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Geophysical Research Letters     Full-text available via subscription   (Followers: 77, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 9, SJR: 3.239, h-index: 119)
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Journal Cover Global Biogeochemical Cycles
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Origins, Seasonality and Fluxes of Organic Matter in the Congo River
    • Authors: Robert G. M. Spencer; Peter J. Hernes, Bienvenu Dinga, Jose N. Wabakanghanzi, Travis W. Drake, Johan Six
      Abstract: The Congo River in central Africa represents a major source of organic matter (OM) to the Atlantic Ocean. This study examined elemental (%OC, %N, C:N), stable isotopic (δ13C, δ15N) and biomarker composition (lignin phenols) of particulate OM (POM) and dissolved OM (DOM) across the seasonal hydrograph. Even though the Congo exhibits an extremely stable intraannual discharge regime, seasonal variability in OM composition was evident. DOM appears predominantly derived from vascular plant inputs with greater relative contribution during the rising limb and peak in discharge associated with the major November‐December discharge maximum. Generally, POM appears to be sourced from soil‐derived mineral‐associated OM (low C:N, low Λ8, higher (Ad:Al)v) but the relative proportion of fresh vascular plant material (higher C:N, higher Λ8, lower (Ad:Al)v) increases with higher discharge. During the study period (September 2009‐November 2010) the Congo exported 29.21 Tg yr‐1 of total suspended sediment (TSS), 1.96 Tg yr‐1 of particulate organic carbon (POC) and 12.48 Tg yr‐1 of dissolved organic carbon (DOC). The Congo exports an order of magnitude lower TSS load in comparison to other major riverine sources of TSS (e.g. Ganges, Brahmaputra), but due to its OM rich character actually exports a comparable amount of POC. The Congo is also 2.5 times more efficient at exporting dissolved lignin per unit volume compared to the Amazon. Including Congo dissolved lignin data in residence time calculations for lignin in the Atlantic Ocean results in an approximately 10% reduction from the existing estimate, suggesting this material is more reactive than previously thought.
      PubDate: 2016-07-05T08:10:41.770855-05:
      DOI: 10.1002/2016GB005427
  • Carbon fate in a large temperate human‐impacted river system: focus
           on benthic dynamics
    • Authors: Lauriane Vilmin; Nicolas Flipo, Nicolas Escoffier, Vincent Rocher, Alexis Groleau
      Abstract: Fluvial networks play an important role in regional and global carbon (C) budgets. The Seine River, from the Paris urban area to the entrance of its estuary (220 km), is studied here as an example of a large human‐impacted river system subject to temperate climatic conditions. We assess organic C (OC) budgets up‐ and downstream from one of the world's largest waste water treatment plants and for different hydrological conditions using a hydro‐biogeochemical model. The fine representation of sediment accumulation on the river bed allows for the quantification of pelagic and benthic effects on OC export towards the estuary and on river metabolism (i.e. net CO2 production). OC export is significantly affected by benthic dynamics during the driest periods, when 25 % of the inputs to the system is transformed or stored in the sediment layer. Benthic processes also substantially affect river metabolism under any hydrological condition. On average, benthic respiration accounts for one third of the total river respiration along the studied stretch (0.27 out of 0.86 gC·m−2·d−1). Even though the importance of benthic processes was already acknowledged by the scientific community for headwater streams, these results stress the major influence of benthic dynamics, and thus of physical processes such as sedimentation and re‐suspension, on C cycling in downstream river systems. It opens the door to new developments in the quantification of C emissions by global models, whereby biogeochemical processing and benthic dynamics should be taken into account.
      PubDate: 2016-06-28T21:40:25.35896-05:0
      DOI: 10.1002/2015GB005271
  • Basin‐wide N2 fixation in the deep waters of the Mediterranean Sea
    • Abstract: Recent findings indicate that N2 fixation is significant in aphotic waters, presumably due to heterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationship between organic matter and heterotrophic N2 fixation remains unknown. Here we explore N2 fixation in the deep chlorophyll maximum (DCM) and underneath deep waters across the whole Mediterranean Sea and relate it to organic matter composition, characterized by optical and molecular methods. Our N2 fixation rates were in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to 0.43 nmol N L‐1 d‐1), and were significantly correlated to the presence of relatively labile organic matter with fluorescence and molecular formula properties representative for peptides and unsaturated aliphatics, and associated with the presence of more oxygenated ventilated water masses. Finally, and despite aphotic N2 fixation contributes largely to total water column diazotrophic activity (>50%), its contribution to overall nitrogen inputs to the basin is negligible (
      PubDate: 2016-06-20T05:45:24.218765-05:
      DOI: 10.1002/2015GB005326
  • Enzyme‐level Interconversion of Nitrate and Nitrite in the Fall
           Mixed Layer of the Antarctic Ocean
    • Authors: P.C. Kemeny; M. A. Weigand, R. Zhang, B.R. Carter, K.L. Karsh, S.E. Fawcett, D.M. Sigman
      Abstract: In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO3‐) have been used to investigate NO3‐ assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the 15N/14N (δ15N) and 18O/16O (δ18O) of NO3‐ and nitrite (NO2‐) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ15N and δ18O of NO3‐ + NO2‐ increase roughly equally, as expected for NO3‐ assimilation; however, the δ15N of NO3‐‐only (measured after NO2‐ removal) increases more than NO3‐‐only δ18O. Differencing indicates that NO2‐ has an extremely low δ15N, often 
      PubDate: 2016-06-20T05:15:26.900053-05:
      DOI: 10.1002/2015GB005350
  • Changes in anthropogenic nitrogen and phosphorus inputs to the St.
           Lawrence sub‐basin over 110 years and impacts on riverine
    • Abstract: Human activities have increased the flow of nitrogen (N) and phosphorus (P) over much of the Earth, leading to increased agricultural production, but also the degradation of air, soil, and water quality. Here, we quantify the sources of anthropogenic N and P inputs to 76 watersheds of the St. Lawrence Basin (SLB) throughout the 20th century using NANI/NAPI (net anthropogenic N/P input to watersheds), a mass balance modeling approach, and estimate the fraction of these inputs exported to adjacent rivers. Our results show that since 1901, NANI and NAPI increased 4.5‐ and 3.8‐fold respectively with a peak in 1991 mainly due to high atmospheric N deposition and P fertilizer application. However the relative increase over the course of the last century was much higher in certain watersheds, particularly those where there was greater urbanization. Ranges in NANI and NAPI vary greatly among watersheds (110 to 9,351 kg N km‐2 yr‐1 and 0.16 to 1,938 kg P km‐2 yr‐1, respectively in 2011) and are strongly related to riverine fluxes (R2 = 0.87 and 0.71 for N and P, respectively). Our results suggest that 22% of NANI (ranging from 11% to 68% across watersheds) and 17% of NAPI (ranging from 3% to 173%) are exported to rivers. Predominant sources of inputs vary spatially and through time largely due to changes in farming practices. By tracking the main sources of inputs to specific watersheds and through time, our work provides insights for N and P management. Reduction strategies will likely need to be watershed specific, although through time, our results clearly show the large‐scale impact of targeted legislation.
      PubDate: 2016-06-15T00:30:38.217089-05:
      DOI: 10.1002/2016GB005384
  • Variability in the sensitivity among model simulations of permafrost and
           carbon dynamics in the permafrost region between 1960 and 2009
    • Abstract: A significant portion of large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO2 and CH4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near‐surface permafrost (within 3 m) area over the region, but there are large differences in the magnitude of the simulated rates of loss among the models (0.2 to 58.8 x 103 km2 y‐1). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954 Tg C y‐1 between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982‐2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. To improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback, and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.
      PubDate: 2016-06-15T00:30:33.346489-05:
      DOI: 10.1002/2016GB005405
  • Quantifying the drivers of ocean‐atmosphere CO2 fluxes
    • Authors: Jonathan M. Lauderdale; Stephanie Dutkiewicz, Richard G. Williams, Michael J. Follows
      Abstract: A mechanistic framework for quantitatively mapping the regional drivers of air–sea CO2 fluxes at a global scale is developed. The framework evaluates the interplay between: (1) surface heat and freshwater fluxes that influence the potential saturated carbon concentration, which depends on changes in sea surface temperature, salinity and alkalinity, (2) a residual, disequilibrium flux influenced by upwelling and entrainment of remineralized carbon‐ and nutrient‐rich waters from the ocean interior, as well as rapid subduction of surface waters, (3) carbon uptake and export by biological activity as both soft tissue and carbonate, and (4) the effect on surface carbon concentrations due to freshwater precipitation or evaporation. In a steady state simulation of a coarse resolution ocean circulation and biogeochemistry model, the sum of the individually determined components is close to the known total flux of the simulation. The leading order balance, identified in different dynamical regimes, is between the CO2 fluxes driven by surface heat fluxes and a combination of biologically‐driven carbon uptake and disequilibrium‐driven carbon outgassing. The framework is still able to reconstruct simulated fluxes when evaluated using monthly‐averaged data and takes a form that can be applied consistently in models of different complexity and observations of the ocean. In this way, the framework may reveal differences in the balance of drivers acting across an ensemble of climate model simulations or be applied to an analysis and interpretation of the observed, real‐world air–sea flux of CO2.
      PubDate: 2016-06-11T00:50:31.538076-05:
      DOI: 10.1002/2016GB005400
  • The Influence of Southern Ocean Winds on the North Atlantic Carbon Sink
    • Authors: Ben Bronselaer; Laure Zanna, David R. Munday, Jason Lowe
      Abstract: Observed and predicted increases in Southern Ocean winds are thought to upwell deep ocean carbon and increase atmospheric CO2. However, Southern Ocean dynamics affect biogeochemistry and circulation pathways on a global scale. Using idealised MITgcm simulations, we demonstrate that an increase in Southern Ocean winds reduces the carbon sink in the North Atlantic sub‐polar gyre. The increase in atmospheric CO2 due to the reduction of the North Atlantic carbon sink is shown to be of the same magnitude as the increase in atmospheric CO2 due to Southern Ocean outgassing. The mechanism can be described as follows: The increase in Southern Ocean winds leads to an increase in upper ocean northward nutrient transport. Biological productivity is therefore enhanced in the tropics, which alters the chemistry of the sub‐thermocline waters that are ultimately upwelled in the subpolar gyre. The results demonstrate the influence of Southern Ocean winds on the North Atlantic carbon sink and show that the effect of Southern Ocean winds on atmospheric CO2 is likely twice as large as previously thought in past, present, and future climates.
      PubDate: 2016-06-08T21:55:41.021524-05:
      DOI: 10.1002/2015GB005364
  • Profiling float‐based observations of net respiration beneath the
           mixed layer
    • Authors: Tyler D. Hennon; Stephen C. Riser, Sabine Mecking
      Abstract: We employ profiling floats with dissolved oxygen sensors to observe in situ temporal oxygen evolution below the mixed layer, allowing us to characterize net respiration of organic carbon in eight distinct regions over the globe. Export and export efficiency are generally high in locations with strong seasonal variability, and low in locations of weak seasonality. Vertically integrated respiration is weakly, yet significantly, correlated with remote observations of chlorophyll, net primary production, and planktonic community size structure. These correlations suggest that regimes of high net primary production and large phytoplankton fuel elevated respiration at depth. Several regions of float‐based observations intersect with sites of other detailed observations (e.g. Hawaii and Sargasso Sea), which allows us to compare our results to independent studies. We find that there is good agreement among export production estimates at highly seasonal locations, and that float‐based observations may be biased low at weakly seasonal locations. We posit that the reason for the low‐latitude discrepancy is the relative steady‐state of oxygen concentration caused by weak seasonality and shallow wintertime mixed layer depths.
      PubDate: 2016-06-08T02:20:58.985053-05:
      DOI: 10.1002/2016GB005380
  • Convergent approaches to determine an ecosystem's transpiration fraction
    • Authors: M. Berkelhammer; D. Noone, T.E. Wong, S.P. Burns, J.F. Knowles, A. Kaushik, P.D. Blanken, M.W. Williams
      Abstract: The transpiration (T) fraction of total terrestrial evapotranspiration (ET), T/ET, can vary across ecosystems between 20‐95% with a global average of ∼60%. The wide range may either reflect true heterogeneity between ecosystems and/or uncertainties in the techniques used to derive this property. Here we compared independent approaches to estimate T/ET at two needle‐leaf forested sites with a factor of three difference in leaf area index (LAI). The first method utilized water vapor isotope profiles and the second derived transpiration through its functional relationship with gross primary production (GPP). We found strong agreement between T/ET values from these two independent approaches although we noted a discrepancy at low vapor pressure deficits (VPD). We hypothesize that this divergence arises because stomatal conductance is independent of humidity at low VPD. Overall, we document significant synoptic‐scale T/ET variability but minimal growing season‐scale variability. This result indicates a high sensitivity of T/ET to passing weather but convergence towards a stable mean state, which is set by LAI. While changes in T/ET could emerge from a myriad of processes, including above‐ (LAI) or below‐ (rooting depth) ground changes, there was only minimal interannual variability and no secular trend in our analysis of T/ET from the 15‐year eddy covariance timeseries at Niwot Ridge. If the lack of trend observed here is apparent elsewhere, it suggests that the processes controlling the T and E fluxes are coupled in a way to maintain a stable ratio.
      PubDate: 2016-06-06T16:36:13.554688-05:
      DOI: 10.1002/2016GB005392
  • Forest harvest contribution to Boreal freshwater methyl mercury load
    • Abstract: Clear‐cutting of Boreal coniferous forests enhances MeHg pool in organic topsoil 7 times Forest clear‐cutting enhances stream export of MeHg more than three times in undulating terrain Forest harvest increases MeHg export to Boreal headwaters by 12‐20% in Sweden and 2 % globally
      PubDate: 2016-06-06T00:50:32.601745-05:
      DOI: 10.1002/2015GB005316
  • Dynamic biogeochemical controls on river pCO2 and recent changes under
           aggravating river impoundment: an example of the subtropical Yangtze River
    • Authors: Shaoda Liu; Xi Xi Lu, Xinghui Xia, Shurong Zhang, Lishan Ran, Xiankun Yang, Ting Liu
      Abstract: This paper highlights two aspects of the dynamic biogeochemical controls of riverine pCO2 in an increasingly impounded large subtropical river (the Yangtze): the terrestrial dominance through internal respiration of land‐derived organic carbon and the influence of increased autotrophic activity in impounded areas on river pCO2. River pCO2 and total organic carbon (TOC) increase downstream on the mainstem (pCO2: 528–1703 µatm; TOC: 137–263 µmol/L) and vary significantly among tributaries (464–3300 µatm; TOC: 109–340 µmol/L). pCO2 displays larger spatial variability than temporal variability and is spatially correlated with river organic carbon across the river (p 
      PubDate: 2016-06-05T21:40:27.553861-05:
      DOI: 10.1002/2016GB005388
  • Heavy silicon isotopic composition of silicic acid and biogenic silica in
           Arctic waters over the Beaufort shelf and the Canada Basin
    • Authors: D. E. Varela; M. A. Brzezinski, C. P. Beucher, J. L. Jones, K. E. Giesbrecht, B. Lansard, A. Mucci
      Abstract: The silicon isotopic composition of silicic acid (δ30Si(OH)4) and biogenic silica (δ30Si‐bSiO2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ30Si(OH)4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30ε = ‐0.97 ± 0.17‰, agrees well with laboratory and global‐ocean estimates. However, the δ30Si dynamics of this region are better represented by closed system isotope models that yield lower values of 30ε, between ‐0.33‰ and ‐0.41‰, depending on how the contribution of sea ice diatoms is incorporated. In the upper 400 m, δ30Si‐bSiO2 values were among the heaviest ever measured in marine suspended bSiO2 (+2.03‰ to +3.51‰). A positive correlation between δ30Si‐bSiO2 and sea‐ice cover implies that heavy signals can result from isotopically‐heavy sea‐ice diatoms introduced to pelagic assemblages. Below the surface bSiO2 production zone, δ30Si(OH)4 distribution followed that of major water masses. Vertical δ30Si(OH)4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125‐200 m) composed of modified Pacific water, and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300‐500 m deep). In the Canada Basin Deep Water (below 2,000 m), δ30Si(OH)4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH)4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ30Si(OH)4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box‐model simulation of the global marine δ30Si(OH)4 distribution successfully reproduced the observed patterns, with the δ30Si(OH)4 of the simulated deep Arctic Ocean being the heaviest of all deep‐ocean basins.
      PubDate: 2016-06-04T16:00:36.778142-05:
      DOI: 10.1002/2015GB005277
  • Different sources and degradation state of dissolved, particulate and
           sedimentary organic matter along the Eurasian Arctic coastal margin
    • Abstract: Thawing Arctic permafrost causes massive fluvial and erosional releases of dissolved and particulate organic carbon (DOC and POC) to coastal waters. Here we investigate how different sources and degradation of remobilized terrestrial carbon may affect large‐scale carbon cycling, by comparing molecular and dual‐isotope composition of waterborne high‐molecular weight DOC (>1kD, a.k.a. colloidal OC), POC and sedimentary OC (SOC) across the East Siberian Arctic Shelves. Lignin phenol fingerprints demonstrate a longitudinal trend in relative contribution of terrestrial sources to coastal OC. Wax lipids and cutins were not detected in COC, in contrast to POC and SOC, suggesting that different terrestrial carbon pools partition into different aquatic carrier phases. The Δ14C signal suggests overwhelmingly contemporary sources for COC, while POC and SOC are dominated by old C from Ice Complex Deposit (ICD) permafrost. Monte Carlo source apportionment (δ13C, Δ14C) constrained that COC was dominated by terrestrial OC from Topsoil permafrost (65%) and marine plankton (25%) with smaller contribution ICD and other older permafrost stocks (9%). This distribution is likely a result of inherent compositional matrix differences, possibly driven by organomineral associations. Modern OC found suspended in the surface water may be more exposed to degradation, in contrast to older OC that preferentially settles to the seafloor where it may be degraded on a longer timescale. The different sources which partition into DOC, POC and SOC appear to have vastly different fates along the Eurasian Arctic coastal margin, and may possibly respond on different timescales to climate change.
      PubDate: 2016-06-02T14:25:25.477014-05:
      DOI: 10.1002/2015GB005307
  • Eroding permafrost coasts release low amounts of dissolved organic carbon
           (DOC) from ground ice into the nearshore zone of the Arctic Ocean
    • Authors: George Tanski; Nicole Couture, Hugues Lantuit, Antje Eulenburg, Michael Fritz
      Abstract: Ice‐rich permafrost coasts in the Arctic are highly sensitive to climate warming and erode at a pace that exceeds the global average. Permafrost coasts deliver vast amounts of organic carbon into the nearshore zone of the Arctic Ocean. Numbers on flux exist for particulate and total soil organic carbon (POC and TOC). However, they do not exist for dissolved organic carbon (DOC), which is known to be highly bioavailable. This study aims to estimate DOC stocks in coastal permafrost as well as the annual flux into the ocean. DOC concentrations in ground ice were analyzed along the ice‐rich Yukon coast (YC) in the western Canadian Arctic. The annual DOC flux was estimated using available numbers for coast length, cliff height, annual erosion rate, and volumetric ice content in different stratigraphic horizons. Our results showed that DOC concentrations in ground ice range between 0.3 and 347.0 mg L‐1 with an estimated stock of 13.6 ± 3.0 g m‐3 along the YC. An annual DOC flux of 54.9 ± 0.9 Mg yr‐1 was computed. These DOC fluxes are low compared to POC fluxes from coastal erosion or POC and DOC fluxes from Arctic rivers. We conclude that DOC fluxes from permafrost coasts play a secondary role in the Arctic carbon budget. However, this DOC is assumed to be highly bioavailable. We hypothesize that DOC from coastal erosion is important for ecosystems in the Arctic nearshore zones, particularly in summer when river discharge is low, and in areas where rivers are absent.
      PubDate: 2016-06-01T21:40:26.783341-05:
      DOI: 10.1002/2015GB005337
  • Net community production at Ocean Station Papa observed with nitrate and
           oxygen sensors on profiling floats
    • Authors: Joshua N. Plant; Kenneth S. Johnson, Carole M. Sakamoto, Hans W. Jannasch, Luke J. Coletti, Stephen C. Riser, Dana D. Swift
      Abstract: Six profiling floats equipped with nitrate and oxygen sensors were deployed at Ocean Station P in the Gulf of Alaska. The resulting six calendar years and ten float years of nitrate and oxygen data were used to determine an average annual cycle for net community production (NCP) in the top 35 meters of the water column. NCP became positive in February as soon as the mixing activity in the surface layer began to weaken, but nearly three months before the traditionally defined mixed layer began to shoal from its winter time maximum. NCP displayed two maxima, one towards the end of May and another in August with a summertime minimum in June corresponding to the historical peak in mesozooplankton biomass. The average annual NCP was determined to be 1.5 ± 0.6 mol C m‐2 yr‐1 using nitrate and 1.5 ± 0.7 mol C m‐2 yr‐1 using oxygen. The results from oxygen data proved to be quite sensitive to the gas exchange model used as well as the accuracy of the oxygen measurement. Gas exchange models optimized for carbon dioxide flux generally ignore transport due to gas exchange through the injection of bubbles and these models yield NCP values that are two to three time higher than the nitrate based estimates. If nitrate and oxygen NCP rates are assumed to be related by the Redfield model, we show that the oxygen gas exchange model can be optimized by tuning the exchange terms to reproduce the nitrate NCP annual cycle.
      PubDate: 2016-06-01T20:55:32.422847-05:
      DOI: 10.1002/2015GB005349
  • Benthic marine calcifiers coexist with CaCO3‐undersaturated seawater
    • Abstract: Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms that build CaCO3 structures. A large proportion of benthic marine calcifiers incorporate Mg2+ into their skeletons (Mg‐calcite), which in general, reduces mineral stability. The relative vulnerability of some marine calcifiers to ocean acidification appears linked to the relative solubility of their shell or skeletal mineralogy, although some organisms have sophisticated mechanisms for constructing and maintaining their CaCO3 structures causing deviation from this dependence. Nevertheless, few studies consider seawater saturation state with respect to the actual Mg‐calcite mineralogy (ΩMg‐x) of a species when evaluating the effect of ocean acidification on that species. Here, a global dataset of skeletal mole % MgCO3 of benthic calcifiers and in situ environmental conditions spanning a depth range of 0 m (subtidal/neritic) to 5600 m (abyssal) was assembled to calculate in situ ΩMg‐x. This analysis shows that 24% of the studied benthic calcifiers currently experience seawater mineral undersaturation (ΩMg‐x 
      PubDate: 2016-05-27T13:30:27.667182-05:
      DOI: 10.1002/2015GB005260
  • Amazon forest response to repeated droughts
    • Abstract: The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin‐wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin‐wide ground‐based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground‐based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha‐1, CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long‐term biomass sink during the baseline pre‐2010 drought period (1998 − pre‐2010) of 1.33 Mg ha‐1 yr‐1 (CI: 0.90, 1.74, p 
      PubDate: 2016-04-30T07:57:00.036414-05:
      DOI: 10.1002/2015GB005133
  • Nitrate uptake across biomes and the influence of elemental stoichiometry:
           A new look at LINX II
    • Abstract: Considering recent increases in anthropogenic N loading it is essential to identify the controls on N removal and retention in aquatic ecosystems because the fate of N has consequences for water quality in streams and downstream ecosystems. Biological uptake of nitrate (NO3‐) is a major pathway by which N is removed from these ecosystems. Here we used data from the second Lotic Intersite Nitrogen eXperiment (LINX II) in a multivariate analysis to identify the primary drivers of variation in NO3‐ uptake velocity among biomes. Across 69 study watersheds in North America, DOC:NO3‐ ratios and photosynthetically active radiation were identified as the two most important predictor variables in explaining NO3‐ uptake velocity. However, within a specific biome the predictor variables of NO3‐ uptake velocity varied, and included various physical, chemical and biological attributes. . Our analysis demonstrates the broad control of elemental stoichiometry on NO3‐ uptake velocity as well as the importance of biome‐specific predictors. Understanding this spatial variation has important implications for biome‐specific watershed management and the downstream export of NO3‐, as well as for development of spatially explicit global models that describe N dynamics in streams and rivers.
  • Oceanic teleconnection for carbon dioxide
    • Abstract: Biogeochemical teleconnection links seemingly unrelated chemical/biological anomalies that are geographically separated by large distances. Bronselaer et al propose a new mechanism for an interhemispheric teleconnection of air‐sea carbon dioxide fluxes in which the upwelling of the Southern Ocean triggers a series of perturbations leading to the alteration of the carbon uptake in the North Atlantic. The westerly wind over the Southern Ocean has a unique role in the climate system. It energizes the strongest ocean current, Antarctic Circumpolar Current, and it lifts up the carbon‐ and nutrient‐rich deep waters all the way to the surface. It is an end point of the ocean's deep overturning circulation and associated biological carbon storage, where the excess carbon from accumulated decomposition of organic material is finally released back into the atmosphere. It is well established that the Southern Ocean upwelling regionally modulates the de‐gassing of carbon dioxide there. However, its global‐scale implication is not yet fully understood. What happens to the carbon uptake in the other parts of the oceans' In this volume of Global Biogeochemical Cycles, Bronselaer et al describes the chain of events that link the increased Southern Ocean wind to the ocean carbon uptake in the northern high latitudes. The authors conducted a set of computational experiments, showing that the Southern Ocean is a starting point of the oceanic teleconnection, where the excess nutrient is transported equatorward through the shallow overturning circulation. The stream of macro‐nutrient then fertilizes the low‐latitude productivity that eventually shifts the carbonate chemistry of the high latitude surface waters. This is an intriguing case of oceanic teleconnection, linking seemingly unrelated biogeochemical anomalies that are geographically separated by large distances. The surprising conclusion is that a stronger Southern Ocean wind increases the de‐gassing of carbon dioxide in both northern and southern high latitudes. This happens because more carbon is upwelling into the northern high latitudes due to the increased low‐latitude biological pump, approximately doubling the de‐gassing intensity relative to the Southern Ocean response alone. There may be more surprises from the Southern Ocean.
  • Oxygen utilization rate (OUR) underestimates ocean respiration – a
           model study
    • Abstract: We use a simple 1D model representing an isolated density surface in the ocean and 3D global ocean biogeochemical models to evaluate the concept of computing the subsurface oceanic oxygen utilization rate (OUR) from the changes of apparent oxygen utilization (AOU) and water age. The distribution of AOU in the ocean is not only the imprint of respiration in the ocean's interior, but is strongly influenced by transport processes and eventually loss at the ocean surface. Since AOU and water age are subject to advection and diffusive mixing, it is only when they are affected both in the same way that OUR represents the correct rate of oxygen consumption. This is the case only when advection prevails or with uniform respiration rates, when the proportions of AOU and age are not changed by transport. In experiments with the 1D‐tube model, OUR underestimates respiration when maximum respiration rates occur near the outcrops of isopycnals, and overestimates when maxima occur far from the outcrops. Given the distribution of respiration in the ocean, i.e. elevated rates near high latitude outcrops of isopycnals and low rates below the oligotrophic gyres, underestimates are the rule. Integrating these effects globally in three coupled ocean biogeochemical and circulation models we find that AOU‐over‐age based calculations underestimate true model respiration by a factor of three. Most of this difference is observed in the upper 1000 m of the ocean with the discrepancies increasing towards the surface where OUR underestimates respiration by as much as factor of four.
  • Issue Information
    • Abstract: No abstract is available for this article.
  • Quantifying Mesoscale‐Driven Nitrate Supply: A Case Study
    • Abstract: The supply of nitrate to surface waters plays a crucial role in maintaining marine life. Physical processes at the mesoscale (~10‐100 km) and smaller have been advocated to provide a major fraction of the global supply. Whilst observational studies have focussed on well‐defined features, such as isolated eddies, the vertical circulation and nutrient supply in a typical 100‐200 km square of ocean will involve a turbulent spectrum of interacting, evolving and decaying features. A crucial step in closing the ocean nitrogen budget is to be able to rank the importance of mesoscale fluxes against other sources of nitrate for surface waters for a representative area of open ocean. While this has been done using models, the vital observational equivalent is still lacking. To illustrate the difficulties that prevent us from putting a global estimate on the significance of the mesoscale observationally, we use data from a cruise in the Iceland Basin where vertical velocity and nitrate observations were made simultaneously at the same high spatial resolution. Local mesoscale nitrate flux is found to be an order of magnitude greater than that due to small‐scale vertical mixing and exceeds coincident nitrate uptake rates and estimates of nitrate supply due to winter convection. However, a non‐zero net vertical velocity for the region introduces a significant bias in regional estimates of the mesoscale vertical nitrate transport. The need for synopticity means that a more accurate estimate can not be simply found by using a larger survey area. It is argued that time‐series, rather than spatial surveys, may be the best means to quantify the contribution of mesoscale processes to the nitrate budget of the surface ocean.
  • Influence of plankton community structure on the sinking velocity of
           marine aggregates
    • Abstract: About 50 gigatons of carbon are fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25 m deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ø 80 – 400 µm) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity (Pestimated) decreased as the picoautotroph (0.2‐2 µm) fraction of the phytoplankton biomass increased. Thus, picoautotroph‐dominated communities may be indicative for food‐webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500 cells/mL accelerate sinking by about 35‐40% which we estimate (by one‐dimensional modelling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging, both of which are controlled by plankton community structure.
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
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