Photophysiological responses of bottom sea-ice algae to fjord dynamics and rapid freshening

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From Winter to Late Summer in the Northwestern Barents Sea Shelf: Impacts of Seasonal Progression of Sea Ice and Upper Ocean on Nutrient and Phytoplankton Dynamics

Strong seasonality is a key feature of high-latitude systems like the Barents Sea. While the interannual variability and long-term changes of the Barents Sea are well-documented, the seasonal progression of the physical and biological systems is less known, mainly due to poor accessibility of the seasonally ice-covered area in winter and spring. Here, we use an extensive set of physical and biological in situ observations from four scientific expeditions covering the seasonal progression from late winter to late summer 2021 in the northwestern Barents Sea, from fully ice-covered to ice-free conditions. We found that sea ice meltwater and the timing of ice-free conditions in summer shape the environment, controlling heat accumulation, light and nutrient availability, and biological activity vertically, seasonally, and meridionally. In March and May, the ocean north of the Polar Front was ice-covered and featured a deep mixed layer. Chlorophyll-a concentrations increased from March to May along with greater euphotic depth, indicating the beginning of the spring bloom despite the absence of surface layer stratification. By July and in September, sea ice meltwater created a shallow low-density surface layer that strengthened stratification. In open water, chlorophyll-a maxima were found at the base of this layer as surface nutrients were depleted, while in the presence of ice, maxima were closer to the surface. Solar heating and the thickness of the surface layer increased with the number of ice-free days. The summer data showed a prime example of an Arctic-like space-for-time seasonal variability in the key physical and biological patterns, with the summer situation progressing northwards following sea ice retreat. The amount of sea ice melt (local or imported) has a strong control on the conditions in the northwestern Barents Sea, and the conditions in late 2021 resembled pre-2010 Arctic-like conditions with high freshwater content and lower ocean heat content.acceptedVersio

Interannual differences in sea ice regime in the north-western Barents Sea cause major changes in summer pelagic production and export mechanisms

The Barents Sea is a highly dynamic and productive marine ecosystem and a hotspot of global warming. Variability in sea ice extent is a common feature in the Barents Sea with substantial movements of the sea ice edge on short-term, seasonal to interannual time scales. Historically the northern Barents Sea (north of 75◦N) has been ice-covered in winter, but recently it has become the area with most winter ice loss in the Arctic, and year-round ice-free conditions are predicted for the second half of the 21st century. These environmental changes have significant implications for the marine ecosystem. In this study we used contrasting sea ice regimes in August 2018 and August 2019 to explore the response of phytoplankton and bacterial production, microbial abundance, and vertical carbon flux in the north-western Barents Sea (between 76◦N and 83◦N) to the variability of sea ice. While the study area was ice-free in August 2018, extensive areas north of 79◦N were ice-covered in 2019. When the northern parts of the transect were still ice covered, diatoms and other larger phytoplankton were dominant and highest abundances were observed following the receding ice edge. In contrast, under ice-free conditions in 2018, the pelagic ecosystem resembled a post-bloom stage of the seasonal succession with higher abundance of small phytoplankton and heterotrophic protists and low vertical flux throughout the water column. While phytoplankton biomass, bacterial production and downward vertical flux of particulate organic carbon in the upper 60 m were on average higher in 2019, primary production and carbon export below the euphotic layer were comparable between both years. However, overall highest primary production, bacterial production and abundance of both photosynthetic and heterotrophic microorganisms were observed in surface waters (upper 30 m) in 2019, connected to the retreating ice edge, where also vertical particle flux was higher and characterized by a strong attenuation curve. The results clearly demonstrate that differences in ice cover affect the phenology of pelagic primary production and associated biological processes in the Barents Sea.Mellomårlige forskjeller i havisregime i det nordvestlige Barentshavet forårsaker store endringer i den pelagiske produksjonen og eksportmekanismene om sommerenInterannual differences in sea ice regime in the north-western Barents Sea cause major changes in summer pelagic production and export mechanismsacceptedVersio

Seasonal patterns of vertical flux in the northwestern Barents Sea under Atlantic Water influence and sea-ice decline

The northern Barents Sea is a productive Arctic inflow shelf with a seasonal ice cover and as such, a location with an efficient downward export of particulate organic matter through the biological carbon pump. The region is under strong influence of Atlantification and sea-ice decline, resulting in a longer open water and summer period. In order to understand how these processes influence the biological carbon pump, it is important to identify the seasonal and spatial dynamics of downward vertical flux of particulate organic matter. In 2019 and 2021, shortterm sediment traps were deployed between 30 and 200 m depth along a latitudinal transect in the northwestern Barents Sea during March, May, August and December. Vertical flux of particulate organic carbon, δ13C and δ15N values, Chl-a, protists and fecal pellets were assessed. We identified a clear seasonal pattern, with highest vertical flux in May and August (178 ± 202 and 159 ± 79 mg C m− 2 d− 1 , respectively). Fluxes in December and March were − 2 d− 1 . May was characterized by diatom- and Chl a-rich fluxes and high spatial variability, while fluxes in August had a higher contribution of fecal pellets and small flagellates, and were spatially more homogenous. Standing stocks of suspended particulate organic matter were highest in August, suggesting a more efficient retention system in late summer. The strong latitudinal sea-ice gradient and the influence of Atlantic Water probably led to the high spatial variability of vertical flux in spring, due to their influence on primary productivity. We conclude that the efficiency of the biological carbon pump in a prolonged open-water period depends on the reworking of small, slow sinking material into efficiently sinking fecal pellets or aggregates, and the occurrence of mixing

Seasonal dynamics of sea-ice protist and meiofauna in the northwestern Barents Sea

The rapid decline of Arctic sea ice makes understanding sympagic (ice-associated) biology a particularly urgent task. Here we studied the poorly known seasonality of sea-ice protist and meiofauna community composition, abundance and biomass in the bottom 30 cm of sea ice in relation to ice properties and ice drift trajectories in the northwestern Barents Sea. We expected low abundances during the polar night and highest values during spring prior to ice melt. Sea ice conditions and Chlorophyll a concentrations varied strongly seasonally, while particulate organic carbon concentrations were fairly stable throughout the seasons. In December to May we sampled growing first-year ice, while in July and August melting older sea ice dominated. Low sea-ice biota abundances in March could be related to the late onset of ice formation and short time period for ice algae and uni- and multicellular grazers to establish themselves. Pennate diatoms, such as Navicula spp. and Nitzschia spp., dominated the bottom ice algal communities and were present during all seasons. Except for May, ciliates, dinoflagellates, particularly of the order Gymnodiales, and small-sized flagellates were co-dominant. Ice meiofauna (here including large ciliates and foraminifers) was comprised mainly of harpacticoid copepods, copepod nauplii, rotifers, large ciliates and occasionally acoels and foraminifers, with dominance of omnivore species throughout the seasons. Large ciliates comprised the most abundant meiofauna taxon at all ice stations and seasons (50–90 %) but did not necessarily dominate the biomass. While ice melt might have released and reduced ice algal biomass in July, meiofauna abundance remained high, indicating different annual cycles of protist versus meiofauna taxa. In May highest Chlorophyll a concentrations (29.4 mg m− 2 ) and protist biomass (107 mg C m− 2 ) occurred, while highest meiofauna abundance was found in August (23.9 × 103 Ind. m− 2 ) and biomass in December (0.6 mg C m− 2 ). The abundant December ice biota community further strengthens the emerging notion of an active biota during the dark Arctic winter. The data demonstrated a strong and partially unexpected seasonality in the Barents Sea ice biota, indicating that changes in ice formation, drift and decay will significantly impact the functioning of the ice-associated ecosystem