacial transition provides a useful example for understanding the interactions and feedbacks among Earth's climate, biosphere carbon cycling, and atmospheric chemistry. However, the causes of CH4 doubling during the last deglaciation are still uncertain and debated. Although the ice‐core data consistently suggest a dominant contribution from northern high‐latitude wetlands in the early Holocene, identifying the actual sources from the ground‐based data has been elusive. Here we present data syntheses and a case study from Alaska to demonstrate the importance of northern wetlands in contributing to high atmospheric CH4 concentration in the early Holocene. Our data indicate that new peatland formation as well as peat accumulation in northern high‐latitude regions increased more than threefold in the early Holocene in response to climate warming and the availability of new habitat as a result of deglaciation. Furthermore, we show that marshes and wet fens that represent early stages of wetland succession were likely more widespread in the early Holocene. These wetlands are associated with high CH4 emissions due to high primary productivity and the presence of emergent plant species that facilitate CH4 transport to the atmosphere. We argue that early wetland succession and rapid peat accumulation and expansion (not simply initiation) contributed to high CH4 emissions from northern regions, potentially contributing to the sharp rise in atmospheric CH4 at the onset of the Holocene.
- Seasonal variability in concentration, composition, age and fluxes of particulate organic carbon exchanged between the floodplain and Amazon River
- Abstract: The composition, sources and age of particulate organic matter were determined in an Amazonian river‐floodplain system during rising, high, falling and low water periods over seven years (1999‐2006), and a mass balance for total organic carbon (dissolved and particulate) was estimated. The Curuai floodplain, comprised of several temporally interconnected lakes, is permanently connected to the Amazon River via channels. Organic matter (OM) is imported to the floodplain from the Amazon River mainly during the rising water period and produced in the floodplain and exported to the river during high and falling water periods. No significant exchanges occurred during low water periods. The OM produced in the floodplain is characterized by low C/N ratios and by high chlorophyll a concentrations (Chl‐a). The δ13C signature has a seasonal trend, with more negative δ13C values during the high water period than other periods. Δ14C results indicate that the bulk OM present in floodplain lakes is predominantly post‐bomb (i.e., post‐1950). Particulate organic carbon (POC) and dissolved organic carbon (DOC) fluxes exported by the Curuai floodplain represent 1.3 % and 0.1 %, respectively, of the POC and DOC annual fluxes in the mainstem Amazon River at Óbidos but may reach up to 3.3 % and 0.8 % during falling water. Based on Δ14C, δ13C, Chl‐a and elemental analysis of the particulate organic matter, we demonstrate that floodplain lakes have intense phytoplankton and macrophyte primary production, which is partly exported to the main river channel. Floodplains are thus a significant source of modern and labile organic carbon to the river mainstem, where it can be rapidly degraded and recycled back to the atmosphere.
- Abstract: The composition, sources and age of particulate organic matter were determined in an Amazonian river‐floodplain system during rising, high, falling and low water periods over seven years (1999‐2006), and a mass balance for total organic carbon (dissolved and particulate) was estimated. The Curuai floodplain, comprised of several temporally interconnected lakes, is permanently connected to the Amazon River via channels. Organic matter (OM) is imported to the floodplain from the Amazon River mainly during the rising water period and produced in the floodplain and exported to the river during high and falling water periods. No significant exchanges occurred during low water periods. The OM produced in the floodplain is characterized by low C/N ratios and by high chlorophyll a concentrations (Chl‐a). The δ13C signature has a seasonal trend, with more negative δ13C values during the high water period than other periods. Δ14C results indicate that the bulk OM present in floodplain lakes is predominantly post‐bomb (i.e., post‐1950). Particulate organic carbon (POC) and dissolved organic carbon (DOC) fluxes exported by the Curuai floodplain represent 1.3 % and 0.1 %, respectively, of the POC and DOC annual fluxes in the mainstem Amazon River at Óbidos but may reach up to 3.3 % and 0.8 % during falling water. Based on Δ14C, δ13C, Chl‐a and elemental analysis of the particulate organic matter, we demonstrate that floodplain lakes have intense phytoplankton and macrophyte primary production, which is partly exported to the main river channel. Floodplains are thus a significant source of modern and labile organic carbon to the river mainstem, where it can be rapidly degraded and recycled back to the atmosphere.
- Spatiotemporal variations of pCO2 and δ13C‐DIC in subarctic streams in northern Sweden
- Abstract: Current predictions of climate‐related changes in high‐latitude environments suggest major effects on the C export in streams and rivers. To what extent this will also affect the stream water CO2 concentrations is poorly understood. In this study we examined the spatiotemporal variation in partial pressure of CO2 (pCO2) and in stable isotopic composition of dissolved inorganic carbon (δ13C‐DIC) in sub‐arctic streams in northern Sweden. The selected watersheds are characterized by large variations in high‐latitude boreal forest, tundra and differences in bedrock. We found that all streams generally were supersaturated in pCO2 with an average concentration of 850 µatm. The variability in pCO2 across streams was poorly related to vegetation cover and carbonaceous bedrock influence was manifested in high DIC concentrations but not reflected in either stream pCO2 or δ13C‐DIC. Stream water pCO2 values were highest during winter baseflow when we also observed the lowest δ13C‐DIC values and this pattern are interpreted as a high contribution from CO2 from soil respiration. Summer base flow δ13C‐DIC values probably are more affected by in‐situ stream processes such as aquatic production/respiration and degassing. A challenge for further studies will be to disentangle the origin of stream water CO2 and quantify their relative importance.
- Abstract: Current predictions of climate‐related changes in high‐latitude environments suggest major effects on the C export in streams and rivers. To what extent this will also affect the stream water CO2 concentrations is poorly understood. In this study we examined the spatiotemporal variation in partial pressure of CO2 (pCO2) and in stable isotopic composition of dissolved inorganic carbon (δ13C‐DIC) in sub‐arctic streams in northern Sweden. The selected watersheds are characterized by large variations in high‐latitude boreal forest, tundra and differences in bedrock. We found that all streams generally were supersaturated in pCO2 with an average concentration of 850 µatm. The variability in pCO2 across streams was poorly related to vegetation cover and carbonaceous bedrock influence was manifested in high DIC concentrations but not reflected in either stream pCO2 or δ13C‐DIC. Stream water pCO2 values were highest during winter baseflow when we also observed the lowest δ13C‐DIC values and this pattern are interpreted as a high contribution from CO2 from soil respiration. Summer base flow δ13C‐DIC values probably are more affected by in‐situ stream processes such as aquatic production/respiration and degassing. A challenge for further studies will be to disentangle the origin of stream water CO2 and quantify their relative importance.
- Phosphorus fertilisation by active dust deposition in a super‐humid, temperate environment – soil phosphorus fractionation and accession processes
- Abstract: The inventory of soil phosphorus (P) is subject to significant changes over time. The main primary form, bedrock‐derived apatite P, becomes progressively lost through leaching, or transformed into more immobile and less plant‐accessible, secondary organic and mineral forms. Here we studied the rejuvenating effect of dust deposition on soil P along an active dust flux gradient downwind of a braided river. Along the gradient, we measured soil P fractions to 50 cm depth of six Spodosols and one Inceptisol, supplemented by tree foliage P concentrations. While an increasing dust flux correlates with a 2‐fold increase of foliar P and soil organic P along the gradient, apatite P declines from ~50 to 3 g m‐2 and total P shows no response. Compared to dust‐unaffected Spodosols, depth distribution of total P becomes increasingly uniform and organic P propagates deeper into the soil under a dust flux. Further, the effect of topsoil P eluviation attenuates due to higher organic P content and the zone of high apatite P associated with unweathered subsoil becomes progressively removed from the upper 50 cm. We interpret these patterns as being consistent with upbuilding pedogenesis; however, dust‐derived mineral P is assimilated in the organic surface horizon and does not reach the mineral soil. Dust‐derived mineral P is temporarily stored in the living biomass and returns to the soil with plant and microbial detritus as organic P, which is subsequently buried by further dust deposition. We conclude that (1) the efficiency of P fertilisation of the ecosystem by dust accession is higher than through P advection in dust‐unaffected Spodosols, and (2) organic P may serve as an important source of labile P in a high‐leaching environment.
- Abstract: The inventory of soil phosphorus (P) is subject to significant changes over time. The main primary form, bedrock‐derived apatite P, becomes progressively lost through leaching, or transformed into more immobile and less plant‐accessible, secondary organic and mineral forms. Here we studied the rejuvenating effect of dust deposition on soil P along an active dust flux gradient downwind of a braided river. Along the gradient, we measured soil P fractions to 50 cm depth of six Spodosols and one Inceptisol, supplemented by tree foliage P concentrations. While an increasing dust flux correlates with a 2‐fold increase of foliar P and soil organic P along the gradient, apatite P declines from ~50 to 3 g m‐2 and total P shows no response. Compared to dust‐unaffected Spodosols, depth distribution of total P becomes increasingly uniform and organic P propagates deeper into the soil under a dust flux. Further, the effect of topsoil P eluviation attenuates due to higher organic P content and the zone of high apatite P associated with unweathered subsoil becomes progressively removed from the upper 50 cm. We interpret these patterns as being consistent with upbuilding pedogenesis; however, dust‐derived mineral P is assimilated in the organic surface horizon and does not reach the mineral soil. Dust‐derived mineral P is temporarily stored in the living biomass and returns to the soil with plant and microbial detritus as organic P, which is subsequently buried by further dust deposition. We conclude that (1) the efficiency of P fertilisation of the ecosystem by dust accession is higher than through P advection in dust‐unaffected Spodosols, and (2) organic P may serve as an important source of labile P in a high‐leaching environment.
- River discharge influences on particulate organic carbon age structure in the Mississippi/Atchafalaya River System
- Abstract: Applying ramped pyrolysis radiocarbon analysis to suspended river sediments, we generate radiocarbon (14C) age spectra for particulate organic carbon (POC) from the lower Mississippi‐Atchafalaya River system (MARS) to better understand a major river system's role in carbon transport. Ramped pyrolysis 14C analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components. Our results indicate higher proportions of older material in the POC during higher discharge. Ages increase throughout the high‐discharge age spectra, indicating that no single component of the POC is responsible for the overall age increases observed. Instead, older material is contributed across the POC age spectrum and unrelated to increased bedload suspension. In this comparison of 2 spring discharges, less than half of the POC transported during higher discharge is less than 1000 14C years in age, constraining of the role of the MARS as a flux of atmospheric CO2 toward longer‐term sedimentary sinks in the Mississippi delta and the Gulf of Mexico. Delta‐building processes that benefit disproportionately from high discharge events carrying larger amounts of sediment involve both a higher proportion of millennially‐aged carbon from floodplain exchange of POC and transport and a potentially higher proportion of petrogenic carbon (30–530% increase). Overall, an internally consistent picture of PO14C age distributions from a major river system emerges, as differences in space and time are small compared to the range of ages of POC sources in such a large basin. © 2013 American Geophysical Union. All rights reserved.
- Abstract: Applying ramped pyrolysis radiocarbon analysis to suspended river sediments, we generate radiocarbon (14C) age spectra for particulate organic carbon (POC) from the lower Mississippi‐Atchafalaya River system (MARS) to better understand a major river system's role in carbon transport. Ramped pyrolysis 14C analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components. Our results indicate higher proportions of older material in the POC during higher discharge. Ages increase throughout the high‐discharge age spectra, indicating that no single component of the POC is responsible for the overall age increases observed. Instead, older material is contributed across the POC age spectrum and unrelated to increased bedload suspension. In this comparison of 2 spring discharges, less than half of the POC transported during higher discharge is less than 1000 14C years in age, constraining of the role of the MARS as a flux of atmospheric CO2 toward longer‐term sedimentary sinks in the Mississippi delta and the Gulf of Mexico. Delta‐building processes that benefit disproportionately from high discharge events carrying larger amounts of sediment involve both a higher proportion of millennially‐aged carbon from floodplain exchange of POC and transport and a potentially higher proportion of petrogenic carbon (30–530% increase). Overall, an internally consistent picture of PO14C age distributions from a major river system emerges, as differences in space and time are small compared to the range of ages of POC sources in such a large basin. © 2013 American Geophysical Union. All rights reserved.
- Annual Cycle of Air–Sea CO2 Exchange in an Arctic Polynya Region
- Abstract: During the Canadian International Polar Year projects in the Cape Bathurst polynya region, we measured a near–complete annual cycle of sea surface CO2 (pCO2sw), atmospheric CO2 (pCO2atm), sea surface temperature (SST), salinity (S), and wind speed (U). In this paper, we combine these data with ancillary measurements of sea ice concentration (Ci) to estimate the mean annual (September 2007–September 2008) air–sea CO2 exchange for the region.For the non–freezing seasons the exchange was calculated using a standard bulk aerodynamic approach, whereas during the freezing seasons we extrapolated eddy covariance measurements of CO2 exchange. Our results show that in 2007–08 the region served as a net sink of atmospheric CO2 at a mean rate of ‐10.1 ± 6.5 mmol m− 2 d− 1. The strongest calculated uptake rate occurred in the fall when wind velocities were highest, pCO2sw was significantly lower than pCO2atm, and ice was beginning to form. Atmospheric CO2 uptake was calculated to occur (at lower rates) throughout the rest of the year, except for a brief period of outgassing during late July. Using archival U, Ci, and pCO2sw data for the region, we found that winds in 2007–08 were 25–35 % stronger than the decadal mean and were predominately easterly, which appears to have induced a relatively late freeze–up (by ∼3 weeks relative to mean conditions) and an early polynya opening (by ∼4 weeks). In turn, these conditions may have given rise to a higher CO2 uptake than normal. Estimated winter CO2 exchange through leads and small polynya openings made up more than 50% of the total CO2 uptake, consistent with recent observations of enhanced CO2 exchange associated with open water components of the winter icescape. Our calculations for the Cape Bathurst polynya region are consistent with past studies thatestimated the total winter CO2 uptake in Arctic coastal polynyas to be on the order of 1012 g C yr− 1. © 2013 American Geophysical Union. All rights reserved.
- Abstract: During the Canadian International Polar Year projects in the Cape Bathurst polynya region, we measured a near–complete annual cycle of sea surface CO2 (pCO2sw), atmospheric CO2 (pCO2atm), sea surface temperature (SST), salinity (S), and wind speed (U). In this paper, we combine these data with ancillary measurements of sea ice concentration (Ci) to estimate the mean annual (September 2007–September 2008) air–sea CO2 exchange for the region.For the non–freezing seasons the exchange was calculated using a standard bulk aerodynamic approach, whereas during the freezing seasons we extrapolated eddy covariance measurements of CO2 exchange. Our results show that in 2007–08 the region served as a net sink of atmospheric CO2 at a mean rate of ‐10.1 ± 6.5 mmol m− 2 d− 1. The strongest calculated uptake rate occurred in the fall when wind velocities were highest, pCO2sw was significantly lower than pCO2atm, and ice was beginning to form. Atmospheric CO2 uptake was calculated to occur (at lower rates) throughout the rest of the year, except for a brief period of outgassing during late July. Using archival U, Ci, and pCO2sw data for the region, we found that winds in 2007–08 were 25–35 % stronger than the decadal mean and were predominately easterly, which appears to have induced a relatively late freeze–up (by ∼3 weeks relative to mean conditions) and an early polynya opening (by ∼4 weeks). In turn, these conditions may have given rise to a higher CO2 uptake than normal. Estimated winter CO2 exchange through leads and small polynya openings made up more than 50% of the total CO2 uptake, consistent with recent observations of enhanced CO2 exchange associated with open water components of the winter icescape. Our calculations for the Cape Bathurst polynya region are consistent with past studies thatestimated the total winter CO2 uptake in Arctic coastal polynyas to be on the order of 1012 g C yr− 1. © 2013 American Geophysical Union. All rights reserved.
- Mercury isotopes in a forested ecosystem: implications for air‐surface exchange dynamics and the global mercury cycle
- Abstract: Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10‐year‐old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had δ202Hg values of −0.74 to 0.06‰ and ∆199Hg values of 0.16 to 0.82‰. Atmospheric THg(g) had δ202Hg values of 0.48 to 0.93‰ and ∆199Hg values of −0.21 to −0.15‰. Uptake of THg(g) by foliage resulted in a large (−2.89‰) shift in δ202Hg values; foliage displayed δ202Hg values of −2.53 to −1.89‰ and ∆199Hg values of −0.37 to −0.23‰. Forest floor samples had δ202Hg values of −1.88 to −1.22‰ and ∆199Hg values of −0.22 to −0.14‰. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for ~16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in δ202Hg values and MIF of even isotopes (∆200Hg and ∆204Hg). Mercury evasion from the forest floor may have arisen from air‐surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re‐emission of wet‐deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopes during foliar uptake and air‐surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive δ202Hg values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.
- Abstract: Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10‐year‐old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had δ202Hg values of −0.74 to 0.06‰ and ∆199Hg values of 0.16 to 0.82‰. Atmospheric THg(g) had δ202Hg values of 0.48 to 0.93‰ and ∆199Hg values of −0.21 to −0.15‰. Uptake of THg(g) by foliage resulted in a large (−2.89‰) shift in δ202Hg values; foliage displayed δ202Hg values of −2.53 to −1.89‰ and ∆199Hg values of −0.37 to −0.23‰. Forest floor samples had δ202Hg values of −1.88 to −1.22‰ and ∆199Hg values of −0.22 to −0.14‰. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for ~16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in δ202Hg values and MIF of even isotopes (∆200Hg and ∆204Hg). Mercury evasion from the forest floor may have arisen from air‐surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re‐emission of wet‐deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopes during foliar uptake and air‐surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive δ202Hg values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.
- Climate Warming Shifts Carbon Allocation from Stemwood to Roots in Calcium‐Depleted Spruce Forests
- Abstract: Increased greening of northern forests, measured by the Normalized Difference Vegetation Index (NDVI), has been presented as evidence that a warmer climate has increased both net primary productivity (NPP) and the carbon sink in boreal forests. However, higher production and greener canopies may accompany changes in carbon allocation that favor foliage or fine roots over less decomposable woody biomass. Furthermore, tree core data throughout mid‐ and northern latitudes have revealed a divergence problem (DP), a weakening in tree ring responses to warming over the past half century that is receiving increasing attention, but remains poorly understood. Often, the same sites exhibit trend inconsistency phenomenon (TIP), namely positive, or no trends in growing season NDVI where negative trends in tree ring indexes are observed.
Here we studied growth of two Norway spruce (Picea abies) stands in western Russia that exhibited both the DP and TIP but were subject to soil acidification and calcium depletion of differing timing and severity. Our results link the decline in radial growth starting in 1980 to a shift in carbon allocation from wood to roots driven by a combination of two factors: (a) soil acidification that depleted calcium and impaired root function and (b) earlier onset of the growing season that further taxed the root system. The latter change in phenology appears to act as a trigger at both sites to push trees into nutrient limitation as the demand for Ca increased with the longer growing season, thereby causing the shift in carbon allocation. © 2013 American Geophysical Union. All rights reserved.
- Abstract: Increased greening of northern forests, measured by the Normalized Difference Vegetation Index (NDVI), has been presented as evidence that a warmer climate has increased both net primary productivity (NPP) and the carbon sink in boreal forests. However, higher production and greener canopies may accompany changes in carbon allocation that favor foliage or fine roots over less decomposable woody biomass. Furthermore, tree core data throughout mid‐ and northern latitudes have revealed a divergence problem (DP), a weakening in tree ring responses to warming over the past half century that is receiving increasing attention, but remains poorly understood. Often, the same sites exhibit trend inconsistency phenomenon (TIP), namely positive, or no trends in growing season NDVI where negative trends in tree ring indexes are observed.
Here we studied growth of two Norway spruce (Picea abies) stands in western Russia that exhibited both the DP and TIP but were subject to soil acidification and calcium depletion of differing timing and severity. Our results link the decline in radial growth starting in 1980 to a shift in carbon allocation from wood to roots driven by a combination of two factors: (a) soil acidification that depleted calcium and impaired root function and (b) earlier onset of the growing season that further taxed the root system. The latter change in phenology appears to act as a trigger at both sites to push trees into nutrient limitation as the demand for Ca increased with the longer growing season, thereby causing the shift in carbon allocation. © 2013 American Geophysical Union. All rights reserved.
- Inter‐ and Intra‐annual Variability of Vegetation in the Northern Hemisphere and its Association with Precursory Meteorological Factors
- Abstract: Determination of phenological variation is one of the most critical challenges in dynamic vegetation modeling, given the lack of a strong theoretical framework. Previous studies generally focused on the timing of a phenological event (e.g., bud‐burst or onset of growing season) and its atmospheric prompts, but not on the interactive variations across phenological stages. This study, therefore, investigated the inter‐ and intra‐annual variability existing in all the phenological stages and the relations of the variability with four meteorological variables (surface temperature (Ts), shortwave radiation (SW), vapor pressure deficit (VPD), and precipitation (PRCP)) using a 25‐year (1982‐2006) dataset of leaf area index (LAI) from the Advanced Very High Resolution Radiometer (AVHRR). Our six study sites of each 4° × 4° grids (mixed forest in China, deciduous needle‐leaf forest in Siberia, evergreen needle‐leaf forest in western Canada, grass in Gobi, and crops in the Central United States and southeastern Europe) include various vegetation types, local climates, and land‐use types in the mid‐latitudes of the northern hemisphere. Empirical orthogonal function (EOF) analysis with detrended LAI anomalies identified the two leading EOF modes that account for the amplitude and phase of the monthly LAI variations. The inter‐annual correlation between the principle components (PCs) of the two modes and the meteorological variables for spring and summer showed that the amplitude and phase modes (AM and PM, respectively) were affected by different dominant meteorological factors. Over most of the study regions, AM was positively correlated with PRCP and negatively with Ts, SW, and VPD, while PM was predominantly positively correlated with Ts. The contrasting responses of the two EOF modes to Ts reflect environmental limitations on plant growth such as early start of growth, but with a reduced value of maximum LAI in a year with a warm spring. In addition, insignificant correlations between AM and PRCP, as well as negative correlations between PM and PRCP, in the crop regions suggest that human interventions such as advanced irrigation systems also play a key role in vegetative activity.
- Abstract: Determination of phenological variation is one of the most critical challenges in dynamic vegetation modeling, given the lack of a strong theoretical framework. Previous studies generally focused on the timing of a phenological event (e.g., bud‐burst or onset of growing season) and its atmospheric prompts, but not on the interactive variations across phenological stages. This study, therefore, investigated the inter‐ and intra‐annual variability existing in all the phenological stages and the relations of the variability with four meteorological variables (surface temperature (Ts), shortwave radiation (SW), vapor pressure deficit (VPD), and precipitation (PRCP)) using a 25‐year (1982‐2006) dataset of leaf area index (LAI) from the Advanced Very High Resolution Radiometer (AVHRR). Our six study sites of each 4° × 4° grids (mixed forest in China, deciduous needle‐leaf forest in Siberia, evergreen needle‐leaf forest in western Canada, grass in Gobi, and crops in the Central United States and southeastern Europe) include various vegetation types, local climates, and land‐use types in the mid‐latitudes of the northern hemisphere. Empirical orthogonal function (EOF) analysis with detrended LAI anomalies identified the two leading EOF modes that account for the amplitude and phase of the monthly LAI variations. The inter‐annual correlation between the principle components (PCs) of the two modes and the meteorological variables for spring and summer showed that the amplitude and phase modes (AM and PM, respectively) were affected by different dominant meteorological factors. Over most of the study regions, AM was positively correlated with PRCP and negatively with Ts, SW, and VPD, while PM was predominantly positively correlated with Ts. The contrasting responses of the two EOF modes to Ts reflect environmental limitations on plant growth such as early start of growth, but with a reduced value of maximum LAI in a year with a warm spring. In addition, insignificant correlations between AM and PRCP, as well as negative correlations between PM and PRCP, in the crop regions suggest that human interventions such as advanced irrigation systems also play a key role in vegetative activity.
- Temperate reservoirs are large carbon sinks and small CO2 sources: Results from high‐resolution carbon budgets
- Abstract: Sediment organic carbon (C) burial and CO2 fluxes in inland waters are quantitatively important in regional and global carbon budgets. Estimates of C fluxes from inland waters are typically based on limited temporal resolution despite potential large variations with season and weather events. Further, most freshwater C budget studies have focused on natural soft‐water lakes, while reservoirs and hard‐water systems are globally numerous. Our study quantifies C fluxes in two hard‐water, human constructed reservoirs (Ohio, USA) of contrasting watershed land use (agriculture vs. forest) using high‐resolution mass balance budgets. We show that during a dry summer, C retention and export via the dam were reduced compared to a wet summer. Both reservoirs were net CO2 sources during a wet summer, but CO2 sinks during a dry summer. Despite weather‐related summer differences, annual C fluxes within each reservoir were similar between years. Both reservoirs appear to be net autotrophic despite often being CO2 sources based on budgets. This is likely because CO2 fluxes in our hard‐water reservoirs were more strongly associated with DIC than DOC. Using our C fluxes and statewide watershed land use, we determined the regional importance of Ohio reservoirs in OC burial and CO2 emissions. We estimate that Ohio reservoirs bury up to 4 times more OC, but emit
- Abstract: Sediment organic carbon (C) burial and CO2 fluxes in inland waters are quantitatively important in regional and global carbon budgets. Estimates of C fluxes from inland waters are typically based on limited temporal resolution despite potential large variations with season and weather events. Further, most freshwater C budget studies have focused on natural soft‐water lakes, while reservoirs and hard‐water systems are globally numerous. Our study quantifies C fluxes in two hard‐water, human constructed reservoirs (Ohio, USA) of contrasting watershed land use (agriculture vs. forest) using high‐resolution mass balance budgets. We show that during a dry summer, C retention and export via the dam were reduced compared to a wet summer. Both reservoirs were net CO2 sources during a wet summer, but CO2 sinks during a dry summer. Despite weather‐related summer differences, annual C fluxes within each reservoir were similar between years. Both reservoirs appear to be net autotrophic despite often being CO2 sources based on budgets. This is likely because CO2 fluxes in our hard‐water reservoirs were more strongly associated with DIC than DOC. Using our C fluxes and statewide watershed land use, we determined the regional importance of Ohio reservoirs in OC burial and CO2 emissions. We estimate that Ohio reservoirs bury up to 4 times more OC, but emit
