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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 towards 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 N2 fixation, 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.

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Winter to summer CO2 dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO2 measurements, air-snow-ice CO2 fluxes, 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 pCO2 supersaturation drove CO2 release to the atmosphere while spring-summer pCO2 undersaturation led to CO2 uptake most of the time. CO2 fluxes 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 pCO2 decrease 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., SCOPUS: ar.j, info:eu-repo/semantics/published

Observations over recent decades suggest that sea ice plays a significant role in global biogeochemical cycles, providing an active biogeochemical interface at the ocean-atmosphere boundary. Sea ice is a semisolid matrix permeated by a network of channels and pores. The brine-filled spaces are colonized by sympagic (ice-associated) communities that are both taxonomically diverse and metabolically active, with multiple trophic levels, efficiently consuming, reprocessing, and redistributing chemicals within the ice and exchanging with both the overlying atmosphere and the underlying ocean. Analyzing biogeochemical properties in sea ice is fundamentally complicated by its inherent heterogeneity and multiphase nature. This is especially illustrated by the lack of robust estimates for the most basic biogeochemical fluxes, such as primary production and carbon export. Measurements of sea-ice primary production are scarce and challenging. Accumulation of organic matter being trapped within sea ice during the growth season is likely to provide a conservative estimate of the net community production. More than 20 years ago, Legendre et al. [1] used the few available observations to infer Antarctic sea-ice primary productivity. We will revisit this estimation by using a much larger database (n = 421 ice-cores). Based on this compilation, a preliminary estimate for Antarctic sea-ice primary productivity is 35 Tg C yr-1, representing roughly 20% of the primary productivity in the seasonal ice zone. Sympagic communities are exposed to major biogeochemical and physical changes during their lifetime into the ice. A key adaptive response is the formation of biofilms, which play multiple roles in the entrapment, retention and survival of microorganisms. We will review the growing body of evidence that suggests that the biofilm is also playing a major role in sea-ice biogeochemical dynamics (e.g. macro- and micro-nutrient storage; microenvironments with distinct biogeochemical proper, info:eu-repo/semantics/nonPublished

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ContextEnvironmental changes are now vastly documented for large areas of the Southern Ocean. The response of marine organisms and ecosystem processes to these changes are still not well understood. In this framework, insights in resilience, thresholds and tipping points for species, communities and ecosystems are of paramount importance. Addressing these endpoints is however highly complex, and requires robust, interdisciplinary and collaborative research networks to progressively reach a holistic understanding.By design, the vERSO project was intended to forge ahead and identify key areas of research in this respect, and in line with the SCAR priorities. Building on the experience gathered by its long-term partners and taking advantage of the overlap period with another BELSPO-funded project (rECTO), the vERSO developments were intentionally novel and ambitious (in experimental, field and integration facets).ObjectivesThe goal of the vERSO project was to assess the impact of the main stressors driven by global change on benthic Antarctic ecosystems using an integrated multiscale approach including different representative size classes of the benthos. Two principal regions with current contrasting responses to global change have been considered, namely the Western Antarctic Peninsula and the East Antarctic. To reach this goal, researches on connectivity and adaptation, trophic ecology and sensitivity and resilience were conducted by all partners and integrated using state of the art modelling techniques.Results and conclusionsvERSO has globally reached its proposed objectives, paving the way for its sister project, rECTO. vERSO members have participated in 11 expeditions to the Antarctic. Specific efforts were devoted to involving several consortium partners in the fieldwork to optimize the exchange of know-how and crosslinkages using the obtained results. This is reflected in the authorships of the papers and datasets published so far. Most of the experimental wor, info:eu-repo/semantics/published