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Seasonal cycling in carbon, alkalinity, and nutrients in landfast sea ice in Hangar Cove, Adelaide Island, West Antarctic Peninsula, were investigated during winter, spring, and summer 2014–2015. Temporal dynamics were driven by changes in the sea‐ice physicochemical conditions, ice‐algal community composition, and organic matter production. Winter sea ice was enriched with dissolved inorganic carbon (DIC) and inorganic nutrients from organic matter remineralization. Variations in alkalinity (Alk) and DIC indicated that abiotic calcium carbonate (ikaite) precipitation had taken place. Relative to other nutrients, low phosphate (PO4) concentrations potentially resulted from co‐precipitation with ikaite. Seawater flooding and meltwater induced variability in the physical and biogeochemical properties in the upper ice in spring where nutrient resupply supported haptophyte productivity and increased particulate organic carbon (POC) in the interstitial layer. Rapid nitrate (NO3) and DIC (< 165 μmol kg−1) uptake occurred alongside substantial build‐up of algal biomass (746 μg chlorophyll a L−1) and POC (6191 μmol L−1) during summer. Silicic acid drawdown followed NO3 depletion by approximately 1 month with a shift to diatom‐dominated communities. Accumulation of PO4 in the lower ice layers in summer likely resulted from PO4 released during ikaite dissolution in the presence of biofilms. Increased Alk : DIC ratios in the lower ice and under‐ice water suggested that ikaite dissolution buffered against meltwater dilution and enhanced the potential for atmospheric CO2 uptake. This study revealed strong seasonality in carbon and nutrient cycling in landfast sea ice and showed the importance of sea ice in biogeochemical cycling in seasonally ice‐covered waters around Antarctica. [ABSTRACT FROM AUTHOR]

Tides significantly affect polar coastlines by modulating ice shelf melt and modifying shelf water properties through transport and mixing. However, the effect of tides on the marine carbonate chemistry in such regions, especially around Antarctica, remains largely unexplored. We address this topic with two case studies in a coastal polynya in the south-eastern Weddell Sea, neighbouring the Ekström Ice Shelf. The case studies were conducted in January 2015 (PS89) and January 2019 (PS117), capturing semi-diurnal oscillations in the water column. These are pronounced in both physical and biogeochemical variables for PS89. During rising tide, advection of sea ice meltwater from the north-east created a fresher, warmer, and more deeply mixed water column with lower dissolved inorganic carbon (DIC) and total alkalinity (TA) content. During ebbing tide, water from underneath the ice shelf decreased the polynya's temperature, increased the DIC and TA content, and created a more stratified water column. The variability during the PS117 case study was much smaller, as it had less sea ice meltwater input during rising tide and was better mixed with sub-ice shelf water. The contrasts in the variability between the two case studies could be wind and sea ice driven, and they underline the complexity and highly dynamic nature of the system. The variability in the polynya induced by the tides results in an air–sea CO 2 flux that can range between a strong sink (-24 mmol m -2 d -1) and a small source (3 mmol m -2 d -1) on a semi-diurnal timescale. If the variability induced by tides is not taken into account, there is a potential risk of overestimating the polynya's CO 2 uptake by 67 % or underestimating it by 73 %, compared to the average flux determined over several days. Depending on the timing of limited sampling, the polynya may appear to be a source or a sink of CO 2. Given the disproportionate influence of polynyas on heat and carbon exchange in polar oceans, we recommend future studies around the Antarctic and Arctic coastlines to consider the timing of tidal currents in their sampling strategies and analyses. This will help constrain variability in oceanographic measurements and avoid potential biases in our understanding of these highly complex systems. [ABSTRACT FROM AUTHOR]

Coastal zones of Antarctica harbor rich but highly variable phytoplankton communities. The mechanisms that control the dynamics of these communities are not well defined. Here we elucidate the mechanisms that drive seasonal species succession, based on algal photophysiological characteristics and environmental factors. For this, phytoplankton community structure together with oceanographic parameters was studied over a 5‐year period (2012–2017) at Rothera Station at Ryder Bay (Western Antarctic Peninsula). Algal pigment patterns and photophysiological studies based on fluorescence analyses were combined with data from the Rothera Time‐Series program. Considerable interannual variation was observed, related to variations in wind‐mixing, ice cover and an El Niño event. Clear patterns in the succession of algal classes became manifest when combining the data collected over the five successive years. In spring, autotrophic flagellates with a high light affinity were the first to profit from increasing light and sea ice melt. These algae most likely originated from sea‐ice communities, stressing the role of sea ice as a seeding vector for the spring bloom. Diatoms became dominant towards summer in more stratified and warmer surface waters. These communities displayed significantly lower photoflexibility than spring communities. There are strong indications for mixotrophy in cryptophytes, which would explain much of their apparently random occurrence. Climate models predict continuing retreat of Antarctic sea‐ice during the course of this century. For the near‐future we predict that the marginal sea‐ice zone will still harbor significant communities of haptophytes and chlorophytes, whereas increasing temperatures will mainly be beneficial for diatoms. [ABSTRACT FROM AUTHOR]

The Southern Ocean is a hotspot of the climate-relevant organic sulfur compound dimethyl sulfide (DMS). Spatial and temporal variability in DMS concentration is higher than in any other oceanic region, especially in the marginal ice zone. During a one-week expedition across the continental shelf of the West Antarctic Peninsula (WAP), from the shelf break into Marguerite Bay, in January 2015, spatial heterogeneity of DMS and its precursor dimethyl sulfoniopropionate (DMSP)was studied and linked with environmental conditions, including sea-ice melt events. Concentrations of sulfur compounds, particulate organic carbon (POC) and chlorophyll a in the surface waters varied by a factor of 5-6 over the entire transect. DMS and DMSP concentrations were an order of magnitude higher than currently inferred in climatologies for the WAP region. Particulate DMSP concentrations were correlated most strongly with POC and the abundance of haptophyte algae within the phytoplankton community, which, in turn, was linked with sea-ice melt. The strong sea-ice signal in the distribution of DMS(P) implies that DMS(P) production is likely to decrease with ongoing reductions in sea-ice cover along the WAP. This has implications for feedback processes on the region's climate system. This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'. [ABSTRACT FROM AUTHOR]

The effect of sea ice melt on the carbonate chemistry of surface waters in the Weddell-Scotia Confluence, Southern Ocean, was investigated during January 2008. Contrasting concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA) and the fugacity of carbon dioxide ( fCO) were observed in and around the receding sea ice edge. The precipitation of carbonate minerals such as ikaite (CaCO·6HO) in sea ice brine has the net effect of decreasing DIC and TA and increasing the fCO in the brine. Deficits in DIC up to 12 ± 3 μmol kg in the marginal ice zone (MIZ) were consistent with the release of DIC-poor brines to surface waters during sea ice melt. Biological utilization of carbon was the dominant processes and accounted for 41 ± 1 μmol kg of the summer DIC deficit. The data suggest that the combined effects of biological carbon uptake and the precipitation of carbonates created substantial undersaturation in fCO of 95 μatm in the MIZ during summer sea ice melt. Further work is required to improve the understanding of ikaite chemistry in Antarctic sea ice and its importance for the sea ice carbon pump. [ABSTRACT FROM AUTHOR]

The Western Antarctic Peninsula has recently undergone rapid climatic warming, with associated decreases in sea ice extent and duration, and increases in precipitation and glacial discharge to the ocean. These shifts in the freshwater budget can have significant consequences on the functioning of the regional ecosystem, feedbacks on regional climate, and sea-level rise. Here we use shelf-wide oxygen isotope data from cruises in four consecutive Januaries (2011–2014) to distinguish the freshwater input from sea ice melt separately from that due to meteoric sources (precipitation plus glacial discharge). Sea ice melt distributions varied from minima in 2011 of around 0 % up to maxima in 2014 of around 4–5%. Meteoric water contribution to the marine environment is typically elevated inshore, due to local glacial discharge and orographic effects on precipitation, but this enhanced contribution was largely absent in January 2013 due to anomalously low precipitation in the last quarter of 2012. Both sea ice melt and meteoric water changes are seen to be strongly influenced by changes in regional wind forcing associated with the Southern Annular Mode and the El Niño–Southern Oscillation phenomenon, which also impact on net sea ice motion as inferred from the isotope data. A near-coastal time series of isotope data collected from Rothera Research Station reproduces well the temporal pattern of changes in sea ice melt, but less well the meteoric water changes, due to local glacial inputs and precipitation effects. [ABSTRACT FROM AUTHOR]

The Scotia Sea region contains some of the most productive waters of the Southern Ocean. It is also a dynamic region through the interaction of deep water masses with the atmosphere. We present a first seasonally-resolved time series of the fugacity of CO 2 ( f CO 2 ) from spring 2006, summer 2008, autumn 2009 and winter (potential temperature minimum) along a 1000 km transect from the pack ice to the Polar Front to quantify the effects of biology and temperature on oceanic f CO 2 . Substantial spring and summer decreases in sea surface f CO 2 occurred in phytoplankton blooms that developed in the naturally iron-fertilised waters downstream (north) of South Georgia island (54–55°S, 36–38°W) and following sea ice melt (in the seasonal ice zone). The largest seasonal f CO 2 amplitude (Δ f CO 2 ) of − 159 μatm was found in the South Georgia bloom. In this region, biological carbon uptake dominated the seasonal signal, reducing the winter maxima in oceanic f CO 2 by 257 μatm by the summer. In the Weddell–Scotia Confluence, the southern fringe of the Scotia Sea, the shift from wintertime CO 2 -rich conditions in ice covered waters to CO 2 undersaturation in the spring blooms during and upon sea ice melt created strong seasonality in oceanic f CO 2 . Temperature effects on oceanic f CO 2 ranged from Δ f CO 2 sst of ~ 55 μatm in the seasonal ice zone to almost double that downstream of South Georgia (98 μatm). The seasonal cycle of surface water f CO 2 in the high-nutrient low-chlorophyll region of the central Scotia Sea had the weakest biological control and lowest seasonality. Basin-wide biological processes dominated the seasonal control on oceanic f CO 2 (Δ f CO 2 bio of 159 μatm), partially compensated (43%) by moderate temperature control (Δ f CO 2 sst of 68 μatm). The patchwork of productivity across the Scotia Sea creates regions of seasonally strong biological uptake of CO 2 in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Abstract: The spawning success of Antarctic krill (Euphausia superba) is generally assumed to depend on substantial winter sea ice extent, as ice biota can serve as a food source during winter/spring and the seasonal ice melt conditions the upper water column for extensive phytoplankton blooms. However, direct observations during spring are rare. Here we studied krill body condition and maturity stage in relation to feeding (i.e. stomach fullness, diet, absorption of individual fatty acids and defecation rate) across the Scotia Sea in November 2006. The phytoplankton concentrations were low at the marginal ice zone (MIZ) in the southern Scotia Sea (Stn. 1, 2, and 3), high in open waters of the Southern Antarctic Circumpolar Current Front (SACCF) in the central Scotia Sea (Stn. 5), and moderate further north (Stn. 6 and 7). Krill had low lipid reserves (∼6.5% of dry mass, DM), low mass:length ratios (∼1.7mgDMmm−1), and small digestive glands (∼7% of total DM) near the ice edge. The stomachs contained lithogenic particles, diatom debris, and bacterial fatty acids, but low proportions of diatom-indicating fatty acids, which suggest that these krill were feeding on detritus rather than on fresh ice algae. In the SACCF, krill had higher lipid reserves (∼10% of DM), high mass:length ratios (∼2.2mgDMmm−1), and large digestive glands (∼16% of total DM). Stomach content and tissue composition indicate feeding on diatoms. In the north, moderate food concentrations co-occurred with low lipid reserves in krill, and moderate mass:length ratios and digestive gland sizes. Only in the phytoplankton bloom in the SACCF had the mating season already started and some females were about to spawn. Based on the way krill processed their food at the different stations, we indicate two mechanisms that can lead to fast regeneration of body reserves and oocyte maturation in E. superba. One is “superfluous” feeding at high food concentrations, which maximises the overall nutrient gain. The other is a preferential absorption of crucial fatty acids: 20:5(n−3) and 22:6(n−3) when regaining body condition after the winter, and 14:0 and 16:1(n−7) during accumulation of oocyte yolk. Multi-year time series of phytoplankton distribution show that an early spring bloom, as found in 2006, is not unusual for the central Scotia Sea and coincides with high krill larval concentrations in this area. In conclusion, the Scotia Sea is a main spawning ground of Antarctic krill and this is linked to phytoplankton blooms in open waters of the SACCF rather than to ice edge blooms. [Copyright &y& Elsevier]

The carbon cycle in seasonally sea-ice covered waters remains poorly understood due to both a lack of observational data and the complexity of the system. Here we present three consecutive seasonal cycles of upper ocean dissolved inorganic carbon (DIC) and total alkalinity measurements from Ryder Bay on the West Antarctic Peninsula. We attribute the observed changes in DIC to four processes: mixing of water masses, air–sea CO 2 flux, calcium carbonate precipitation/dissolution and photosynthesis/respiration. This approach enables us to resolve the main drivers of the seasonal DIC cycle and also investigate the mechanisms behind interannual variability in the carbonate system. We observe a strong, asymmetric seasonal cycle in the carbonate system, driven by physical processes and primary production. In summer, melting glacial ice and sea ice and a reduction in mixing with deeper water reduce the concentration of DIC in surface waters. The dominant process affecting the carbonate system is net photosynthesis which reduces DIC and the fugacity of CO 2 , making the ocean a net sink of atmospheric CO 2 . In winter, mixing with deeper, carbon-rich water and net heterotrophy increase surface DIC concentrations, resulting in pH as low as 7.95 and aragonite saturation states close to 1. We observe no clear seasonal cycle of calcium carbonate precipitation/dissolution but some short-lived features of the carbonate time series strongly suggest that significant precipitation of calcium carbonate does occur in the Bay. The variability observed in this study demonstrates that changes in mixing and sea-ice cover significantly affect carbon cycling in this dynamic environment. Maintaining this unique time series will allow the carbonate system in seasonally sea-ice covered waters to be better understood. [ABSTRACT FROM AUTHOR]

Nutrient supply, uptake and cycling underpin high primary productivity over the continental shelf of the west Antarctic Peninsula (WAP). Here we use a suite of biogeochemical and isotopic data collected over five years in northern Marguerite Bay to examine these macronutrient dynamics and their controlling biological and physical processes in the WAP coastal ocean. We show pronounced nutrient drawdown over the summer months by primary production which drives a net seasonal nitrate uptake of 1.83 mol N m -2 yr -1 , equivalent to net carbon uptake of 146 g C m -2 yr -1 . High primary production fuelled primarily by deep-sourced macronutrients is diatom-dominated, but non-siliceous phytoplankton also play a role. Strong nutrient drawdown in the uppermost surface ocean has the potential to cause transient nitrogen limitation before nutrient resupply and/or regeneration. Interannual variability in nutrient utilisation corresponds to winter sea ice duration and the degree of upper ocean mixing, implying susceptibility to physical climate change. The nitrogen isotope composition of nitrate (δ 15 N NO3 ) shows a utilisation signal during the growing seasons with a community-level net isotope effect of 4.19 ± 0.29‰. We also observe significant deviation of our data from modelled and observed utilisation trends, and argue that this is driven primarily by water column nitrification and meltwater dilution of surface nitrate. This study is important because it provides a detailed description of the nutrient biogeochemistry underlying high primary productivity at the WAP, and shows that surface ocean nutrient inventories in the Antarctic sea ice zone can be affected by intense recycling in the water column, meltwater dilution and sea ice processes, in addition to utilisation in the upper ocean. [ABSTRACT FROM AUTHOR]

The polar oceans are particularly vulnerable to ocean acidification; the lowering of seawater pH and carbonate mineral saturation states due to uptake of atmospheric carbon dioxide (CO 2 ). High spatial variability in surface water pH and saturation states (Ω) for two biologically-important calcium carbonate minerals calcite and aragonite was observed in Ryder Bay, in the coastal sea-ice zone of the West Antarctic Peninsula. Glacial meltwater and melting sea ice stratified the water column and facilitated the development of large phytoplankton blooms and subsequent strong uptake of atmospheric CO 2 of up to 55 mmol m -2 day -1 during austral summer. Concurrent high pH (8.48) and calcium carbonate mineral supersaturation (Ωaragonite ~3.1) occurred in the meltwater-influenced surface ocean. Biologically-induced increases in calcium carbonate mineral saturation states counteracted any effects of carbonate ion dilution. Accumulation of CO 2 through remineralisation of additional organic matter from productive coastal waters lowered the pH (7.84) and caused deep-water corrosivity (Ωaragonite ~0.9) in regions impacted by Circumpolar Deep Water. Episodic mixing events enabled CO 2 -rich subsurface water to become entrained into the surface and eroded seasonal stratification to lower surface water pH (8.21) and saturation states (Ωaragonite ~1.8) relative to all surface waters across Ryder Bay. Uptake of atmospheric CO 2 of 28 mmol m -2 day -1 in regions of vertical mixing may enhance the susceptibility of the surface layer to future ocean acidification in dynamic coastal environments. Spatially-resolved studies are essential to elucidate the natural variability in carbonate chemistry in order to better understand and predict carbon cycling and the response of marine organisms to future ocean acidification in the Antarctic coastal zone. [ABSTRACT FROM AUTHOR]

The Western Antarctic Peninsula warmed significantly during the second half of the twentieth century, with a concurrent retreat of the majority of its glaciers, and marked changes in the sea-ice field. These changes may affect summertime upper-ocean stratification, and thereby the seasonal dynamics of phytoplankton and bacteria. In the present study, we examined coastal Antarctic microbial community dynamics by pigment analysis and applying molecular tools, and analysed various environmental parameters to identify the most important environmental drivers. Sampling focussed on the austral summer of 2009–2010 at the Rothera oceanographic and biological Time Series (RaTS) site in northern Marguerite bay, Antarctica. The Antarctic summer was characterized by a salinity decrease (measured at 15 m depth) coinciding with increased meteoric water fraction. Maximum Chl-a values of 35 µg l -1 were observed during midsummer and mainly comprised of diatoms. Microbial community fingerprinting revealed four distinct periods in phytoplankton succession during the summer while bacteria showed a delayed response to the phytoplankton community. Non-metric multidimensional scaling analyses showed that phytoplankton community dynamics were mainly directed by temperature, mixed layer depth and wind speed. Both high and low N/P ratios might have influenced phytoplankton biomass accumulation. The bacterioplankton community composition was mainly governed by Chl-a, suggesting a link to phytoplankton community changes. High-throughput 16S and 18S rRNA amplicon sequencing revealed stable eukaryotic and bacterial communities with regards to observed species, yet varying temporal relative contributions. Eukaryotic sequences were dominated by pennate diatoms in December followed by polar centric diatoms in January and February. Our results imply that the reduction of mixed layer depth during summer, caused by meltwater-related surface stratification, promotes a succession in diatoms rather than in nanophytoflagellates in northern Marguerite Bay, which may favour higher trophic levels. [ABSTRACT FROM AUTHOR]

Abstract: Mesopelagic fish are a key component of the pelagic ecosystem throughout the world’s oceans. Opening and closing nets were used to investigate patterns in the distribution and abundance of mesopelagic fish from the surface to 1000m on a series of transects across the Scotia Sea from the ice-edge to the Antarctic Polar Front. A total of 141 non-target net hauls were undertaken during three cruises (Nov 2006, Jan 2008 and Mar 2009), with 7852 teleost fish captured, representing 43 species in 17 families. A further 1517 fish were caught in targeted net hauls. The dominant families were the Myctophidae (6961 specimens; 21 species) and Bathylagidae (1467 specimens; 4 species). Few fish were caught in the upper 400m during daylight, which was attributed to a combination of net avoidance and diurnal vertical migration. Species composition was linked to depth and location and was closely associated with oceanographic features. Diversity was lowest in cold water at the most southerly stations, which were dominated by Electrona antarctica, Gymnoscopelus braueri and Bathylagus antarcticus. Further north, diversity increased with the addition of species such as Krefftichthys anderssoni, Protomyctophum bolini and Electrona carlsbergi. The depth integrated biomass of myctophids was similar across the latitudinal transect and produced an estimate of 4.5 million tonnes in the Scotia Sea. Bathylagids were patchily distributed, but were abundant in the lower mesopelagic zone (>400m) and are potentially significant zooplankton consumers. Given the biomass of the myctophids and bathylagids coupled with the vertical migrations of many species, these fish are likely to play a significant role in carbon export from the surface waters to the deep ocean. [Copyright &y& Elsevier]

Abstract: In recent decades, the west Antarctic Peninsula (WAP) has warmed more rapidly than anywhere else in the Southern Hemisphere. Associated with this, there has been a marked shortening of the sea ice season, a retreat of the majority of glaciers, and an increase in precipitation. Each of these changes in the freshwater system has the potential to exert significant influence on the ecosystem, via processes such as stabilisation of the upper water column, and supply of micronutrients to the mixed layer. Here we use a time series of hydrographic and stable oxygen isotope (δ 18O) measurements collected at a near-coastal site in Marguerite Bay to quantify the prevalence of meteoric freshwater (glacial melt plus precipitation) separately from sea ice melt. During 2002–2009, meteoric water dominated, with summer water column inventories of order 4–6m. Summer sea ice melt inventories were lower, ranging from −1 to 0.5m (where a negative value indicates net sea ice formation from this water). In the near-surface layers, we find highest meteoric water prevalence in February 2006 (∼6%) and lowest in October 2007 (∼1%), whilst sea ice melt is highest in February 2005 (2%) and lowest in July 2002 (−2%). The ranges in both meteoric water and sea ice melt are significantly larger than derived previously using a subset of the data, reflecting the strong interannual variability present. The largest single determinant of the near-surface freshwater percentages is found to be changes in mixed layer depth. Notably deep layers occurred in the winters of 2003, 2007 and 2008, due to northerly winds associated with El Niño/Southern Oscillation and the Southern Annular Mode. These led to greatly reduced sea ice cover in northern Marguerite Bay, and allowed persistent air-sea heat fluxes and stronger rates of sea ice production, which is a key factor in controlling mixed layer depth. We also discuss the possible role of interannual changes in wind-induced mixing in this context. As climate change at the WAP continues, we expect further changes in each of the components of the freshwater budget, and also changes in the vertical redistribution of this freshwater by oceanographic processes. Our ongoing δ 18O monitoring will help track these changes, and elucidate their consequences for the operation of the marine ecosystem. [Copyright &y& Elsevier]