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The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate δ15N and δ18O is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate δ18O indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate δ15N correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the 15N enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual 15N‐enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4–3.8‰, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters.

In estuarine ecosystems, large spatial and seasonal variations in δ13C values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic levels. Obtaining clean phytoplankton samples for isotope analysis is usually impossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton δ13C variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestrial detritus and phytoplankton) and the δ13C of the terrestrial component, we here present a simple model to estimate the phytoplankton δ13C variations using an existing dataset on the δ13C and elemental (C:N) composition of suspended organic matter from an estuarine mangrove ecosystem in southeast India.These variations are related to the monthly rainfall pattern during the sampling period. It is stressed that this method estimates approximate phytoplankton δ13C values, which should not be used in e.g., mixing models. However, we propose that in cases where sufficiently large datasets are available, the described procedure could provide a valuable method to semi-quantitatively estimate the seasonal or spatial variations of the phytoplankton δ13C signal.

Suspended particulate barium was measured in the Western Mediterranean along 4 profiles sampled during the PHYCEMED 1 cruise in 1981. The non-terrigenous fraction of particulate barium (i.e. excess Ba; Baxs) can account for up to 96% of total barium. This fraction follows the barite settling and dissolution rate model we described earlier for the Atlantic Ocean, confirming barite as the carrier of excess barium. We propose that these Baxs concentrations, which are higher in the Western Mediterranean than in the overall Atlantic, may represent an adaptation of the microplankton to the high densities and density gradients of surface Mediterranean waters.

During austral spring and summer 1988 the upper 500 m of water column in the Scotia-Weddell Confluence was sampled for the elemental composition of total suspended matter. For particulate organic carbon surface water concentrations ranged between 2.5 and 15 µmol/l, with an estimated 19 to 47% of this pool being detrital carbon. In late November, the highest surface water particulate organic carbon concentrations (15 µmol/l) occurred in the Confluence area where they coincided with a maximum in particulate Si (1.7 µmol/l). Later in the season particulate Si in the Confluence area decreased to =0.3 µmol/l. In the Scotia Sea on the contrary, surface water particulate Si increased with time and reached 3 µmol/l in late December. For particulate Ca and Sr in surface water, strong gradients are observed across the Scotia Front (e.g. Ca: from 230 to 10 nmol/l; Sr: from 1.0 to 0.1 nmol/l), with highest concentrations in the Scotia Sea. In general, these distributions are confirmed by the observations on plankton species composition, done by other participants. In the Scotia Sea heavily calcified coccolithophorids and diatoms occurred throughout the season, while in the Confluence area heavily calcified coccolithophorids were absent and a switch-over from diatom to naked flagellate dominance was observed following a krill event. In the surface waters, the lithogenic Si fraction represents on average only 4% of the total particulate Si content. However, this fraction reaches 60% below 100 m depth in the Confluence area, due mainly to the presence of a sub-surface maximum in the aluminosilicate load (particulate Al content up to 30 pmol/l), probably reflecting advection of resuspended shelf sediments. Subsurface Ba/barite concentrations are highest in the Scotia Sea (280 pmol/l) and decrease through the Scotia Front to reach values of 100 pmol/l and less in the Confluence area, the marginal ice zone and the closed pack ice zone.

To realize chlorophyll a analysis, a method is proposed that combines the sensitivity of fluorimetric determinations and the inherent selectivity and accuracy of chromatographic separations. A partial application of the overlapping resolution mapping (O.R.M.) method led to the use of a methanol:acetone:water (75:22:3, v/v) mixture for the isocratic separation by high pressure liquid chromatography on a reserved phase C-18 column of at least sixteen chloropigments and degradation products in a natural extract.Ten replicate measurements of a 50μg. 1-1 standard showed a reproducibility of 3%, while a detection limit of 84 pg chlorophyll a was estimated.

In this study we report full‐depth water column profiles for nitrogen and oxygen isotopic composition (δ15N and δ18O) of nitrate (NO3−) during the GEOTRACES GA01 cruise (2014). This transect intersects the double gyre system of the subtropical and subpolar regions of the North Atlantic separated by a strong transition zone, the North Atlantic Current. The distribution of NO3−δ15N and δ18O shows that assimilation by phytoplankton is the main process controlling the NO3−isotopic composition in the upper 150 m, with values increasing in a NO3−δ18O versus δ15N space along a line with a slope of one toward the surface. In the subpolar gyre, a single relationship between the degree of NO3−consumption and residual NO3−δ15N supports the view that NO3−is supplied via Ekman upwelling and deep winter convection, and progressively consumed during the Ekman transport of surface water southward. The co‐occurrence of partial NO3−assimilation and nitrification in the deep mixed layer of the subpolar gyre elevates subsurface NO3−δ18O in comparison to deep oceanic values. This signal propagates through isopycnal exchanges to greater depths at lower latitudes. With recirculation in the subtropical gyre, cycles of quantitative consumption‐nitrification progressively decrease subsurface NO3−δ18O toward the δ18O of regenerated NO3−. The low NO3−δ15N observed south of the Subarctic Front is mostly explained by N2fixation, although a contribution from the Mediterranean outflow is required to explain the lower NO3−δ15N signal observed between 600 and 1500 m depth close to the Iberian margin. Nitrate assimilation controls the nitrate isotopic composition in surface waters of the North Atlantic subpolar gyreNitrate isotopic composition reveals that nitrate is supplied from subpolar to subtropical gyre by Ekman transport and isopycnal mixingN2fixation and Mediterranean outflow have an impact on nitrate isotopic composition in the North Atlantic Ocean Nitrate assimilation controls the nitrate isotopic composition in surface waters of the North Atlantic subpolar gyre Nitrate isotopic composition reveals that nitrate is supplied from subpolar to subtropical gyre by Ekman transport and isopycnal mixing N2fixation and Mediterranean outflow have an impact on nitrate isotopic composition in the North Atlantic Ocean

Winter to summer CO2dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO2measurements, air‐snow‐ice CO2fluxes, dissolved inorganic carbon (DIC), total alkalinity (TA), and ikaite saturation state. Our results suggest depth‐dependent biotic and abiotic controls that led us to discriminate the ice column in three layers. At the surface, winter pCO2supersaturation drove CO2release to the atmosphere while spring‐summer pCO2undersaturation led to CO2uptake most of the time. CO2fluxes showed a diel pattern superimposed upon this seasonal pattern which was potentially assigned to either ice skin freeze‐thaw cycles or diel changes in net community production. In the ice interior, the pCO2decrease across the season was driven by physical processes, mainly independent of the autotrophic and heterotrophic phases. Bottom sea ice was characterized by a massive biomass build‐up counterintuitively associated with transient heterotrophic activity and nitrate plus nitrite accumulation. This inconsistency is likely related to the formation of a biofilm. This biofilm hosts both autotrophic and heterotrophic activities at the bottom of the ice during spring and may promote calcium carbonate precipitation. Sea ice participates actively in the regional cycling of CO2both as a source and a sink at different times of the year depending on ice physics, ice chemistry, and ice trophic status (autotrophic vs. heterotrophic). We identified the key processes driving the CO2dynamics in each sea ice layer (surface, interior, and bottom) from McMurdo Sound (Antarctica) from late winter to summer. At the surface, CO2release from the ice to the atmosphere occurred in late winter while CO2uptake occurred in summer. Superimposed upon this seasonal pattern, we observed a diurnal pattern with both release and uptake occurring over 24 hr period. This diurnal pattern can be related to physical processes (nocturnal freeze‐up and diurnal melting) or biotic processes (autotrophy or heterotrophy). In the ice interior, a succession of autotrophic and heterotrophic phases took place. At the sea ice bottom, a particular assemblage of microbial cells and organic matter, called biofilm, enabled the accumulation of biomass and nitrate plus nitrite simultaneously leading to both autotrophic and heterotrophic activities. In addition, this biofilm is suggested to promote calcium carbonate precipitation. First long‐term monitoring of both pCO2and CO2fluxes at sea ice interfaces in McMurdo Sound (Antarctica) from late winter to summerLarge biomass build‐up is strikingly associated with transient heterotrophy and nitrate plus nitrite accumulationNew conceptual approach involving biofilm formation can explain observed inconsistencies: accumulation of nitrate plus nitrite and CaCO3