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  • Articles  (922)
  • Wiley  (922)
  • American Physical Society
  • Global Biogeochemical Cycles  (922)
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  • 1
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    Is atmospheric phosphorus pollution altering global alpine lake stoichiometry? (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: Anthropogenic activities have significantly altered atmospheric chemistry and changed the global mobility of key macronutrients. Here, we show that contemporary global patterns in nitrogen (N) and phosphorus (P) emissions drive large hemispheric variation in precipitation chemistry. These global patterns of nutrient emission and deposition (N:P) are in turn closely reflected in the water chemistry of naturally oligotrophic lakes ( r 2 =0.81, p 〈0.0001). Observed increases in anthropogenic N deposition play a role in nutrient concentrations ( r 2 =0.20, p 〈0.05); however, atmospheric deposition of P appears to be major contributor to this pattern ( r 2 =0.65, p 〈0.0001). Atmospheric simulations indicate a global increase in P deposition by 1.4 times the preindustrial rate largely due to increased dust and biomass burning emissions. Although changes in the mass flux of global P deposition are smaller than for N, the impacts on primary productivity may be greater because, on average, one unit of increased P deposition has 16X the influence of one unit of N deposition. These stoichiometric considerations, combined with the evidence presented here, suggest that increases in P deposition may be a major driver of alpine lake trophic status, particularly in the Southern Hemisphere. These results underscore the need for the broader scientific community to consider the impact of atmospheric phosphorus deposition on the water quality of naturally oligotrophic lakes.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 2
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    The imbalance of new and export production in the Western Antarctic Peninsula, a potentially “leaky” ecosystem (2015)
    Wiley
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    Publication Date: 2015-08-13
    Description: To quantify the balance between new production and vertical nitrogen export of sinking particles, we measured nitrate uptake, net nitrate drawdown, ΔO 2 /Ar-based net community production, sediment trap flux, and 234 Th export at a coastal site near Palmer Station, Antarctica during the phytoplankton growing season from October 2012 to March 2013. We also measured nitrate uptake and 234 Th export throughout the northern western Antarctic Peninsula (WAP) region on a cruise in January 2013. We used a non-steady state 234 Th equation with temporally-varying upwelling rates and an irradiance-based phytoplankton production model to correct our export and new production estimates in the complex coastal site near Palmer Station. Results unequivocally showed that nitrate uptake and net community production were significantly greater than the sinking particle export on region-wide spatial scales and season-long temporal scales. At our coastal site, new production (105±17.4 mg N m −2 d −1 , mean±st.err.) was 5.3 times greater than vertical nitrogen export (20.4±2.4 mg N m −2 d -1 ). On the January cruise in the northern WAP, new production (47.9±14.4 mg N m −2 d -1 ) was 2.4 times greater than export (19.9±1.4 mg N m −2 d −1 ). Much of this imbalance can be attributed to diffusive losses of particulate nitrogen from the surface ocean due to diapycnal mixing, indicative of a “leaky” WAP ecosystem. If these diffusive losses are common in other systems where new production exceeds export, it may be necessary to revise current estimates of the ocean's biological pump.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 3
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    Issue Information (2015)
    Wiley
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    Publication Date: 2015-08-16
    Description: Cover: In Somes and Oschlies [doi 10.1002/2014GB005050 ], comparison of surface (0–50m) (a) map and (b) zonally averaged DON observations [Letscher et al., 2013] with annual semirecalcitrant DON from the model experiments (c) Redfield DOM (RedDOM), (d) preferential DOP remineralization (pref_DOP_remin), (e) preferential DOP recycling and phytoplankton DOP uptake (nonRedDOP), (f ) non-Redfield DOP with low DOM production (low_nonRedDOP), (g) non-Redfield DOP with high DOM production (high_nonRedDOP), and (h) fast recycling non-Redfield DOP (fast_nonRedDOP). Note that the zonally averaged model results in (b) are taken only from locations where observations exist. See pp. 973–993.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 4
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    The multiple fates of sinking particles in the North Atlantic Ocean (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here, we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle-attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle-attached microbial communities each spanned more than an order of magnitude. Rates of substrate-specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 d -1 . A comparison of these substrate-specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m -2 d -1 , on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth-integrated mesopelagic respiration and carbon inputs.
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  • 5
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    Climate extremes dominate seasonal and interannual variations in carbon export from the Mississippi River Basin (2015)
    Wiley
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    Publication Date: 2015-08-07
    Description: Knowledge about the annual and seasonal patterns of organic and inorganic carbon (C) exports from the major rivers of the world to the coastal ocean are essential for our understanding and potential management of the global C budget so as to limit anthropogenic modification of global climate. Unfortunately our predictive understanding of what controls the timing, magnitude and quality of carbon export is still rudimentary. Here we use a process-based coupled hydrologic/ecosystem biogeochemistry model (the Dynamic Land Ecosystem Model, DLEM) to examine how climate variability and extreme events, changing land use, and atmospheric chemistry have affected the annual and seasonal patterns of C exports from the Mississippi River basin to the Gulf of Mexico. Our process-based simulations estimate that the average annual exports of dissolved organic C (DOC), particulate organic C (POC), and dissolved inorganic C (DIC) in the 2000s was 2.6 ± 0.4 Tg C yr −1 , 3.4 ± 0.3 Tg C yr −1 and 18.8 ± 3.4 Tg C yr −1 , respectively. Although land-use change was the most important agent of change in C export over the past century, climate variability and extreme events (such as flooding and drought) were primarily responsible for seasonal and interannual variations in C export from the basin. The maximum seasonal export of DIC occurred in summer while for maximum DOC and POC occurred in winter. Relative to the 10-year average (2001–2010), our modeling analysis indicates that the years of maximal and minimal C export co-occurred with wet and dry years (2008: 32% above average and 2006: 32% below average). Given IPCC-predicted changes in climate variability and the severity of rain events and droughts of wet and dry years for the remainder of the 21 st Century, our modeling results suggest major changes in the riverine link between the terrestrial and oceanic realms, which are likely to have a major impact on carbon delivery to the coastal ocean.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 6
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    An observational assessment of the influence of mesoscale and submesoscale heterogeneity on ocean biogeochemical reactions (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: Numerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100 km -1 km). The causes and consequences of physical processes at these scales (‘eddy advection’) influencing biogeochemistry have received much attention. Less studied, the non-linear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed ‘eddy reactions’. Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modelling work has previously suggested that the neglected ‘eddy reactions’ may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates, and not taken as a justification for ignoring eddy reactions. Compared to modelling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions.
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  • 7
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    Seasonal variations, origin and fate of settling diatoms in the Southern Ocean tracked by silicon isotope records in deep sediment traps (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: The Southern Ocean plays a pivotal role in the control of atmospheric CO 2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace limiting nutrient and dissolved silicon (DSi) as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here, we use δ 30 Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ and Subantarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ 30 Si composition reveal a range of insights, including that i) the sinking rate of BSi exceeds 200 m d −1 in summer in the AZ, yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, ii) occasional vertical mixing events affect the δ 30 Si composition of exported BSi in both the SAZ and AZ, iii) the δ 30 Si signature of diatoms is well conserved through the water column despite strong BSi and POC attenuation at depth, and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.
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  • 8
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    Strong dependence of CO2 emissions from anthropogenic land cover change on initial land cover and soil carbon parametrization (2015)
    Wiley
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    Publication Date: 2015-08-13
    Description: The quantification of sources and sinks of carbon from land use and land cover changes (LULCC) is uncertain. We investigated how the parametrization of LULCC and of organic matter decomposition, as well as initial land cover affect the historical and future carbon fluxes in an Earth System Model (ESM). Using the land component of the Max-Planck-Institute ESM, we found that the historical (1750–2010) LULCC flux varied up to 25% depending on the fraction of biomass which enters the atmosphere directly due to burning or is used in short-lived products. We found an uncertainty in the decadal LULCC fluxes of the recent past due to the parametrization of decomposition and direct emissions of 0.6 Pg C yr −1 , which is three times larger than the un-certainty previously attributed to model and method in general. Pre-industrial natural land cover had a larger effect on decadal LULCC fluxes than the aforementioned parameter sensitivity (1.0 Pg C yr −1 ). Re-gional differences between reconstructed and dynamically-computed land cover, in particular at low-latitudes, led to differences in historical LULCC emissions of 84–114 Pg C, globally. This effect is larger than the effects of forest regrowth, shifting cultivation or climate feedbacks and comparable to the effect of differences among studies in the terminology of LULCC. In general, we find that the practice of calibrating the net land carbon balance to provide realistic boundary conditions for the climate component of an ESM hampers the applicability of the land component outside its primary field of application.
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  • 9
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    Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Summertime carbon, nitrogen and biogenic silica export was examined using 234 Th: 238 U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, PC and PN concentrations were more rapidly attenuated relative to 234 Th activities in small particles compared to large particles, converging to 1–5 µmol·dpm −1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/ 234 Th ratio, regional trends were consistent across measurement techniques. Highest C fixation rates were associated with diatom dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton and diazotroph dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234 Th fluxes into carbon or other elemental fluxes, including 〈50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.
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  • 10
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    Climatological distribution of aragonite saturation state in the global oceans (2015)
    Wiley
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    Publication Date: 2015-09-19
    Description: Aragonite saturation state (Ω arag ) in surface and subsurface waters of the global oceans was calculated from up-to-date (through the year of 2012) ocean station dissolved inorganic carbon (DIC) and total alkalinity (TA) data. Surface Ω arag in the open ocean was always supersaturated (Ω〉1), ranging between 1.1 and 4.2. It was above 2.0 (2.0-4.2) between 40°N and 40°S, but decreased towards higher latitude to below 1.5 in polar areas. The influences of water temperature on the TA/DIC ratio, combined with the temperature effects on inorganic carbon equilibrium and apparent solubility product ( K ’ sp ), explain the latitudinal differences in surface Ω arag . Vertically, Ω arag was highest in the surface mixed layer (SML). Higher hydrostatic pressure, lower water temperature, and more CO 2 buildup from biological activity in the absence of air-sea gas exchange helped maintain lower Ω arag in the deep ocean. Below the thermocline, aerobic decomposition of organic matter along the pathway of global thermohaline circulation played an important role in controlling Ω arag distributions. Seasonally, surface Ω arag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open-ocean waters of both hemispheres. Decadal changes of Ω arag in the Atlantic and Pacific Oceans showed that Ω arag in waters shallower than 100 m depth decreased by 0.10±0.09 (−0.40±0.37% yr −1 ) on average from the decade spanning 1989–1998 to the decade spanning 1998–2010.
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