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This study traces dissolved organic matter (DOM) in different water masses of the Arctic Ocean and its effect on the distributions of trace elements (TEs; Fe, Cu, Mn, Ni, Zn, Cd) using fluorescent properties of DOM and the terrigenous biomarker lignin. The Nansen, Amundsen, and Makarov Basins were characterized by the influence of Atlantic water and the fluvial discharge of the Siberian Rivers with high concentrations of terrigenous DOM (tDOM). The Canada Basin and the Chukchi Sea were characterized by Pacific water, modified through contact with productive shelf sediments with elevated levels of marine DOM. Within the surface layer of the Beaufort Gyre, meteoric water (river water and precipitation) was characterized by low concentrations of lignin and tDOM fluorescence proxies as DOM is removed during freezing. High‐resolution in situ fluorescence profiles revealed that DOM distribution closely followed isopycnals, indicating the strong influence of sea‐ice formation and melt, which was also reflected in strong correlations between DOM fluorescence and brine contributions. The relationship of DOM and hydrography to TEs showed that terrigenous and marine DOM were likely carriers of dissolved Fe, Ni, Cu from the Eurasian shelves into the central Arctic Ocean. Chukchi shelf sediments were important sources of dCd, dZn, and dNi, as well as marine ligands that bind and carry these TEs offshore within the upper halocline in the Canada Basin. Our data suggest that tDOM components represent stronger ligands relative to marine DOM components, potentially facilitating the long‐range transport of TE to the North Atlantic. Plain Language Summary: The Arctic Ocean receives a disproportionate large amount of global river discharge and has limited but well‐constrained exchanges with other oceans. This makes the Arctic Ocean unique in terms of dissolved organic matter (DOM) and trace element (TE) sources and distribution. We used data collected during two expeditions spanning the entire Arctic Ocean to characterize the unique distribution of DOM and to study its potential as a water mass tracer and its role in the transport of TE. While the Atlantic‐dominated Nansen Basin was characterized by low levels of the DOM and TE, the central Arctic was dominated by the Transpolar Drift, a current that connects the Eurasian shelves to the Fram Strait and transports DOM from the Siberian Rivers toward the North Atlantic. In contrast, the Chukchi shelf‐Canada Basin region was characterized by the dominance of Pacific water that is enriched by marine DOM from the shallow and productive Chukchi shelf. The distribution of DOM from these different sources was affected by freezing and thawing processes and, therefore, can be used to study water mass transformations and pathways in the Arctic Ocean. Key Points: Dissolved Organic Matter (DOM) distribution in the Arctic Ocean is largely controlled by sea ice formation and melt processesDOM distribution in the Arctic Ocean reveals its potential as a tracer for halocline formation and freshwater source assignmentsTerrigenous and marine DOM are carriers of trace elements from shelves to the open Arctic Ocean, but terrigenous DOM represent stronger ligands [ABSTRACT FROM AUTHOR]

The ocean contains a large reservoir of dissolved organic matter (DOM) that persists for millennia. Both the very dilute concentrations of individual DOM molecules and intrinsic recalcitrance to microbial decay imparted by molecular structure are suggested mechanisms for this long residence time. Here, we report an experiment comparing the responses of surface and deep prokaryotes to DOM isolated and enriched by solid‐phase extraction from surface and deep waters of the subtropical North Atlantic Ocean. Extracts from both depths were qualitatively characterized as biologically recalcitrant given their similarly high C : N ratios of 26. Surface prokaryotes measurably drew down extracted dissolved organic carbon (DOC) concentrations, but the drawdown was only 4% of the initial enriched DOC concentration regardless of enrichment level or depth. Deep microbes, in contrast, did not cause observable changes in DOC concentrations. Surface and deep prokaryotes had similar temperature‐normalized growth responses to extracts from each depth. Biological indicators (e.g., kinetics) suggest that prokaryotes were less efficient at catalyzing surface than deep DOM (catalytic efficiencies of 0.003–0.005 vs. 0.02–0.03 h−1, respectively). These values indicate qualitative differences in extracted DOM from the two depths, perhaps suggesting a variable nature of the refractory DOC depending on depth. Moreover, only a small portion of the extracted DOM was biologically utilizable, regardless of concentration factor or depth, and essentially only a small fraction of it was incorporated into biomass. Microbial selection against substrates that meet modest energy but no growth demands may be a factor contributing to the long‐term stability of marine DOM. [ABSTRACT FROM AUTHOR]

Between 2013 and 2016, a series of warm events induced by ocean atmosphere oscillations negatively impacted productivity in the northeast Pacific Ocean. For two consecutive winters (2013–2014 and 2014–2015), suppressed wind stress and warm near‐surface ocean temperature anomalies restricted vertical mixing between the surface and underlying nutrient‐enriched waters. Here we assess historical data of sea surface temperature and sea level pressure, along with nearly a decade of biogeochemical float data to evaluate the impact of these warm events on organic carbon production. The first stratified winter experienced little apparent impact on the magnitude of net organic carbon production in the growing season relative to prior years, suggesting an immediate resilience from reduced new nutrients, apparently depending on recycled iron. However, the subsequent winter experienced virtually zero net production; a loss of resilience, perhaps due to net iron removal with export, was evident. We find that consistently enhanced winter stratification decreased carbon production much more so than a single warm winter. This study highlights the sensitivity of marine productivity to ocean atmosphere oscillations, reducing deep ocean carbon sequestration with prolonged ocean warming and stratification. Key Points: Warm events impacted the North Pacific Ocean and increased the upper ocean's stratification, preventing nutrient renewalNet organic carbon production drastically decreased in the second warm year due to a loss in the ecosystem's resilienceA more stratified ocean decreases the prospects for organic carbon production and subsequent export to the deep ocean [ABSTRACT FROM AUTHOR]

Most dissolved organic carbon (DOC) sequestered in the deep ocean has residence times of decades to thousands of years, with clear implications for climate regulation, though some net removal is typically observed with increasing water mass age. Here, a high‐quality‐high‐resolution data set has allowed us to identify net additions of recalcitrant DOC in specific water masses of the deep South Atlantic. Overall, the South Atlantic is a net source of recalcitrant DOC, adding 0.027 ± 0.019 Pg C/year, while the North Atlantic is a net sink that removes 0.298 ± 0.141 Pg C/year. We find that the balance of addition/removal of recalcitrant DOC depends not only on the origin but also on the temperature, age, and depth of the water masses that circulate and mix in the Atlantic Ocean. Future changes in the water mass composition and circulation patterns due to climate change would eventually affect that balance, altering the carbon cycle. Key Points: Identification of net additions of recalcitrant DOC into the deep AtlanticThe South Atlantic is a net source of recalcitrant DOC, while the North Atlantic is a net sink [ABSTRACT FROM AUTHOR]

From seasonal cruises in the NE Pacific Ocean during 2017, we (1) determined dissolved organic carbon concentrations; (2) calculated net community production (NCP) from nitrate drawdown; and (3) established relationships between NCP and seasonal dissolved organic carbon (DOC) accumulation in the upper 75 m. The fraction of NCP that accumulated as DOC, hereafter referred to as the net dissolved production ratio, was calculated for several stations during spring and summer. The net dissolved production ratio was about 0.26 at the oceanic station Ocean Station Papa during different seasons and years. Using nitrate concentration profiles obtained from Bio‐Argo floats during 2009–2018 operating near Ocean Station Papa, we calculated NCP at high temporal resolution and then applied the 0.26 constant in order to (4) estimate DOC variability for the 9‐year period. We found strong seasonality near Ocean Station Papa, with NCP maxima during summers ranging from 0.3 to 2.9 mol C/m2 and surface DOC concentrations estimated from 56 μmol/kg in winters to 73 μmol/kg in summers. There was a 10‐fold interannual variability in the seasonally accumulated inventory of DOC, ranging from 0.078 to 0.75 mol C/m2. This study reinforces the value of deploying floats equipped with chemical sensors in order to better understand marine biogeochemical cycles, especially when high resolution data cannot be obtained otherwise. Given that ~26% of NCP accumulates as DOC in the central Gulf of Alaska, the remaining balance of ~74% is available for export as sinking biogenic particles. Key Points: Dissolved organic carbon accumulates seasonally along the Line P section with maximum concentrations during late summerApproximately 26% of net community production at Ocean Station Papa accumulates as dissolved organic carbonBio‐Argo floats equipped with nitrate sensors are used to estimate variability in dissolved organic carbon concentrations [ABSTRACT FROM AUTHOR]

Abstract: To obtain qualitative information on the Black Sea's dissolved organic matter (DOM) pool, the optical properties (absorption and fluorescence) were measured in 111 samples collected across the basin. Chromophoric dissolved organic matter (CDOM) was considered at three wavelengths (254 nm, 280 nm, and 325 nm), along with the spectral slope between 275 nm and 295 nm (S275–295) and the specific ultraviolet absorbance at 254 nm (SUVA254). Using parallel factor analysis, a five‐component model identified three humic‐like components, a protein‐like component, and a polycyclic aromatic hydrocarbon‐like component. In the basin's oxic layer (upper ∼ 100 m), protein‐like CDOM was elevated, likely due to the production of this labile material, while humic‐like material was low, suggesting its removal by photo‐oxidation. In the underlying waters, the protein‐like material decreased, perhaps due to the utilization of this nitrogen‐containing DOM, while humic‐like material increased, suggesting its production at depth. In the anoxic layer (lower ∼ 2000 m), dissolved organic carbon (DOC) varied by only ∼ 10% while CDOM increased with depth by a factor of approximately two; the optical properties correlated well with the H2S‐equivalence of mineralization, referred to here as apparent carbon mineralization (ACM), while DOC did not. The strong correlation between CDOM and ACM is similar to correlations previously identified in the open ocean that compared CDOM with apparent oxygen utilization, suggesting that CDOM accumulates as a function of mineralization, independently of the oxidizing agent (i.e., oxygen or sulfate). [ABSTRACT FROM AUTHOR]

Taurine (Tau), an amino acid-like compound, is present in almost all marine metazoans including crustacean zooplankton. It plays an important physiological role in these organisms and is released into the ambient water throughout their life cycle. However, limited information is available on the release rates by marine organisms, the concentrations and turnover of Tau in the ocean. We determined dissolved free Tau concentrations throughout the water column and its release by abundant crustacean mesozooplankton at two open ocean sites (Gulf of Alaska and North Atlantic). At both locations, the concentrations of dissolved free Tau were in the low nM range (up to 15.7 nM) in epipelagic waters, declining sharply in the mesopelagic to about 0.2 nM and remaining fairly stable throughout the bathypelagic waters. Pacific amphipod-copepod assemblages exhibited lower dissolved free Tau release rates per unit biomass (0.8 ± 0.4 μmol g−1 C-biomass h−1) than Atlantic copepods (ranging between 1.3 ± 0.4 μmol g−1 C-biomass h−1 and 9.5 ± 2.1 μmol g−1 C-biomass h−1), in agreement with the well-documented inverse relationship between biomass-normalized excretion rates and body size. Our results indicate that crustacean zooplankton might contribute significantly to the dissolved organic matter flux in marine ecosystems via dissolved free Tau release. Based on the release rates and assuming steady state dissolved free Tau concentrations, turnover times of dissolved free Tau range from 0.05 d to 2.3 d in the upper water column and are therefore similar to those of dissolved free amino acids. This rapid turnover indicates that dissolved free Tau is efficiently consumed in oceanic waters, most likely by heterotrophic bacteria. [ABSTRACT FROM AUTHOR]

Antarctica's continental shelves generate the densest waters in the world and are responsible for the formation of Antarctic Bottom Water (AABW), a water mass with the potential to sequester carbon in the deep ocean for millennia. One such form of marine carbon is dissolved organic carbon (DOC), the ocean's largest standing stock of reduced carbon. In this study, we quantify DOC enrichment in dense shelf waters (DSW) in the Ross Sea and assess the potential for DOC to be sequestered from Antarctic shelves into AABW. We find that Ross Sea DSW is enriched in DOC by ∼7 μmol kg−1 relative to the incoming source waters (initial conditions), which is primarily caused by vertical mixing. The total DOC excess in DSW suggests that 4.0 ± 0.6 Tg DOC yr−1 is exported off the shelf. However, this exported fraction does not appear to persist in newly formed AABW and is likely remineralized, sequestering this carbon as TCO2 in the deep ocean. [ABSTRACT FROM AUTHOR]

The phytoplankton bloom in the Ross Sea is the largest in spatial extent and one of the most productive in Antarctica, yet the fate of the summer bloom remains poorly understood. Here we present carbon system data from the first biogeochemical process cruise to be conducted in both the western and central Ross Sea during late summer (February-March 2013). Using one-dimensional carbon budgets, we found evidence for substantial positive net community production (425 ± 204 mmol C m−2 d−1) during the late summer in Terra Nova Bay (TNB) of the western Ross Sea, which was rapidly exported to below 200 m. In addition, seasonally integrated carbon export was higher in diatom-dominated TNB (7.3 ± 0.9 mol C m−2) compared to the Phaeocystis antarctica-dominated central Ross Sea (3.4 ± 0.8 mol C m−2). Substantial late summer productivity and export may be a widespread phenomenon in Antarctic coastal regions that is not accounted for in regional carbon models. [ABSTRACT FROM AUTHOR]

Marine dissolved organic matter (DOM) contains one of the largest exchangeable organic carbon pools on Earth. Riverine input represents an important source of DOM to the oceans, yet much remains to be learned about the fate of the DOM linking terrestrial to oceanic carbon cycles through rivers at the global scale. Here we use ultrahigh-resolution mass spectrometry to identify 184 molecular formulae that are indicators of riverine inputs (referred to as t-Peaks) and to track their distribution in the deep North Atlantic and North Pacific Oceans. The t-Peaks were found to be enriched in the Amazon River, to be highly correlated with known tracers of terrigenous input, and to be observed in all samples from four different rivers characterized by vastly different landscapes and vegetation coverage spanning equatorial (Amazon and Congo), subtropical (Altamaha), and Arctic (Kolyma) regions. Their distribution reveals that terrigenous organic matter is injected into the deep ocean by the global meridional overturning circulation, indicating that a fraction of the terrigenous DOM introduced by rivers contributes to the DOM pool observed in the deep ocean and to the storage of terrigenous organic carbon. This novel molecular approach can be used to further constrain the transfer of DOM from land to sea, especially considering that Fourier transform ion cyclotron resonance mass spectrometer analysis is becoming increasingly frequent in studies characterizing the molecular composition of DOM in lakes, rivers, and the ocean. [ABSTRACT FROM AUTHOR]

The global ocean contains a massive reservoir (662 ± 32 Pg C) of dissolved organic carbon (DOC), and its dynamics, particularly in the deepest zones, are only slowly being understood. DOC in the deep ocean is ubiquitously low in concentration (~35 to 48 µmol kg−1) and aged (4000 to 6000 years), persisting for multiple meridional overturning circulations. Deep waters relatively enriched in DOC form in the North Atlantic, migrate to the Southern Ocean to mix with waters from Antarctic shelves and the deep Pacific and Indian Oceans, in turn forming the voluminous waters of the Circumpolar Deep Water. Here we seek evidence for local (autochthonous) versus distant (allochthonous) processes in determining the distribution of DOC in the deep Southern Ocean. Prior analyses on DOC in the deep Southern Ocean have conflicted, describing both conservative and nonconservative traits: the deep DOC field has been reported as uniform in distribution, yet local inputs have been suggested as quantitatively important. We use multiple approaches (multiple linear regression, mass transport, and mass balance calculations) with data from Climate Variability and Predictability Repeat Hydrography sections to evaluate the system. We find that DOC concentrations in the deep Southern Ocean largely reflect the conservative mixing of the several deep waters entering the system from the north. Mass balance suggests that the relatively depleted DOC radiocarbon content in the deep Southern Ocean is a conserved property as well. These analyses advance our understanding of the controls on the DOC reservoir of the Southern Ocean. [ABSTRACT FROM AUTHOR]

The global ocean holds one of Earth's major carbon reservoirs as dissolved organic matter (662 ± 32 PgC). Most of this material (>95%) is termed refractory dissolved organic carbon (RDOC) as Williams and Druffel (1987) found it to be old relative to the circulation time of the ocean. While RDOC within the modern ocean is thus perceived as vast and only slowly renewed, its mobilization has been implicated by Sexton et al. (2011) to explain Earth's transient warming events (i.e., hyperthermals) of the Paleocene and Eocene epochs (65-34 million years ago). Assessing this proposed function of RDOC as a rapidly (~5-10 kyr) exchangeable carbon reservoir is presently limited by insufficient knowledge of the responsible processes. Here we investigate the dynamics of RDOC in the deep Pacific Ocean, previously characterized by concentration gradients thought to be established by slow but systematic RDOC removal with circulation and aging of the water masses. We demonstrate that RDOC is instead conserved during much of its circulation, but that there exist localized sinks in the deep, far North Pacific and at mid depth in the subtropical South Pacific. Water mass mixing into these sink regions creates the observed RDOC gradients. Together, the Pacific sinks remove 7-29% of the 43 Tg RDOC added to the deep global ocean each year with overturning circulation, and point to an important but still unidentified control on the RDOC inventory of deep marine systems. [ABSTRACT FROM AUTHOR]

The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean's oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphoric zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphoric zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphoric zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface-accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphoric zone. [ABSTRACT FROM AUTHOR]

Marine dissolved organic matter is a massive reservoir of carbon holding >200x the ocean biomass inventory. Primarily produced at the ocean surface and then exported to depth with overturn of the water column, this carbon can be sequestered in the ocean interior for centuries. Understanding the loss of dissolved organic carbon (DOC) upon export has been data limited, but recent global ocean surveys are overcoming that problem. Here we characterize three fractions of exported carbon by apparent continuity in removal rates: semi-labile and semi-refractory, summing to 20 PgC, and the balance as refractory DOC. Distinct lifetimes coupled with ocean circulation control where the fractions are exported to depth, and thus the carbon sequestration time scales. Maximum remineralization rates of exported DOC occur in the convergent subtropical gyres, where a range of ∼500 to <1500 mmol C m-2 yr-1 can exceed remineralization of sinking biogenic particles. Regions of high particle export production and highly stratified systems exhibit minimal exported DOC remineralization. [ABSTRACT FROM AUTHOR]

The Gulf Stream provides a 'nutrient Stream,' an advective flux of nutrients carried in sub-surface waters, redistributing nutrients from the tropics to the mid latitudes. There is a dramatic downstream strengthening in the full depth, volume and nitrate transport diagnosed from synoptic measurements along three sections: 32 Sv and 300 kmol s-1 at 27°N, increasing to 66.7 Sv and 940 kmol s-1 at 35.5°N, and 149.5 Sv and 2100 kmol s-1 at 36.5°N; the transport estimates have uncertainties reaching 10% of their values. The transport-weighted nitrate concentration carried by the Gulf Stream generally decreases downstream in light layers (σϑ< 26.8), but increases in denser layers between 35.5°N and 36.5°N. The fraction of nutrients from regeneration within the upper thermocline slightly decreases downstream between 35.5°N and 36.5°N. Hence, the downstream variations in the nitrate concentrations carried by the Gulf Stream are probably due to lateral exchange along isopycnals, rather than diapycnal transfer or biological consumption and regeneration. The Gulf Stream also transports dissolved organic nitrogen (DON) northward within lighter upper waters, providing up to 26% of the total nitrogen transport. An accompanying model study reveals coherent nitrate and DON streams, including significant lateral exchange, running from the tropics along the western boundary of the subtropical gyre and following the separated jet along the inter-gyre boundary. The combined nitrate and DON transport along light surfaces (σϑ < 26.8) remains within the subtropical gyre, while the larger nitrate transport along denser surfaces passes into the subpolar gyre, sustaining high-latitude productivity. [ABSTRACT FROM AUTHOR]

Recently there has been discussion about the metabolic state of the ocean, with arguments questioning whether the open ocean is net autotrophic or net heterotrophic. Accurately determining the metabolic balance of a marine system depends on fully defining the system being evaluated and on quantifying the inputs and outputs to that system. Here, a net northward transport of dissolved organic carbon (DOC) (across 24.5°N) of 3.3 ± 1.9 Tmol C yr-1 was determined using basin-wide transport estimates of DOC. This flux, coupled with DOC inputs from the Arctic Ocean (2.2 ± 0.8 Tmol C yr-1), the atmosphere (0.6 ± 0.08 Tmol C yr-1), and rivers (3.1 ± 0.6 Tmol C yr-1), indicates net heterotrophy in the North Atlantic (full depth, 24.5-72°N) of 9.2 ± 2.2 Tmol C yr-1 This rate is small (<2%) compared to autochthonous production (∼494 Tmol C yr-1) and consumption (production:respiration of 0.98), indicating that the North Atlantic is essentially metabolically balanced and that autochthonous production is remineralized within the basin. The upper layer of the subtropical gyre has previously been reported to exhibit high rates of net heterotrophy, but our analysis does not support those findings. Instead, allochthonous inputs of organic carbon to the upper subtropical gyre are an order of magnitude less than required by the elevated rates of net heterotrophy reported. We find, too, that net mineralization of allochthonous DOC within the basin could account for 10% of the preindustrial inorganic carbon exported from the basin to the south. Two factors, the import of organic matter and the unique thermohaline circulation pattern of the North Atlantic, are primary in ensuring net heterotrophy in the basin. [ABSTRACT FROM AUTHOR]

At the Bermuda Atlantic Time-Series Study (BATS) site, the field observations of dissolved organic carbon (DOC) dynamics indicate that seasonally produced "semilabile" DOC is resistant to rapid microbial degradation in the surface waters but available for microbial remineralizarion once it is delivered into the mesopelagic zone after convective overturn. In this study, we employed an experimental simulation of convective overturn events to determine whether the remineralization of semilabile DOC would occur in a controlled laboratory setting. Seawater culture experiments were conducted in which surface (≤10 m) and mesopelagic (250 m) 0.2-µm filtrates were inoculated with unfiltered water from ≤10 and 250 m in an assortment of combinations to simulate various mixtures of nutrients, DOC quantity and quality, and microbial assemblages. Results indicate that (1) microbial inocula from the upper euphoric zone were incapable of remineralizing the seasonally accumulated semilabile DOC (µmol C L-1 resolution) on the timescales of the incubations; (2) the utilization of semilabile DOC was greatest when the inoculum source was from 250 m and the filtrate source was from the upper 10 m; and (3) the decrease in bacterioplankton diversity, estimated with the Shannon-Wiener diversity index, was greater in treatments in which inoculum from 250 m was mixed with filtrate from 10 m than in treatments in which the surface inoculum was mixed with the surface filtrate. Our findings are that a portion of the surface semilabile DOC can be metabolized by microorganisms in a laboratory setting and that mesopelagic nutrients alone are insufficient to stimulate DOC drawdown > 1.3 µmol L-1 Transformations of microbial community structure were associated with the drawdown of surface DOC in simulated mixing events and suggest that microbial community structure is a factor in surface-layer DOC dynamics. [ABSTRACT FROM AUTHOR]

We demonstrate that the distribution of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in the deep North Pacific is a unique tracer for the accumulation of biochemically identifiable organic residue of the export flux. RuBisCO is found both dissolved and assembled in microgels in a dynamic gel-to-dissolved-to-gel continuum that may protect RuBisCO from degradation in the water column. High concentrations are located below biologically productive equatorial and subarctic systems, and low concentrations are associated with the subtropical gyre. RuBisCO tracks the advective transport of export products along deep circulation pathways of the ocean interior, serving as a quantifiable biochemical tracer of modern organic carbon exported to and resident in the deep ocean. [ABSTRACT FROM AUTHOR]

Considerable drawdown of total dissolved inorganic carbon (CT) and oversaturation of oxygen (O2) within a cold (15°C) oligotrophic eddy in the extratropical North Atlantic Ocean (46°N, 20.5°W) indicate that, despite the absence of nitrate (NO3-), the eddy was highly productive. Estimates of net community production using the mass balances of CT and O2 were two to five times greater than those obtained using the mass balance of NO3-. The remineralization rates obtained using the integrated rates of CT and NO3- accumulation and O2 utilization for the upper thermocline waters (35-300-m depth) were in agreement with CT- and O2-based net community production over the same period; however, all the estimates exceeded the NO3--based net community production by a factor of two to five, pointing to a considerable accumulation of NO3- in the upper thermocline in excess of changes in the mixed-layer NO3- inventory. The amount of this excess NO3- suggests that a considerable fraction of the net community production was not supported by the mixed-layer NO3- inventory and that an external source of NO3- must be present. Of the various mechanisms that might explain the inequity between NO3- drawdown in the surface layer and NO3- accumulation in the upper thermocline, N2 fixation is the most viable yet surprising mechanism for producing such excess NO3- in this oligotrophic eddy. A significant fraction of net community production in oligotrophic extratropical waters could be supported by processes that are not fully explored or to date have been considered to be insignificant. [ABSTRACT FROM AUTHOR]

Each spring and summer, the Chukchi Sea in the western Arctic Ocean experiences a short but intense phytoplankton bloom that is controlled by sea ice cover, nutrient availability, and river runoff. During the bloom, dissolved inorganic carbon (DIC) concentrations are drawn down in the mixed layer (shelf, ∼Δ100 µmol kg-1; shelf-break, ∼Δ50 µmol kg-1; basin, <Δ5 µmol kg-1) and converted to particulate organic carbon (POC) and dissolved organic carbon (DOC). We use d18O and salinity to estimate DOC inputs by river and sea ice melt, allowing the marine portion of the DOC pool to be compared between seasons and to facilitate estimates of net DOC production in the region. We estimate that ∼10% of the DIC consumed during net community production is converted to DOC. The remaining carbon deficit (90% of DIC drawdown) is most likely exported from the mixed layer as POC. In 2002 a carbon (C) mass balance was developed for the Chukchi Sea that showed that rates of vertical export were highest over the Chukchi Shelf near Point Barrow (64 mmol C m- d-1), but more commonly ranged from 10 to 35 mmol C m- d-1 over the remainder of the shelf. At the shelf-break and in the Canada Basin export production was <10 mmol C m- d-1. DOC plays a relatively small role as a carbon sink in the system, but quantifying its contributions provides more robust estimates of export production in the Arctic. [ABSTRACT FROM AUTHOR]