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A paradox is commonly observed in productive sea ice in which an accumulation in the macro-nutrients nitrate and phosphate coincides with an accumulation of autotrophic biomass. This paradox requires a new conceptual understanding of the biogeochemical processes operating in sea ice. In this study, we investigate this paradox using three time series in Antarctic landfast sea ice, in which massive algal blooms are reported (with particulate organic carbon concentrations up to 2,600 µmol L–1) and bulk nutrient concentrations exceed seawater values up to 3 times for nitrate and up to 19 times for phosphate. High-resolution sampling of the bottom 10 cm of the cores shows that high biomass concentrations coexist with high concentrations of nutrients at the subcentimeter scale. Applying a nutrient-phytoplankton-zooplankton-detritus model approach to this sea-ice system, we propose the presence of a microbial biofilm as a working hypothesis to resolve this paradox. By creating microenvironments with distinct biogeochemical dynamics, as well as favoring nutrient adsorption onto embedded decaying organic matter, a biofilm allows the accumulation of remineralization products (nutrients) in proximity to the sympagic (ice-associated) community. In addition to modifying the intrinsic physicochemical properties of the sea ice and providing a substrate for sympagic community attachment, the biofilm is suggested to play a key role in the flux of matter and energy in this environment., info:eu-repo/semantics/published

While representing less than 5% of the total ice cover around Antarctica, landfast sea ice is nevertheless an important habitat known to exhibit high biomass levels at the ocean/ice interface, with particulate organic carbon (POC) concentrations easily reaching 2000 μmol C L–1 during spring bloom. Surprisingly, together with the POC increase in bottom ice, fieldwork measurements performed in East Antarctica (Adélie Land 2011, McMurdo Sound 2012, Prydz Bay 2015) of nitrate and phosphate concentrations report a simultaneous increase with concentrations exceeding those of underlying seawater, suggesting an intense remineralization and nitrification processes within the ice. This goes against the classic view of nutrients being consumed during the growth season and regenerated after the height of the bloom. Regardless of the high nitrate levels available in the ice, increasing total nitrogen concentrations also suggest still more nitrogen from the underlying seawater was brought into the ice. Results of a NPZD-model indicates that a second nutrient pool, in addition to the brine pool, is essential to successfully model and reproduce field observations. The presence of a biofilm attached to the ice walls could act as a water-retaining substrate forming microenvironments with chemical gradients within the brine channels. The effect of biofilm on nitrogen dynamics (concentration and isotopic composition) in sea ice will be discussed as well as potential implications for other parameters (phosphate, carbon, oxygen). This calls for the integration of the biofilm concept into the current view of sea-ice biogeochemistry.

Antarctic sea ice is one of the largest ecosystems on Earth, with an extent varying between a maximum of 19x106 km2in late winter and a minimum of 3x106 km2in late summer, most of which consists of annual pack ice. Direct measurements of primary production in Antarctic sea ice, using either oxygen-based or tracer incubation methods, remain scare. Thus, to estimate large-scale Antarctic sea ice primary productivity, two approaches have been used. First, sea ice biogeochemical models suggest that Antarctic pack ice contributes to a small but significant fraction (10 to 28%) of the primary production in the ice-covered area of the Southern Ocean. Second, accumulation of organic matter being trapped within sea ice during the growth season is likely to be representative of the net community production. More than 20 years ago, Legendre et al. (1992) used the few available observations to infer Antarctic sea ice primary productivity. We believe that it is time to revisit this estimation by accounting from a much larger compilation of data (historical to present). Here, we present the first results using an updated dataset of historical ice cores sampled between 1989 and 2017. These allow us to provide an updated estimation of the sea ice primary production based on in-situ data, and its contribution to the SIZ and Southern Ocean. A comparison between pack and fast ice will be also briefly discussed.

With an extent varying between a maximum of 19 × 106 km2 in late winter and a minimum of 3 × 106 km2 in late summer, Antarctic sea ice is one of the largest ecosystems on Earth, most of which consists of annual pack ice. Primary production in-situ measurements in Antarctic sea ice, using either oxygen-based or tracer incubation methods, are relatively tricky to achieve and remain scarce. Thus, to estimate large-scale Antarctic sea-ice primary productivity, two approaches have been used. First, the use of sea-ice biogeochemical models suggest that Antarctic pack ice contributes to a small but significant fraction (10–28%) of the primary production in the ice-covered area of the Southern Ocean. Second, accumulation of organic matter trapped within sea ice during the growth season is likely to be representative of the net community production. More than 20 years ago, Legendre et al. (1992) used the few available observations to infer Antarctic sea-ice primary productivity. We believe that it is time to revisit this estimation by accounting from a much larger compilation of data (historical to present). Here, we present the first results using an updated dataset of historical ice cores sampled between 1989 and 2017 (± 400 pack-ice cores). These allow us to provide an updated estimation of the sea-ice primary production based on in-situ data, and its contribution to the SIZ and Southern Ocean. A comparison between pack and fast ice (± 110 fast- ice cores) will be also briefly discussed.

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special-issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue.

Antarctic landfast sea ice exhibits a large accumulation of biomass in the bottom few centimetres of the ice, with particulate organic carbon (POC) concentrations reaching 2 mmol C l-1 during spring bloom. Surprisingly, fieldwork measurements (Adélie Land 2011, McMurdo Sound 2012, Prydz Bay 2015) of nitrate and phosphate concentrations show a simultaneous increase, with values exceeding those of underlying seawater, suggesting an intense remineralisation process within the ice. Such co-evolution of nutrients and biomass contradicts the classical view of nutrient consumption during the growth period followed by remineralisation after the bloom peak, and suggests an intense supply of N from the underlying water column. Results of a NPZD-model indicates that a second nutrient pool, in addition to the brine pool, is essential to successfully model and reproduce field observations. The presence of a biofilm attached to the ice walls could act as a water-retaining substrate forming microenvironments with chemical gradients within the brine channels. The effect of biofilm on nitrogen dynamics (concentration and isotopic composition) in sea ice will be discussed as well as potential implications for other parameters (phosphate, carbon, oxygen).

In this study we report particulate organic carbon (POC) export fluxes for different biogeochemical basins in the North Atlantic as part of the GEOTRACES GA01 expedition (GEOVIDE, May–June 2014). Surface POC export fluxes were deduced by combining export fluxes of total Thorium-234 (234Th) with the ratio of POC to 234Th of sinking particles at the depth of export. Particles were collected in two size classes (>53 and 1–53 µm) using in situ pumps and the large size fraction was considered representative of sinking material. Surface POC export fluxes revealed latitudinal variations between provinces, ranging from 1.4 mmol m−2 d−1 in the Irminger basin, where the bloom was close to its maximum, to 12 mmol m−2 d−1 near the Iberian Margin, where the bloom had already declined. In addition to the state of progress of the bloom, variations of the POC export fluxes were also related to the phytoplankton size and community structure. In line with previous studies, the presence of coccolithophorids and diatoms appeared to enhance the POC export flux, while the dominance of picophytoplankton cells, such as cyanobacteria, resulted in lower fluxes. The ratio of POC export to primary production (PP) strongly varied regionally and was generally low (≤14 %), except at two stations located near the Iberian Margin (35 %) and within the Labrador basin (38 %), which were characterized by unusual low in situ PP. We thus conclude that during the GEOVIDE cruise, the North Atlantic was not as efficient in exporting carbon from the surface, as reported earlier by others. Finally, we also estimated the POC export at 100 m below the surface export depth to investigate the POC transfer efficiencies. This parameter was also highly variable amongst regions, with the highest transfer efficiency at sites where coccolithophorids dominated.

Over the East Siberian Sea and common to the western Arctic Ocean, a subsurface nutrient maximum is reported in the halocline, and generally attributed to both nutrient-rich Pacific inflow and intensive remineralization in shelf bottom waters being advected into the central Arctic basin. We report nitrogen and oxygen isotopic measurement of nitrate in the water column. A large decoupling between nitrate δ15N and δ18 O is reported, increasing and decreasing upward from the Atlantic T°C maximum into the halocline, respectively. Nitrate δ18O follows the decrease in water δ18O, harboring low-δ18O from large Arctic river discharge. This imprint is transmitted with nitrification to the ambient nitrate δ18O, suggesting that most of the nitrate being supplied into the Arctic Ocean has been reprocessed at least once within the Arctic. The associated increase in nitrate δ15N, correlated with the fixed N deficit, indicates that a significant share of benthic denitrification is supplied from nitrate produced by partial nitrification in the reactive sediment layer. Following an imbalance between remineralization and nitrification, the residual high-δ15N ammonium is accumulated in pore waters and diffuses into shelf bottom waters to be ultimately nitrified. Following the advection into the central basin, this processes could explain both the nutrient maximum, with high-δ15N and low-δ18O nitrate, and the accentuated fixed N deficit in the western Arctic halocline in comparison to the Pacific inflow. A sedimentary isotope effect is reported for benthic denitrification, 2.3-3.1‰, in the middle range given in the literature.

The dual nitrogen (N) and oxygen (O) isotopic composition of nitrate is a powerful tool to study the oceanic N biogeochemical cycle. Internal cycling (e.g. nitrate assimilation and nitrification) as well as sources (N2-fixation) and sinks (denitrification) can be identified. We report N and O isotopic compositions of nitrate in the water column along the GEOTRACES GA01 section (GEOVIDE; May-June 2014) in North Atlantic (from Iberian margin to Greenland) and Labrador Sea (from Greenland to Newfoundland). All stations exhibit an increase of both δ15N and δ18O signals in surface waters. This indicates a partial nitrate assimilation by phytoplankton during the growing season. Signals decrease with depth reaching the mean Atlantic Ocean deep values (δ15N = 4.8 ‰; δ18O = 1.8 ‰). However, in the Western European basin (between 20°W and 10°W), a significant δ15N-NO3- minimum is observed in subsurface waters (150 – 600 m). This imprint of low δ15N nitrogen input to the nitrate pool can be attributed to remineralisation of nitrogen coming from N2-fixation. When plotting δ18O versus δ15N, we observe a clear offset from the 1:1 slope anchored on the mean Atlantic Ocean deep values, suggesting that part of nitrate used in surface waters by phytoplankton is coming from remineralisation of nitrogen initially brought into the oceanic pool through N2-fixation. While significant diazotroph activity is indeed observed in West European basin (see abstract of Fonseca-Batista et al.), it is also likely that the N2-fixation imprint is advected with water mass circulation.

Antarctic sea ice is one of the largest ecosystems on Earth, with an extent varying between a maximum of 19x106 km2 in late winter and a minimum of 3x106km2 in late summer, most of which consists of annual pack ice. Actual measurements of primary production in Antarctic pack ice remain scare. Two approaches have been used to estimate Antarctic sea ice primary productivity. First, sea ice biogeochemical models suggest that Antarctic pack ice contributes to a small but significant fraction (10 to 28%) of the primary production in the ice-covered area of the Southern Ocean. Second, accumulation of organic matter being trapped within sea ice during the growth season is likely to be representative of the net community production. More than 20 years ago, Legendre et al. used the few available observations to infer Antarctic sea ice primary productivity. We believe that it is time to revisit this estimation by accounting from a much larger compilation of data (historical to present). In this poster, we present the first trends using a preliminary dataset of 109 historical ice cores sampled between 1989 and 2012, and provide an updated, in-situ data based estimation of the sea ice primary production and its contribution to the SIZ and Southern Ocean.

A time series study was performed during the Austral spring of 2015 (Oct. 27th – Dec. 11th), in the vicinity of Davis station (68°35’ S, 77°58’ E, Prydz Bay, Antarctica). Landfast sea ice, snow cover and underlying seawater were sampled, with both spatial and temporal high resolution. Different parameters such as nutrients, particulate matter (PM) and isotopic signatures for nitrate, ammonium and PM have been measured with focus on the nitrogen cycle. Primary production and uptake rates for different nitrogen substrates were also determined using in-situ stable isotope incubation experiments. Our study aims to estimate the growth rates of sea-ice algae, follow their evolution with the annual melting of the sea ice cover and highlight their pivotal role for higher tropic levels. Temperature profiles in the ice showed a clear change halfway the sampling campaign with increasing temperatures, likely accompanied by an increase in permeability throughout the ice. Results show a large accumulation of biomass in the bottom few 4 cm (particulate organic carbon reaching up to 1300 μmol l-1). Nutrients profiles (NO3-, NH4+, PO43-) also present a maximum in bottom ice, with concentrations exceeding those in the underlying water. The δ15N and δ18O of bottom ice nitrate show values higher than 18‰ and 16‰ respectively and are located on the 1:1 slope anchored on the nitrate isotopic signature of the underlying seawater (with δ15N and δ18O values of 5,3‰ and 2,2‰). The results of isotopic composition of nitrate will be presented and discussed.

Antarctic fast ice is known for its high biomass levels at the ocean/ice interface where particulate organic carbon (POC) concentrations can easily exceed 2mmol C l-1 during spring bloom. Coinciding with these high POC levels, high concentrations of nitrate and phosphate were observed during three different fieldwork campaigns in the East Antarctic. At Davis Station, Dumont D’Urville Station and Scott Base, bulk concentrations for nitrate were up to 10-times higher than observed in the underlying seawater suggesting a strong remineralisation and nitrification in the ice. However a long time series at Scott Base showed that the accumulation of inorganic nutrients in the bottom ice started early in the growth season while biomass was still growing. This goes against the classic view of nutrients being consumed during the growth season and regenerated after the height of the bloom. Regardless of the high nitrate levels available in the ice, increasing total nitrogen concentrations also suggest still more nitrogen from the underlying seawater was brought into the ice. A qualitative NPZD-model was elaborated and used to understand these observations. Implementation of a second nutrient pool proved essential in successfully modelling and reproducing the field observations. A biofilm could act as a water retaining barrier and result in chemical gradients in the brine channels and create microenvironments. It can also explain other interesting observations for carbon and phosphate that will be discussed in more detail during the conference.

While representing less than 5% of the total ice cover around Antarctica, fast ice is nevertheless an important habitat with highest biomass and production occurring in the bottom (Archer et al., 1996). Sea-ice algae are playing a key role in structuring the ecosystem, as they are an essential food source for krill. With the annual melting of the sea ice there is a strong release of nutrients and algae to the water, possibly fertilizing and/or seeding algae the underlying water column. A time series was sampled during the Austral spring of 2015 (Oct. 27th - Dec. 11th), in the vicinity of Davis station (68°35' S, 77°58' E, Prydz Bay, Antarctica). Fast ice, snow cover and underlying water column were sampled with focus on the nitrogen and carbon cycle and the pivotal role of sea-ice algae for higher tropic levels. Different parameters such as nutrients, particulate matter (PM) and isotopic signatures for nitrate, ammonium and PM will be measured. Primary production and uptake rates for different nitrogen substrates were also measured using in-situ stable isotope incubation experiments to study the change in algae growth rates. That campaign was performed in close relation with colleagues from the Institute of Marine and Antarctic Studies (Hobart, Australia) to link our findings with the availability of trace metals such as iron. Temperature profiles in the ice showed a clear change halfway the sampling campaign with increasing temperatures, likely accompanied by an increase in permeability throughout the ice. Results show a large accumulation of biomass in the bottom few 5 cm (particulate organic carbon reaching up to 1300 _mol l-1). Preliminary data also shows a large accumulation of nitrate at the bottom with concentrations exceeding those in the underlying water. Preliminary results will be presented and discussed.

The rapid decline of sea ice extent in the Arctic Ocean will likely impact the overall Arctic ecosystem and associated biogeochemical cycles. As a first indicator, Arctic primary production has significantly increased during the last decades. All evidences highlights the necessity to better constrain the mod- ern, yet overlooked, Arctic nitrogen (N) cycle to make future primary production projections more reliable. We analysed nitrate δ15N and δ18O and water δ18O on full water column profiles during two summer cruises (July-August 2014) in the Arctic Ocean: SWERUS (R. V. Oden) consisting of across-slope sections over the East-Siberian and Laptev shelves; and ARK-XXVII (R. V. Polarstern) in the vicinity of Fram Strait. The coupling of the N and O isotopes is especially useful to constrain nitrification, the mi- crobially-mediated oxidation of ammonium into nitrate. Preliminary interpretations indicate a strong decoupling between nitrate δ15N and δ18O in the upper ocean, increasing and decreasing upward, respectively. This feature is unique in comparison to the other oceanic basins, and indicates that most of the nitrate in the upper ocean has been produced by nitrification within the Arctic Ocean, either over the shelves or in the deep basins.

To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO3-- and N-2-fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N-2-fixation was observed to sustain 45-85% of the HCO3--fixation in the picoplanktonic fraction performing 47% of the total C-fixation at the deep chlorophyll maximum north and south of the AzF. N2-fixation rates as high as 10.9 mu mol N m(-3) d(-1) and surface nitrate delta N-15 as low as 2.7 parts per thousand were found in the warm (18-24 degrees C), most saline (36.5-37.0) and least productive waters south of the AzF, where UCYN were the least abundant. However, picoplanktonic UCYN abundances up to 55 cells mL(-1) were found at 45-200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N-2-fixation rates up to 4.5 mu mol N m(-3) d(-1) were detected, associated with depth-integrated (HCO3-)-C-13-fixation rates at least 50% higher than observed south of the AzF. The numerous eddies generated at the NW-AzC/AzF seem to enhance exchanges of plankton between water masses, as well as vertical and horizontal diapycnal diffusion of nutrients, whose increase probably enhances the growth of diazotrophs and the productivity of C-fixers.

The international GEOVIDE expedition (GEOTRACES GA01, May-June 2014, spring period) had notably investigated the link between trace element cycling and the production, export and remineralization of particulate organic matter in the North Atlantic Ocean. Sampling was undertaken within different biogeochemical provinces including the Iberian Margin, the West European Basin, Reykjanes Ridge, Irminger Sea, Greenland Margin and the Labrador Sea showing contrasted physical, biological and chemical characteristics. Some of these areas are known to present an important spring phytoplankton bloom leading to the export of particles. Here, we report Particulate Organic Carbon (POC) export fluxes estimated using the 234Th-based approach at 12 stations as part of the GEOVIDE expedition. POC export fluxes were deduced by combining export fluxes of 234Thtotal with the POC/234Th ratio of sinking particles at the depth of export. Profiles of POC to 234Th ratio of particles were obtained for two size classes (>53µm and 1-53 µm) using in-situ pumps. Considering the large (>53 µm) particle fraction as representative of sinking material, POC export fluxes revealed latitudinal variations between provinces ranging from 25 mgC.m-2.d-1 in oligotrophic waters of the Iberian Plain to 62 mgC.m-2.d-1 in the Labrador Sea. Comparison between export and production, defined as ThE ratio, will be addressed in order to study POC export efficiencies. Preliminary data for ThE ratio show a range from 1% (Irminger Sea) to 12% (Labrador Sea) and suggest a relatively low-efficiency biological carbon pump in the North-Atlantic during late spring conditions.

One of the aims of the GEOVIDE expedition in the North- Atlantic (May–July 2014; Lisbon, Portugal – St John, Canada) is to understand the role of the biological carbon pump on the distribution of trace elements and their isotopes (TEI). In this study we determined the natural nitrogen and oxygen isotopic composition of nitrate to trace the impact of biogeochemical processing and watermasses mixing on this nutrient. The N, O isotopic composition of nitrate was determined using the denitrifier method (Sigman et al., 2001), whereby nitrate is converted into nitrous oxide, using denitrifying bacteria. We will present results from contrasted biogeochemical regions of the North Atlantic: the Iberian margin, the West European Basin, Reykjanes Ridge, the Irminger Sea, the Greenland margin and the Labrador Sea. These regions strongly differ in terms of deep water ventilation and are presently still poorly documented regarding their nitrate isotopic composition.

Nitrate is an important nitrogen source for organisms as it is the largest pool of fixed nitrogen available in ocean water and also in sea ice. During biological processing small isotope fractionations occur which affect the isotopic signature of nitrate (δ15N-NO3- and δ18O-NO3-) and contain information on past and ongoing processes influencing nitrate. Although isotopic signature analysis for nitrate has become a common tool for investigating the N-cycling in freshwater and marine systems, measurements in sea ice are still extremely scarce. During the SIPEX II expedition (Sept.-Oct. 2012; R/V: Aurora Australis), samples were taken in both sea ice and the underlying water column. Nitrate isotopic signals in the water column show little variation and were close to deep ocean values (δ15N = 4.8‰, δ18O = 2.4‰). Close to the surface of the water column a small elevation was observed for both nitrogen and oxygen suggesting some biological activity was ongoing. In the sea-ice, slightly higher values of δ15N = 5.5‰ were observed at the water-ice interface and which increased to 14‰ in the center and top layers of the ice cores. Increasing δ15N values coincide with an increase of δ18O and decrease of nitrate concentration suggesting assimilation or denitrification of nitrate being one of the main processes. Solely based on nitrate isotopy differentiating between these two processes is difficult, however anoxic conditions and denitrification have been observed before. At some stations the δ15N and δ18O results deviate from the 1:1 slope generally linked with assimilation and denitrification, indicating possibly other processes are also taking place. Further analysis of the data is required to better constrain the processes taking place.

Nitrate is an important nitrogen source for organisms as it is the largest pool of fixed nitrogen available in ocean water and also in sea ice. During biological processing small isotope fractionations occur which affect the isotopic signature of nitrate (δ15N-NO3- and δ18O-NO3-) and contain information on past and ongoing processes influencing nitrate. Although isotopic signature analysis for nitrate has become a common tool for investigating the N-cycling in freshwater and marine systems, measurements in sea ice are still extremely scarce. During the SIPEX II expedition (Sept.-Oct. 2012; R/V: Aurora Australis), samples were taken in both sea ice and the underlying water column. Nitrate isotopic signals in the water column show little variation and were close to deep ocean values (δ15N = 4.8‰, δ18O = 2.4‰). Close to the surface of the water column a small elevation was observed for both nitrogen and oxygen suggesting some biological activity was ongoing. In the sea-ice, slightly higher values of δ15N = 5.5‰ were observed at the water-ice interface and which increased to 14‰ in the center and top layers of the ice cores. Increasing δ15N values coincide with an increase of δ18O and decrease of nitrate concentration suggesting assimilation or denitrification of nitrate being one of the main processes. Solely based on nitrate isotopy differentiating between these two processes is difficult, however anoxic conditions and denitrification have been observed before. At some stations the δ15N and δ18O results deviate from the 1:1 slope generally linked with assimilation and denitrification, indicating possibly other processes are also taking place. Further analysis of the data is required to better constrain the processes taking place.