Net Biological Production Research Articles

Carbon Outgassing in the Antarctic Circumpolar Current Is Supported by Ekman Transport From the Sea Ice Zone in an Observation‐Based Seasonal Mixed‐Layer Budget

AbstractDespite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high‐latitude Southern Ocean. This is due in part to a historical paucity of observations in this remote, turbulent, and seasonally ice‐covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly climatology of the mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different zones of the Southern Ocean—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon‐rich waters at the base of the mixed layer supplies DIC which is, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern Antarctic Circumpolar Current (ACC), which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind‐driven transport of DIC from the SIZ. These results stem from the first observation‐based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region.

Seasonal variation and controlling factors of carbon balance over dry semi-humid cropland in Guanzhong Plain

A theoretical basis for optimizing mitigation and adaptation to future climate change can be obtained by determining the carbon balance and their control mechanism in agroecosystems. To better understand summer maize (Zea mays L.)/winter wheat (Triticum aestivum L.) agroecosystems in dry semi-humid carbon balance mechanism, we conducted an in-depth analysis of three-year (2019.6–2022.6) seasonal dynamics of carbon fluxes and their driving factors in summer maize/winter wheat cropland in the Guanzhong Plain combined with path analysis. The results showed that net radiation (Rn) was the main direct controlling factor of the net ecosystem exchange (NEE) in the summer maize growing season, followed by leaf area index (LAI). However, LAI was the main factor controlling the seasonal variation of NEE in the winter wheat growing season, followed by Rn. Furthermore, LAI was the dominant direct driving factor for the gross primary productivity (GPP) in the summer maize and winter wheat growing seasons. In addition, LAI was the dominant driving factor for the total ecosystem respiration (TER) in the summer maize season, while Rn (indirect effect) and LAI (direct effect) were the main controlling factors of TER in the winter wheat season. The NEE values of the summer maize system, winter wheat system, and summer maize/winter wheat system in the study area were from 119 to 221, − 601 to − 595, and − 433 to − 291 g C m−2, respectively (the net biological production (NBP) were − 709 to − 437, 220–303, and − 496 and − 260 g C m−2, respectively). Notably, the average carbon emission rate of the summer maize/winter wheat agroecosystem in the Guanzhong Plain, considering the carbon output at harvest is 386.2 g C m−2year−1. Therefore, this study may guide assessment of carbon balance of agroecosystems in dry sub-humid regions, and provide a strong reference for achieving carbon neutrality in local agroecosystems.

A Pseudo‐Lagrangian Transformation to Study a Chlorophyll‐a Patch in the Ría de Vigo (NW Iberian Peninsula)

AbstractBecause of the difficulties in tracking a water mass over time and conducting synoptic surveys over large spatial scales, measurements of biological variables in the ocean are often restricted to isolated Eulerian observations. Here a proof‐of‐concept of a pseudo‐Lagrangian transformation was applied to a time series of chlorophyll‐a profiles acquired at a single fixed station to diagnose the shape of a phytoplankton patch, and its physical‐biological rates of transformation. During August 2013, a 27‐hr time series of observations of horizontal currents, hydrographic properties, turbulent mixing and chlorophyll‐a was acquired at a central station of the Ría de Vigo (NW Iberian Peninsula). A chlorophyll‐a patch, tilted upward toward the inner part of the Ría, was observed moving back and forth past the sampling station. Its shape and position during the sampling period were modulated by the semi‐diurnal (M2) tidal currents and the vertically sheared upwelling circulation. The pseudo‐Lagrangian transformation allowed estimation of chlorophyll‐a net rates of change. These rates were positive between 12 and 14 m depth, and negative elsewhere, with a mean value of −0.001 ± 0.449 days−1 within the patch maximum. After accounting for the effects of diffusion and sinking, the mean net biological production rate in the upper 15 m (0.53 ± 0.25 days−1) was comparable with in vitro estimates of the balance between phytoplankton growth and mortality obtained from dilution experiments carried out under similar conditions (0.2 ± 0.7 days−1). This pseudo‐Lagrangian transformation is complementary to traditional analyses for the quantification of ecological processes and biogeochemical budgets.

Controls on surface water carbonate chemistry along North American ocean margins

Syntheses of carbonate chemistry spatial patterns are important for predicting ocean acidification impacts, but are lacking in coastal oceans. Here, we show that along the North American Atlantic and Gulf coasts the meridional distributions of dissolved inorganic carbon (DIC) and carbonate mineral saturation state (Ω) are controlled by partial equilibrium with the atmosphere resulting in relatively low DIC and high Ω in warm southern waters and the opposite in cold northern waters. However, pH and the partial pressure of CO2 (pCO2) do not exhibit a simple spatial pattern and are controlled by local physical and net biological processes which impede equilibrium with the atmosphere. Along the Pacific coast, upwelling brings subsurface waters with low Ω and pH to the surface where net biological production works to raise their values. Different temperature sensitivities of carbonate properties and different timescales of influencing processes lead to contrasting property distributions within and among margins.

Dynamics of dissolved organic matter in headwaters: comparison of headwater streams with contrasting DOM and nutrient composition

Dissolved organic matter (DOM) in freshwaters is recognised as a significant and active component of the global carbon budget. DOM exported from terrestrial ecosystems may be compositionally and functionally altered by chemical and biological reactions as it is transported downstream. The processes affecting DOM in headwater streams remain uncertain but are potentially highly variable depending on DOM composition and nutrient availability as a function of soils, land-use, and human pressures. To investigate variability of DOM reactivity we took water samples from two contrasting headwater catchments, considered functional ‘end-members’ for DOM export: one rich in aromatic DOM and low in inorganic nutrients (peatland), and another (nearby) catchment characterized by less aromatic DOM and high nutrient loadings (agricultural grassland). Under controlled laboratory conditions, we evaluated the effects of light, presence/absence of aquatic biota and nutrient enrichment on short-term changes in DOM quantity and quality in these samples. For the peat stream, exposure to sunlight (with UV) resulted in net abiotic DOM removal, whereas in the agricultural stream it led to net biological DOM production. Nutrient addition accelerated DOM production in both streams. We conclude that in-stream changes in DOM quantity and quality represent the net effect of multiple consumption and production processes whose relative importance is strongly influenced by source-dependent DOM composition and environmental factors such as inorganic nutrient content and sunlight exposure. Our findings suggest that headwater streams may be more active processors of carbon and nutrients than presumed hitherto.

Air‐Sea Gas Transfer: Determining Bubble Fluxes With In Situ N2 Observations

AbstractOxygen measurements by in situ sensors on remote platforms are used to determine net biological oxygen fluxes in the surface ocean. On an annual basis these fluxes are stoichiometrically related to the export of organic carbon from the upper ocean (the ocean's biological carbon pump). In situ measurements on remote platforms make it feasible to observe the annual biological oxygen flux globally, but the accuracy of these estimates during periods of high winds depends on model‐determined fluxes by bubble processes created by breaking waves. We verify the importance of bubble processes in the gas exchange model of Liang et al. (2013, https://doi.org/10.1002gbc.20080) using surface‐ocean N2 gas measurements determined from observations of dissolved gas pressure and oxygen concentrations every 3 hr on a mooring in the northeast subarctic Pacific at Ocean Station Papa. The changes in N2 concentration during 10 separate monthlong periods in the winters between 2007 and 2016 indicate that bubble processes in the gas exchange model are over predicted by about a factor of 3 at this location. (The bubble mass transfer coefficients must be multiplied by 0.37 ± 0.14 to match the observations.) These results can be used to adjust model‐determined bubble fluxes to yield more accurate measurements of net biological oxygen production until the next generation gas‐exchange models are developed.

Significant Biologically Mediated CO2 Uptake in the Pacific Arctic During the Late Open Water Season

AbstractShifting baselines in the Arctic atmosphere‐sea ice‐ocean system have significant potential to alter biogeochemical cycling and ecosystem dynamics. In particular, the impact of increased open water duration on lower trophic level productivity and biological CO2 sequestration is poorly understood. Using high‐resolution observations of surface seawater dissolved O2/Ar and pCO2 collected in the Pacific Arctic in October 2011 and 2012, we evaluate spatial variability in biological metabolic status (autotrophy vs heterotrophy) as constrained by O2/Ar saturation (∆O2/Ar) as well as the relationship between net biological production and the sea‐air gradient of pCO2 (∆pCO2). We find a robust relationship between ∆pCO2 and ∆O2/Ar (correlation coefficient of −0.74 and −0.61 for 2011 and 2012, respectively), which suggests that biological production in the late open water season is an important determinant of the air‐sea CO2 gradient at a timeframe of maximal ocean uptake for CO2 in this region. Patchiness in biological production as indicated by ∆O2/Ar suggests spatially variable nutrient supply mechanisms supporting late season growth amidst a generally strongly stratified and nutrient‐limited condition.

Seasonal Dynamics of Dissolved Organic Carbon Under Complex Circulation Schemes on a Large Continental Shelf: The Northern South China Sea

AbstractWe examined the distribution and seasonality of dissolved organic carbon (DOC) based on a large data set collected from the northern South China Sea (NSCS) shelf under complex circulation schemes influenced by river plume, coastal upwelling, and downwelling. The highest surface values of ∼117 μmol L−1 were observed nearshore in summer suggesting high DOC supplies from the river inputs, whereas the lowest surface values of ∼62 μmol L−1 were on the outer shelf in winter due to entrainment of DOC‐poor subsurface water under strengthened vertical mixing. While the summer coastal upwelling brought lower DOC from offshore depth to the nearshore surface, the winter coastal downwelling delivered higher surface DOC to the midshelf deep waters from the inner shelf fueled by the China Coastal Current (CCC) transporting relatively high DOC from the East China Sea to the NSCS. The intensified winter downwelling generated a cross‐shelf DOC transport of 3.1 × 1012 g C over a large shelf area, which induced a significant depression of the NSCS DOC inventory in winter relative to in autumn. In addition to the variable physical controls, net biological production of DOC was semiquantified in both the river plume (2.8 ± 3.0 μmol L−1) and coastal upwelling (3.1 ± 1.3 μmol L−1) in summer. We demonstrated that the NSCS shelf had various origins of DOC including riverine inputs, inter‐shelf transport and in situ production. Via cross‐shelf transport, the accumulated DOC would be exported to and stored in the deep ocean, suggesting that continental shelves are a potentially effective carbon sink.

Time series pCO2 at a coastal mooring: Internal consistency, seasonal cycles, and interannual variability

Marine carbonate system monitoring programs often consist of multiple observational methods that include underway cruise data, moored autonomous time series, and discrete water bottle samples. Monitored parameters include all, or some of the following: partial pressure of CO2 of the water (pCO2w) and air, dissolved inorganic carbon (DIC), total alkalinity (TA), and pH. Any combination of at least two of the aforementioned parameters can be used to calculate the others. In this study at the Gray's Reef (GR) mooring in the South Atlantic Bight (SAB) we: examine the internal consistency of pCO2w from underway cruise, moored autonomous time series, and calculated from bottle samples (DIC-TA pairing); describe the seasonal to interannual pCO2w time series variability and air-sea flux (FCO2), as well as describe the potential sources of pCO2w variability; and determine the source/sink for atmospheric pCO2. Over the ~8.5 years of GR mooring time series, mooring-underway and mooring-bottle calculated-pCO2w strongly correlate with r-values > 0.90. pCO2w and FCO2 time series follow seasonal thermal patterns; however, seasonal non-thermal processes, such as terrestrial export, net biological production, and air-sea exchange also influence variability. The linear slope of time series pCO2w increases by 5.2 ± 1.4µatm y−1 with FCO2 increasing 51–70mmolm−2 y−1. The net FCO2 sign can switch interannually with the magnitude varying greatly. Non-thermal pCO2w is also increasing over the time series, likely indicating that terrestrial export and net biological processes drive the long term pCO2w increase.

Diatom bloom‐derived bottom water hypoxia off the Changjiang estuary, with and without typhoon influence

AbstractDuring the summers of 2009 and 2013, seawater pH and concentrations of dissolved oxygen, inorganic carbon, and nutrients were measured off the Changjiang estuary in the East China Sea. The 2009 cruise captured the effects of Typhoon Morakot; the 2013 cruise sampled more typical conditions (no typhoon). Data from both years indicate a close correlation between high primary productivity in surface waters and hypoxia in bottom waters. Based on these observations, we developed a conceptual model to guide an exploration of processes contributing to the formation of summertime bottom hypoxia. A mixing‐model analysis of the 2009 data identified a surface diatom bloom as the major (70–80%) source of the organic carbon that decomposed and ultimately led to bottom water hypoxia. Within the Changjiang River plume, depth‐integrated net biological production in the water column was 1.8 g C m−2 d−1, indicating strong autotrophic production, which in turn led to a high respiration rate of 1.2 g C m−2 d−1 in the bottom water. During both cruises, strong surface‐to‐bottom physical and metabolic coupling was evident. In 2009, storm‐driven inputs of nutrients from elevated river discharge and strong vertical mixing helped to fuel the rapid development of a surface diatom bloom. Afterwards, stratified conditions re‐established, newly formed labile organic matter sank, and bottom water oxygen was quickly consumed to an extent that hypoxia and acidification developed. To our knowledge, the observed rate of hypoxia and acidification development (within 6 d) is the fastest yet reported for the Changjiang River plume.

Cyanobacterial Diazotrophy and Earth's Delayed Oxygenation.

The redox landscape of Earth’s ocean-atmosphere system has changed dramatically throughout Earth history. Although Earth’s protracted oxygenation is undoubtedly the consequence of cyanobacterial oxygenic photosynthesis, the relationship between biological O2 production and Earth’s redox evolution remains poorly understood. Existing models for Earth’s oxygenation cannot adequately explain the nearly 2.5 billion years delay between the origin of oxygenic photosynthesis and the oxygenation of the deep ocean, in large part owing to major deficiencies in our understanding of the coevolution of O2 and Earth’s key biogeochemical cycles (e.g., the N cycle). For example, although possible links between O2 and N scarcity have been previously explored, the consequences of N2 limitation for net biological O2 production have not been examined thoroughly. Here, we revisit the prevailing view that N2 fixation has always been able to keep pace with P supply and discuss the possibility that bioavailable N, rather than P, limited export production for extended periods of Earth’s history. Based on the observation that diazotrophy occurs at the expense of oxygenesis in the modern ocean, we suggest that an N-limited biosphere may be inherently less oxygenic than a P-limited biosphere—and that cyanobacterial diazotrophy was a primary control on the timing and tempo of Earth’s oxygenation by modulating net biogenic O2 fluxes. We further hypothesize that negative feedbacks inhibit the transition between N and P limitation, with the implication that the pervasive accumulation of O2 in Earth’s ocean-atmosphere system may not have been an inevitable consequence of oxygenic photosynthesis by marine cyanobacteria.

A high-resolution time-depth view of dimethylsulphide cycling in the surface sea.

Emission of the trace gas dimethylsulphide (DMS) from the ocean influences the chemical and optical properties of the atmosphere, and the olfactory landscape for foraging marine birds, turtles and mammals. DMS concentration has been seen to vary across seasons and latitudes with plankton taxonomy and activity, and following the seascape of ocean’s physics. However, whether and how does it vary at the time scales of meteorology and day-night cycles is largely unknown. Here we used high-resolution measurements over time and depth within coherent water patches in the open sea to show that DMS concentration responded rapidly but resiliently to mesoscale meteorological perturbation. Further, it varied over diel cycles in conjunction with rhythmic photobiological indicators in phytoplankton. Combining data and modelling, we show that sunlight switches and tunes the balance between net biological production and abiotic losses. This is an outstanding example of how biological diel rhythms affect biogeochemical processes.

Winds induce CO2 exchange with the atmosphere and vadose zone transport in a karstic ecosystem

AbstractResearch on the subterranean CO2 dynamics has focused individually on either surface soils or bedrock cavities, neglecting the interaction of both systems as a whole. In this regard, the vadose zone contains CO2‐enriched air (ca. 5% by volume) in the first meters, and its exchange with the atmosphere can represent from 10 to 90% of total ecosystem CO2 emissions. Despite its importance, to date still lacking are reliable and robust databases of vadose zone CO2 contents that would improve knowledge of seasonal‐annual aboveground‐belowground CO2 balances. Here we study 2.5 years of vadose zone CO2 dynamics in a semiarid ecosystem. The experimental design includes an integrative approach to continuously measure CO2 in vertical and horizontal soil profiles, following gradients from surface to deep horizons and from areas of net biological CO2 production (under plants) to areas of lowest CO2 production (bare soil), as well as a bedrock borehole representing karst cavities and ecosystem‐scale exchanges. We found that CO2 followed similar seasonal patterns for the different layers, with the maximum seasonal values of CO2 delayed with depth (deeper more delayed). However, the behavior of CO2 transport differed markedly among layers. Advective transport driven by wind induced CO2 emission both in surface soil and bedrock, but with negligible effect on subsurface soil, which appears to act as a buffer impeding rapid CO2 exchanges. Our study provides the first evidence of enrichment of CO2 under plant, hypothesizing that CO2‐rich air could come from root zone or by transport from deepest layers through cracks and fissures.

Marine biological production from in situ oxygen measurements on a profiling float in the subarctic Pacific Ocean

AbstractEvaluating the organic carbon flux from the surface ocean to the interior (the marine biological pump) is essential for predictions of ocean carbon cycle feedback to climate change. One approach for determining these fluxes is to measure the concentration of oxygen in the upper ocean over a seasonal cycle, calculate the net O2 flux using an upper ocean model, and then use a stoichiometric relationship between oxygen evolved and organic carbon produced. Applying this tracer in a variety of ocean areas over seasonal cycles requires accurate O2 measurements on autonomous vehicles. Here we demonstrate this approach using an O2 sensor on a profiling float that is periodically calibrated against atmospheric pO2. Using accurate data and a model that includes all physical and biological processes influencing oxygen, we determine an annual net community production of 0.7 ± 0.5 mol C m−2 yr−1 in the northeast Pacific Ocean (50°N, 145°W) from June 2012 to June 2013. There is a strong seasonal cycle in net biological oxygen production with wintertime fluxes caused by bubble processes critical to determining the annual flux. Approximately 50% of net autotrophic production during summer months is consumed by net respiration during the winter. The result is a biological pump in the subarctic Pacific Ocean that is less than that determined by similar methods in the subtropics to the south. This estimate is significantly lower than that predicted by satellite remote sensing and global circulation models.

Removal of dissolved inorganic carbon in the Yellow River Estuary

The Yellow River of China runs mainly through an arid and semiarid midlatitude region that has experienced substantial anthropogenic and climatic change. This area includes the carbonate‐rich Loess Plateau and carries water of exceptionally high carbonate content. To investigate the processes by which dissolved inorganic carbon (DIC) is biogeochemically modified as the river approaches the sea, a multipronged field investigation was conducted in the Yellow River estuary, 2005–2009. The project included four research cruises (spring and fall), a year of monthly sampling at a lower‐river hydrological station (Lijin), and in situ bottle incubations. Our study revealed that 4–11% of the Yellow River DIC was removed from the water column in the estuarine mixing zone and thus was not transported to the sea. DIC removal was greater in the spring and occurred at a higher salinity range than in the fall. As a unique feature of the Yellow River estuary, calcium carbonate (CaCO3) precipitation was nearly as important as net biological production in the DIC removal. Longer freshwater—seawater mixing distances (and times) and higher DIC concentrations in the freshwater end member also promoted net biological production and CaCO3 precipitation, thus encouraging DIC removal.

Seasonal variability of aragonite saturation state in the Western Pacific

The oceanic uptake of atmospheric carbon dioxide (CO2) since pre-industrial times has increased acidity levels, resulting in a decrease in the pH and aragonite saturation state (Ωar) of surface waters. Aragonite is the predominant biogenic form of calcium carbonate precipitated by calcifying organisms in tropical reef ecosystems. Values of Ωar are often used as a proxy for estimating calcification rates in corals and other calcifying species. We quantify the regional and seasonal variability of Ωar and its main drivers for the Pacific island region (120°E:140°W, 35°S:30°N). The calculation of Ωar uses a seasonal climatology of the surface-water partial pressure of CO2, combined with values of total alkalinity (TA) estimated from a relationship between surface water salinity and total alkalinity that is derived from measurements in the region. This relationship is valid for all phases of El Niño/Southern Oscillation and for mixed layer waters with less than 15μmolkg−1 dissolved nitrate.The influence of seasonal changes in sea surface temperature (SST) on Ωar is small except in the subtropical waters on the northern and southern boundaries of the study region. Here, SST seasonal variability is large (>5°C), causing a change in Ωar of greater than 0.1. Seasonal changes in mixed layer depth and net evaporation (precipitation) do influence the seasonality in TCO2 and TA. However, these processes tend to increase (decrease) the TCO2 (TA) in unison, resulting in a small net effect on seasonal change in Ωar for most of the region. Net biological production and sea–air gas exchange were also found to have only a small impact on the seasonal change in Ωar through the region. Changes in Ωar of between 0.1 and 0.2 occurred where variations in wind-driven upwelling in the Central Equatorial Pacific and the transport of Eastern Pacific waters into the South Equatorial Current region caused a change in the TCO2/TA ratio of the surface waters. In contrast to these two regions, the combined effect of biological and physical processes in the West Pacific Warm Pool and North Equatorial Counter Current subregions resulted in Ωar variability of less than 0.1.

The relative contributions of biological and abiotic processes to carbon dynamics in subarctic sea ice

Knowledge on the relative effects of biological activity and precipitation/dissolution of calcium carbonate (CaCO3) in influencing the air-ice CO2 exchange in sea-ice- covered season is currently lacking. Furthermore, the spatial and temporal occurrence of CaCO3 and other biogeochem- ical parameters in sea ice are still not well described. Here we investigated autotrophic and heterotrophic activity as well as the precipitation/dissolution of CaCO3 in subarctic sea ice in South West Greenland. Integrated over the entire ice season (71 days), the sea ice was net autotrophic with a net carbon fixation of 56 mg C m -2 , derived from a sea-ice- related gross primary production of 153 mg C m -2 and a bacterial carbon demand of 97 mg C m -2 . Primary pro- duction contributed only marginally to the TCO2 depletion of the sea ice (7-25 %), which was mainly controlled by physical export by brine drainage and CaCO3 precipitation. The net biological production could only explain 4 % of this sea-ice-driven CO2 uptake. Abiotic processes contributed to an air-sea CO2 uptake of 1.5 mmol m -2 sea ice day -1 , and dissolution of CaCO3 increased the air-sea CO2 uptake by 36 % compared to a theoretical estimate of melting CaCO3- free sea ice. There was a considerable spatial and temporal variability of CaCO3 and the other biogeochemical param- eters measured (dissolved organic and inorganic nutrients).

A method for in-situ calibration of Aanderaa oxygen sensors on surface moorings

Oxygen-based mass balance estimates of net biological production can be calculated using accurate in-situ O2 measurements. All presently available oxygen sensors drift if deployed for periods of months in the sun-lit surface ocean and must be calibrated to maintain accuracy. We have developed an in-situ calibration system in which Aanderaa optode oxygen sensors periodically determine the pO2 of air. In the moored application described here, the optode is housed in a subsurface chamber on a surface mooring where it alternately measures the pO2 of the water and air. During calibration intervals, atmospheric air is pumped into the optode housing, displacing the water. Subsequent oxygen measurements of the air inside the housing provide a measurement of the atmospheric pO2, which can then be compared to expected atmospheric pO2 based on measured pressure and temperature. We present calibrations for one optode in both water (against Winkler titrations) and air (against atmospheric oxygen). The optode response in lab experiments was different in air than in water, measuring low and exhibiting temperature dependence (−1% at 20°C and −2.3% at 3°C). Initial field tests in a freshwater inlet confirmed this offset and showed a drift of −0.5% over 3months of deployments.

Dissolved organic nitrogen dynamics in the Arctic Ocean

Little is known about the distribution and dynamics of dissolved organic nitrogen (DON) within Arctic Ocean surface waters, though seasonal inputs from both rivers and marine phytoplankton production are likely important. Here we combine multiple datasets to provide the first quasi-synoptic view of DON concentrations in the summertime Arctic Ocean Polar Surface Layer (PSL) and western Arctic shelves, and infer controlling processes. Distributions indicate inputs of DON by Arctic rivers followed by net consumption of up to 70% of this terrigenous material across the resulting salinity gradients over time scales of years. Coupling of DON losses to PSL residence times indicates first order decay constants (λ) of 0.15±0.07yr−1 and 0.08±0.01yr−1 for terrigenous DON in the eastern and western Arctic systems, respectively. Riverine delivery of both inorganic and organic nitrogen have only a minor (<15%) impact on Arctic shelf export production. In the Pacific Ocean-influenced Chukchi Sea, net biological production of ~2μM DON, representing ~8% of net community production, is observed in surface waters following seasonal sea-ice retreat. These findings highlight the contrasting roles and locations of terrigenous versus marine derived DON in the cycling of nitrogen in the surface Arctic Ocean.

Mass, nutrient and oxygen budgets for the northeastern Atlantic Ocean

Abstract. The northeast Atlantic is a key horizontal and vertical crossroads region for the meridional overturning circulation, but basic nutrient and oxygen fluxes are still poorly constrained by observations in the region. A surface to bottom northeast Atlantic Ocean budget for mass, nutrients (nitrate and phosphate) and oxygen is determined using an optimization method based on three surveys of the OVIDE transect (from Greenland to Portugal) completed with the World Ocean Atlas 2009. Budgets are derived for two communicating boxes representing the northeastern European basin (NEEB) and the Irminger Sea. For the NEEB (Irminger) box, it is found that 30% of the mass import (export) across the OVIDE section reach (originate from) the Nordic Seas, while 70% are redistributed between both boxes through the Reykjanes Ridge (9.3 ± 0.7 × 109 kg s−1). Net biological source/sink terms of nitrate point to both the Irminger and NEEB boxes as net organic matter production sites (consuming nitrate at a rate of –7.8 ± 6.5 kmol s−1 and –8.4 ± 6.6 kmol s−1, respectively). Using a standard Redfield ratio of C : N = 106 : 16, nitrate consumption rates indicate that about 40 TgC yr−1 of carbon is fixed by organic matter production between the OVIDE transect and the Greenland–Scotland Ridge. Nutrient fluxes also induce a net biological production of oxygen of 73 ± 60 kmol s−1 and 79 ± 62 kmol s−1 in the Irminger and NEEB boxes, which points to the region as being autotrophic. The abiotic air–sea oxygen flux leads to an oceanic oxygen uptake in the two regions (264 ± 66 kmol s−1 in the north and 443 ± 70 kmol s−1 in the south). The abiotic flux is partitioned into a mixing and a thermal component. It is found that the Irminger Sea oceanic oxygen uptake is driven by an air–sea heat flux cooling increasing the ocean surface oxygen solubility. Over the northeastern European basin the mixing component is about half the thermal flux, presumably because of the oxygen minimum in the subtropical thermocline.