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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 42, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 18)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 5)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 12)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 22)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 13)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 17, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 123, SJR: 1.769, h-index: 110)
Journal Cover Global Biogeochemical Cycles
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     ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.4]   [H-I: 109]
  • Long‐term variability of phytoplankton carbon biomass in the
           Sargasso Sea
    • Authors: Philip J. Wallhead; Véronique C. Garçon, John R. Casey, Michael W. Lomas
      Pages: n/a - n/a
      Abstract: Time series of phytoplankton carbon biomass are scarce yet may provide important insights into ocean productivity and carbon export to depth via the oceanic biological pump. We combine recent flow‐cytometric measurements with pigment concentrations and other standard measurements to reconstruct taxon‐specific phytoplankton carbon biomass in the Sargasso Sea over 22 years, using a multiple regression approach. The reconstructed series reveal an increasing trend (~3% per year) in total phytoplankton carbon, apparently driven by increasing nutrient supply by vertical mixing associated with a shift to a negative phase in the winter North Atlantic Oscillation index. Also, the reconstructed eukaryote biomass fraction shows a multiannual shift from ~45% in the early 1990s/late 2000s to ~70% in the late 1990s/early 2000s. We hypothesize that a multiannual shift in the seasonal pattern of mixing may have stimulated and restructured the eukaryote community while suppressing prokaryote populations by increasing photodamage and grazing mortality.
      PubDate: 2014-07-26T08:59:00.902149-05:
      DOI: 10.1002/2013GB004797
       
  • Worldwide retention of nutrient silicon by river damming: From sparse data
           set to global estimate
    • Authors: Taylor Maavara; Hans H. Dürr, Philippe Van Cappellen
      Pages: n/a - n/a
      Abstract: Damming of rivers represents a major anthropogenic perturbation of the hydrological cycle, with the potential to profoundly modify the availability of nutrient silicon (Si) in streams, lakes and coastal areas. A global assessment of the impact of dams on river Si fluxes, however, is limited by the sparse data set on Si budgets for reservoirs. To alleviate this limitation, we use existing data on dissolved Si (DSi) retention by dams to calibrate a mechanistic model for the biogeochemical cycling of DSi and reactive particulate Si (PSi) in reservoir systems. The model calibration yields a relationship between the annual in‐reservoir siliceous primary productivity and the external DSi supply. With this relationship and an estimate of catchment Si loading, the model calculates the total reactive Si (RSi = DSi + PSi) retention for any given reservoir. A Monte Carlo analysis accounts for the effects of variations in reservoir characteristics, and generates a global relationship that predicts the average reactive Si retention in reservoirs as a function of the water residence time. This relationship is applied to the GRanD database to estimate Si retention by damming worldwide. According to the results, dams retain 163 Gmol yr−1 (9.8 Tg SiO2 yr−1) of DSi and 372 Gmol yr−1 (22.3 Tg SiO2 yr−1) of RSi, or 5.3% of the global RSi loading to rivers.
      PubDate: 2014-07-25T11:44:57.012629-05:
      DOI: 10.1002/2014GB004875
       
  • Evidence for production and lateral transport of dissolved organic
           phosphorus in the eastern subtropical North Atlantic
    • Authors: Sarah Reynolds; Claire Mahaffey, Vassil Roussenov, Richard G. Williams
      Pages: n/a - n/a
      Abstract: The concentration of phosphate and dissolved organic phosphorus (DOP) is chronically low and limits phytoplankton growth in the subtropical North Atlantic relative to other ocean basins. Transport of phosphate and DOP from the productive flanks of the gyre to its interior has been hypothesized as an important phosphorus supply pathway. During a cruise in the eastern Atlantic in spring 2011, the rates of phosphate uptake, alkaline phosphatase (APA) and DOP production were measured in the northwest African shelf region, subtropics and tropics. Rates of DOP production were 6‐fold higher in the shelf region (43 ± 41 nM d−1) relative to the subtropics (6.9 ± 4.4 nM d−1). In contrast, APA production was 3‐fold higher in the subtropics (8.0 ± 7.3 nM d−1), indicative of enhanced DOP utilization, relative to the shelf region (2.6 ± 2.1 nM d−1). Hence, observations suggest net production of DOP in the shelf region and either net consumption of DOP or a near balance in DOP production and consumption in the gyre interior. Eddy‐permitting model experiments demonstrate that (i) DOP accounts for over half the total phosphorus in surface waters, (ii) DOP is transported westward from the shelf region by a combination of gyre and eddy circulations, and (iii) advected DOP supports up to 70% of the particle export over much of the subtropical gyre. Our combined observational and modeling study supports the view that the horizontal transport of DOP from the shelf region is an important mechanism supplying phosphorus to the surface subtropical North Atlantic.
      PubDate: 2014-07-23T08:18:23.989691-05:
      DOI: 10.1002/2013GB004801
       
  • Characterizing the annual cycle of African dust transport to the Caribbean
           
    • Authors: Joseph M. Prospero; François‐Xavier Collard, Jack Molinié, Alexis Jeannot
      Pages: n/a - n/a
      Abstract: Decades of aerosol measurements on Barbados have yielded a detailed picture of African mineral dust transport to the Caribbean Basin that shows a strong seasonal cycle with a maximum in boreal summer and a minimum in winter. Satellite aerosol products suggest that in spring, there is a comparable transport to northeastern South America. Here we characterize the complete annual cycle of dust transport to the western Atlantic by linking the Barbados record to multiyear records of airborne particulate matter less than 10 µm diameter (PM10) measured in air quality programs at Cayenne (French Guiana) and Guadeloupe. Comparisons of PM10 at these sites with concurrent dust measurements at Barbados demonstrate that high PM10 levels are almost entirely due to dust. Cayenne PM10 peaks in spring in a cycle which is consistent with satellite aerosol optical depth and suggests that the Sahel is the dominant source. The persistent transport of dust during much of the year could impact a wide range of environmental processes over a broad region that extends from the southern United States to the Amazon Basin. Finally, the average 24 h PM10 concentrations at Cayenne and Guadeloupe frequently exceed the World Health Organization air quality guideline. Thus soil dust PM10 could be a significant, but generally unrecognized, health factor at western Atlantic sites and also in other relatively remote regions affected by long‐range dust from Africa. Because dust emissions and transport are highly sensitive to climate variability, climate change in coming decades could greatly affect a wide range of biogeochemical processes and human health in this region.
      PubDate: 2014-07-21T10:20:33.500857-05:
      DOI: 10.1002/2013GB004802
       
  • Radium isotopes as a tracer of sediment‐water column exchange in the
           North Sea
    • Authors: W. J. Burt; H. Thomas, J. Pätsch, A. M. Omar, C. Schrum, U. Daewel, H. Brenner, H. J. W. Baar
      Pages: n/a - n/a
      Abstract: Sediment‐water column exchange plays an important role in coastal biogeochemistry. We utilize short‐lived radium isotopes (224Ra and 223Ra) to understand and quantify the dominant processes governing sediment‐water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats, and North Sea sediments as the dominant source. Porewaters show 100‐fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment‐water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass‐balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation‐based Ra flux estimates, and the majority of previously reported Ra flux estimates for other regions. Ra‐based estimates of benthic alkalinity fluxes compare well to observed values and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.
      PubDate: 2014-07-18T08:26:53.862128-05:
      DOI: 10.1002/2014GB004825
       
  • Variability of alkalinity and the alkalinity‐salinity relationship
           in the tropical and subtropical surface ocean
    • Authors: Zong‐Pei Jiang; Toby Tyrrell, David J. Hydes, Minhan Dai, Susan E. Hartman
      Pages: n/a - n/a
      Abstract: The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific, and Indian Oceans, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non‐SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity‐normalized alkalinity with that of the open ocean end‐member. Non‐SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin, and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end‐member can only be reliably derived from the y intercept of TA‐S regression (TAS0) in river‐dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification, etc.) are more influential, TAS0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TAS0 can result from non‐SDC TA removal at the low salinity end (relative to the salinity of the oceanic end‐member) and/or non‐SDC TA addition at high salinities (as occurs in the Mediterranean Sea).
      PubDate: 2014-07-17T17:17:16.055718-05:
      DOI: 10.1002/2013GB004678
       
  • Contribution of anthropogenic phosphorus to agricultural soil fertility
           and food production
    • Authors: B. Ringeval; B. Nowak, T. Nesme, M. Delmas, S. Pellerin
      Pages: n/a - n/a
      Abstract: Agricultural intensification over the last few decades has been accompanied by the extensive use of anthropogenic phosphorus (P) derived from mined phosphate rock. Given the increasing scarcity of P resources, accurate estimates of the reliance of agriculture on anthropogenic P are required. Here we propose a modeling approach for assessing the contribution of anthropogenic P to agricultural soil fertility and food production. We performed computations at country level, and France was chosen as a typical western European country with intensive agriculture. Four soil P pools were identified based on their bioavailability (labile versus stable) and origin (anthropogenic versus natural). Pool evolution between 1948 and 2009 was estimated by combining international databases and a simple biogeochemical model. An optimization procedure demonstrated the necessity of representing a stable P pool capable of replenishing the labile pool within 14 to 33 years in order to match country‐scale observations. Mean simulated P pool sizes for 2009 (0–35 cm soil horizon) were 146, 616, 31, and 156 kgP/ha for natural stable, anthropogenic stable, natural labile, and anthropogenic labile pools, respectively. We found that, on average, 82% (min‐max: 68–91%) of soil P (sum of labile and above defined stable) in that year was anthropogenic. The temporal evolution of this contribution is directly related to the integral of chemical fertilizer use over time, starting from 1948. The contribution of anthropogenic P to food production was similar at 84% (min‐max: 72–91%), which is greater than budget‐based estimates (~50–60%) commonly reported in the literature. By focusing on soil fertility and food production, this study provides a quantitative estimation of human perturbations of the P cycle in agroecosystems.
      PubDate: 2014-07-17T17:12:53.036322-05:
      DOI: 10.1002/2014GB004842
       
  • Western Pacific atmospheric nutrient deposition fluxes, their impact on
           surface ocean productivity
    • Authors: M. Martino; D. Hamilton, A. R. Baker, T. D. Jickells, T. Bromley, Y. Nojiri, B. Quack, P. W. Boyd
      Pages: n/a - n/a
      Abstract: The atmospheric deposition of both macronutrients and micronutrients plays an important role in driving primary productivity, particularly in the low‐latitude ocean. We report aerosol major ion measurements for five ship‐based sampling campaigns in the western Pacific from ~25°N to 20°S and compare the results with those from Atlantic meridional transects (~50°N to 50°S) with aerosols collected and analyzed in the same laboratory, allowing full incomparability. We discuss sources of the main nutrient species (nitrogen (N), phosphorus (P), and iron (Fe)) in the aerosols and their stoichiometry. Striking north–south gradients are evident over both basins with the Northern Hemisphere more impacted by terrestrial dust sources and anthropogenic emissions and the North Atlantic apparently more impacted than the North Pacific. We estimate the atmospheric supply rates of these nutrients and the potential impact of the atmospheric deposition on the tropical western Pacific. Our results suggest that the atmospheric deposition is P deficient relative to the needs of the resident phytoplankton. These findings suggest that atmospheric supply of N, Fe, and P increases primary productivity utilizing some of the residual excess phosphorus (P*) in the surface waters to compensate for aerosol P deficiency. Regional primary productivity is further enhanced via the stimulation of nitrogen fixation fuelled by the residual atmospheric iron and P*. Our stoichiometric calculations reveal that a P* of 0.1 µmol L−1 can offset the P deficiency in atmospheric supply for many months. This study suggests that atmospheric deposition may sustain ~10% of primary production in both the western tropical Pacific.
      PubDate: 2014-07-15T10:36:37.063586-05:
      DOI: 10.1002/2013GB004794
       
  • Issue Information
    • Pages: i - iii
      Abstract: No abstract is available for this article.
      PubDate: 2014-07-12T06:19:06.919551-05:
      DOI: 10.1002/gbc.20084
       
  • Decadal (1994–2008) change in the carbon isotope ratio in the
           eastern South Pacific Ocean
    • Authors: Young Ho Ko; Kitack Lee, Paul D. Quay, Richard A. Feely
      Pages: n/a - n/a
      Abstract: We determined the 14‐year change in the anthropogenic CO2 inventory in the eastern South Pacific Ocean along the 110˚W meridian from 67˚S to 21˚N, using seawater δ13C datasets collected in 1994 and 2008. The vertical integral of the 14‐year δ13C change was assessed in five latitude bands, and found to be greatest (–14.7‰ m yr–1) in the subpolar band (38˚S–55˚S) and smallest (–3.0‰ m yr–1) in the tropical band (21˚N–18˚S). The δ13C change in each of the latitudinal bands was primarily caused by inputs of anthropogenic CO2 via air‐sea exchange and transport. More than 50% of the total anthropogenic CO2 was added to the subpolar band via the northward movement of Antarctic Intermediate Water (AAIW) from the south, and the remaining 50% was added via air‐sea exchange. We also calculated the ratio of the temporal change in δ13C to the change in dissolved inorganic carbon, which is a measure of the efficiency of oceanic uptake of anthropogenic CO2. The ratio for AAIW in 1994 (–0.017‰ (µmol kg–1) –1) was greater than that in 2008 (–0.010‰ (µmol kg–1) –1) based on the change in preformed δ13C and DIC, indicating reduced efficiency of CO2 uptake by the Southern Ocean in 2008 relative to that in 1994. AAIW remained at the surface for a shorter period in 2008 relative to 1994, and thus would have taken up less atmospheric CO2 prior to subduction. The projected reduction in this ratio indicates a weakening of CO2 uptake by the Southern Ocean in the future.
      PubDate: 2014-07-11T01:05:36.589329-05:
      DOI: 10.1002/2013GB004786
       
  • Sunlight‐induced carbon dioxide emissions from inland waters
    • Authors: Birgit Koehler; Tomas Landelius, Gesa A. Weyhenmeyer, Nanako Machida, Lars J. Tranvik
      Pages: n/a - n/a
      Abstract: The emissions of carbon dioxide (CO2) from inland waters are substantial on a global scale. Yet the fundamental question remains open which proportion of these CO2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55°N to 69°N in Sweden. The sunlight‐induced production of dissolved inorganic carbon (DIC) averaged 3.8 ± 0.04 g C m−2 yr−1, which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 ± 1 kt C yr−1 was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO2 emissions from Swedish lakes. With a median depth of 3.2 m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which corresponds to the absorbed photons. Therefore, the slope of the regression line represents the wavelength‐ and depth‐integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr−1 under overcast and clear sky, respectively. Consequently, these directly sunlight‐induced CO2 emissions contribute up to about one tenth to the global CO2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.
      PubDate: 2014-07-10T14:05:24.259288-05:
      DOI: 10.1002/2014GB004850
       
  • Impact of changes in barometric pressure on landfill methane emission
    • Authors: Liukang Xu; Xiaomao Lin, Jim Amen, Karla Welding, Dayle McDermitt
      Pages: n/a - n/a
      Abstract: Landfill methane emissions were measured continuously using the eddy covariance method from June to December 2010. The study site was located at the Bluff Road Landfill in Lincoln, Nebraska, USA. Our results show that landfill methane emissions strongly depended on changes in barometric pressure; rising barometric pressure suppressed the emission, while falling barometric pressure enhanced the emission, a phenomenon called barometric pumping. There was up to a 35‐fold variation in day‐to‐day methane emissions due to changes in barometric pressure. Wavelet coherence analysis revealed a strong spectral coherency between variations of barometric pressure and methane emission at periodicities ranging from 1 day to 8 days. Power spectrum and ogive analysis showed that at least 10 days of continuous measurements was needed in order to capture 90% of the total variance in the methane emission time series at our landfill site. From our results, it is clear that point‐in‐time measurements taken at monthly or longer time intervals using techniques such as the trace plume method, the mass balance method, or the closed‐chamber method will be subject to large variations in measured emission rates because of the barometric pumping phenomenon. Estimates of long‐term integrated methane emissions from landfills based on such measurements could yield uncertainties, ranging from 28.8% underestimation to 32.3% overestimation. Our results demonstrate a need for continuous measurements to quantify annual total landfill emissions. This conclusion may apply to the study of methane emissions from wetlands, peatlands, lakes, and other environmental contexts where emissions are from porous media or ebullition. Other implications from the present study for hazard gas monitoring programs are also discussed.
      PubDate: 2014-07-10T10:15:22.708959-05:
      DOI: 10.1002/2013GB004571
       
  • Effects of soot deposition on particle dynamics and microbial processes in
           marine surface waters
    • Authors: Xavier Mari; Jérôme Lefèvre, Jean‐Pascal Torréton, Yvan Bettarel, Olivier Pringault, Emma Rochelle‐Newall, Patrick Marchesiello, Christophe Menkes, Martine Rodier, Christophe Migon, Chiaki Motegi, Markus G. Weinbauer, Louis Legendre
      Pages: n/a - n/a
      Abstract: Large amounts of soot are continuously deposited on the global ocean. Even though significant concentrations of soot particles are found in marine waters, the effects of these aerosols on ocean ecosystems are currently unknown. Using a combination of in situ and experimental data, and results from an atmospheric transport model, we show that the deposition of soot particles from an oil‐fired power plant impacted biogeochemical properties and the functioning of the pelagic ecosystem in tropical oligotrophic oceanic waters off New Caledonia. Deposition was followed by a major increase in the volume concentration of suspended particles, a change in the particle size spectra that resulted from a stimulation of aggregation processes, a 5% decrease in the concentration of dissolved organic carbon (DOC), a decreases of 33 and 23% in viral and free bacterial abundances, respectively, and a factor ~2 increase in the activity of particle‐attached bacteria suggesting that soot introduced in the system favored bacterial growth. These patterns were confirmed by experiments with natural seawater conducted with both soot aerosols collected in the study area and standard diesel soot. The data suggest a strong impact of soot deposition on ocean surface particles, DOC, and microbial processes, at least near emission hot spots.
      PubDate: 2014-07-09T19:41:07.872005-05:
      DOI: 10.1002/2014GB004878
       
  • The oceanic anthropogenic CO2 sink: Storage, air‐sea fluxes, and
           transports over the industrial era
    • Authors: Tim DeVries
      Pages: n/a - n/a
      Abstract: This study presents a new estimate of the oceanic anthropogenic CO2(Cant) sink over the industrial era (1780 to present), from assimilation of potential temperature, salinity, radiocarbon, and CFC‐11 observations in a global steady state ocean circulation inverse model (OCIM). This study differs from previous data‐based estimates of the oceanic Cant sink in that dynamical constraints on ocean circulation are accounted for, and the ocean circulation is explicitly modeled, allowing the calculation of oceanic Cant storage, air‐sea fluxes, and transports in a consistent manner. The resulting uncertainty of the OCIM‐estimated Cant uptake, transport, and storage is significantly smaller than the comparable uncertainty from purely data‐based or model‐based estimates. The OCIM‐estimated oceanic Cant storage is 160–166 PgC in 2012, and the oceanic Cant uptake rate averaged over the period 2000–2010 is 2.6 PgC yr−1 or about 30% of current anthropogenic CO2 emissions. This result implies a residual (primarily terrestrial) Cant sink of about 1.6 PgC yr−1 for the same period. The Southern Ocean is the primary conduit for Cant entering the ocean, taking up about 1.1 PgC yr−1 in 2012, which represents about 40% of the contemporary oceanic Cant uptake. It is suggested that the most significant source of remaining uncertainty in the oceanic Cant sink is due to potential variability in the ocean circulation over the industrial era.
      PubDate: 2014-07-09T19:32:56.546783-05:
      DOI: 10.1002/2013GB004739
       
  • Global patterns of phytoplankton nutrient and light colimitation inferred
           from an optimality‐based model
    • Authors: Lionel Arteaga; Markus Pahlow, Andreas Oschlies
      Pages: n/a - n/a
      Abstract: The widely used concept of constant ”Redfield” phytoplankton stoichiometry is often applied for estimating which nutrient limits phytoplankton growth in the surface ocean. Culture experiments, in contrast, show strong relations between growth conditions and cellular stoichiometry with often substantial deviations from Redfield stoichiometry. Here we investigate to what extent both views agree by analyzing remote sensing and in situ data with an optimality‐based model of nondiazotrophic phytoplankton growth in order to infer seasonally varying patterns of colimitation by light, nitrogen (N), and phosphorus (P) in the global ocean. Our combined model‐data analysis suggests strong N and N‐P colimitation in the tropical ocean, seasonal light, and N‐P colimitation in the Northern Hemisphere, and strong light limitation only during winter in the Southern Ocean. The eastern equatorial Pacific appears as the only ocean area that is essentially not limited by N, P, or light. Even though our optimality‐based approach specifically accounts for flexible stoichiometry, inferred patterns of N and P limitation are to some extent consistent with those obtained from an analysis of surface inorganic nutrients with respect to the Redfield N:P ratio. Iron is not part of our analysis, implying that we cannot accurately predict N cell quotas in high‐nutrient, low‐chlorophyll regions. Elsewhere, we do not expect a major effect of iron on the relative distribution of N, P, and light colimitation areas. The relative importance of N, P, and light in limiting phytoplankton growth diagnosed here by combining observations and an optimal growth model provides a useful constraint for models used to predict future marine biological production under changing environmental conditions. © 2014. American Geophysical Union. All Rights Reserved.
      PubDate: 2014-07-09T19:25:25.403866-05:
      DOI: 10.1002/2013GB004668
       
  • Impact of Large‐Scale Climate Extremes on Biospheric Carbon Fluxes:
           An Intercomparison Based on MsTMIP Data
    • Authors: Jakob Zscheischler; Anna M. Michalak, Christopher Schwalm, Miguel D. Mahecha, Deborah N. Huntzinger, Markus Reichstein, Gwenaëlle Berthier, Philippe Ciais, Robert B. Cook, Bassil El‐Masri, Maoyi Huang, Akihiko Ito, Atul Jain, Anthony King, Huimin Lei, Chaoqun Lu, Jiafu Mao, Shushi Peng, Benjamin Poulter, Daniel Ricciuto, Xiaoying Shi, Bo Tao, Hanqin Tian, Nicolas Viovy, Weile Wang, Yaxing Wei, Jia Yang, Ning Zeng
      First page: 585
      Abstract: Understanding the role of climate extremes and their impact on the carbon (C) cycle is increasingly a focus of Earth system science. Climate extremes such as droughts, heat waves, or heavy precipitation events can cause substantial changes in terrestrial C fluxes. On the other hand, extreme changes in C fluxes are often, but not always, driven by extreme climate conditions. Here we present an analysis of how extremes in temperature and precipitation, and extreme changes in terrestrial C fluxes are related to each other in 10 state‐of‐the‐art terrestrial carbon models, all driven by the same climate forcing. We use model outputs from the North American Carbon Program (NACP) Multi‐scale synthesis and Terrestrial Model Intercomparison Project (MsTMIP). A global scale analysis shows that both droughts and heatwaves translate into anomalous net releases of CO2 from the land surface via different mechanisms: Droughts largely decrease gross primary production (GPP) and to a lower extent total respiration (TR), while heat waves slightly decrease GPP but increase TR. Cold and wet periods have a smaller opposite effect. Analyzing extremes in C fluxes reveals that extreme changes in GPP and TR are often caused by strong shifts in water availability, but for extremes in TR shifts in temperature are also important. Extremes in net CO2 exchange are equally strongly driven by deviations in temperature and precipitation. Models mostly agree on the sign of the C flux response to climate extremes, but model spread is large. In tropical forests, C cycle extremes are driven by water availability, whereas in boreal forests temperature plays a more important role. Models are particularly uncertain about the C flux response to extreme heat in boreal forests.
      PubDate: 2014-06-06T16:18:36.971395-05:
      DOI: 10.1002/2014GB004826
       
  • Experimental assessment of diazotroph responses to elevated seawater pCO2
           in the North Pacific Subtropical Gyre
    • Authors: Daniela Böttjer; David M. Karl, Ricardo M. Letelier, Donn A. Viviani, Matthew J. Church
      First page: 601
      Abstract: We examined short‐term (24‐72 hours) responses of naturally occurring marine N2 fixing microorganisms (termed diazotrophs) to abrupt increases in the partial pressure of carbon dioxide (pCO2) in seawater during 9 incubation experiments conducted between May 2010 and September 2012 at Station ALOHA (22°45’N, 158˚W) in the North Pacific Subtropical Gyre (NPSG). Rates of N2 fixation, nitrogenase (nifH) gene abundances and transcripts of six major groups of cyanobacterial diazotrophs (including both unicellular and filamentous phylotypes), and rates of primary productivity (as measured by 14C‐bicarbonate assimilation into plankton biomass) were determined under contemporary (~390 ppm) and elevated pCO2 conditions (~1100 ppm). Quantitative polymerase chain reaction (QPCR) amplification of planktonic nifH genes revealed that unicellular cyanobacteria phylotypes dominated gene abundances during these experiments. In the majority of experiments (7 out of 9), elevated pCO2 did not significantly influence rates of dinitrogen (N2) fixation or primary productivity (two‐way ANOVA, P > 0.05). During two experiments, rates of N2 fixation rates and primary productivity were significantly lower (by 79 to 82% and 52 to 72%, respectively) in the elevated pCO2 treatments relative to the ambient controls (two‐way ANOVA, P 
      PubDate: 2014-06-10T15:39:45.55885-05:0
      DOI: 10.1002/2013GB004690
       
  • Reconciling modeled and observed atmospheric deposition of soluble organic
           nitrogen at coastal locations
    • Authors: Akinori Ito; Guangxing Lin, Joyce E. Penner
      First page: 617
      Abstract: Atmospheric deposition of reactive nitrogen (N) species from air pollutants is a significant source of exogenous nitrogen in marine ecosystems. Here we use an atmospheric chemical transport model to investigate the supply of soluble organic nitrogen (ON) from anthropogenic sources to the ocean. Comparisons of modeled deposition with observations at coastal and marine locations show good overall agreement for inorganic nitrogen and total soluble nitrogen. However, previous modeling approaches result in significant underestimates of the soluble ON deposition if the model only includes the primary soluble ON and the secondary oxidized ON in gases and aerosols. Our model results suggest that including the secondary reduced ON in aerosols as a source of soluble ON contributes to an improved prediction of the deposition rates (g N m−2 yr−1). The model results show a clear distinction in the vertical distribution of soluble ON in aerosols between different processes from the primary sources and the secondary formation. The model results (excluding the biomass burning and natural emission changes) suggest an increase in soluble ON outflow from atmospheric pollution, in particular from East Asia, to the oceans in the twentieth century. These results highlight the necessity of improving the process‐based quantitative understanding of the chemical reactions of inorganic nitrogen species with organics in aerosol and cloud water.
      PubDate: 2014-06-13T15:48:18.835644-05:
      DOI: 10.1002/2013GB004721
       
  • Climate Warming Shifts Carbon Allocation from Stemwood to Roots in
           Calcium‐Depleted Spruce Forests
    • Authors: Andrei G. Lapenis; Gregory B. Lawrence, Alexander Heim, Chengyang Zheng, Walter Shortle
      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.
      PubDate: 2013-01-04T00:00:00-05:00
      DOI: 10.1002/gbc.20005
       
 
 
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