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Metazoans comprise multiple physical niches ("microenvironments"), each colonized by unique microbiomes that contribute to their hosts' evolutionary dynamics, influencing their health, physiology, and adaptation to changing environments. Most wildlife microbiome studies focus on higher metazoans and multiple host microenvironments, while studies of lower species often concentrate on a single microenvironment, sometimes pooling whole bodies or specimens. This is particularly evident in small-sized animals, such as freshwater meiofaunal invertebrates, thus impeding a holistic understanding of microbiome assembly across host microenvironments and its relation with host population genetics. Leveraging the anostracan fairy shrimp Branchinecta , which has easily discernible organs and expected high levels of intraspecific genetic divergence, we aimed to investigate the microbiome assembly processes and test the phylosymbiosis signal in two microenvironments (gill and intestine) across four host populations of Branchinecta gaini within Maritime Antarctica, using 16S rRNA metabarcoding. Our results showed that the gill and intestine harbor strikingly different microbiomes resulting from the B. gaini ecological filtering of the surrounding environment microbial community. Both microenvironments exhibit their respective core microbiomes, yet the gill's core microbiome is narrower and constitutes a smaller proportion of the overall bacterial community compared to that of the intestine. Within each host population (i.e. each sampling site), the microbiome assembles through distinct eco-evolutionary processes in both microenvironments, mostly stochastically (ecological drift) in the gill and deterministically (variable selection) in the intestine. Across different B. gaini populations, variable selection dominates in driving compositional divergence of both microenvironment microbiomes, although to a lesser extent in the gill. Lastly, our study reveals robust correlation between host intraspecific genetic structure and intestine microbiome composition, providing evidence of phylosymbiosis in anostracans. Contrastingly, phylosymbiosis was less pronounced in the gill microbiome. We discuss the potential differences in ecological filtering between each host microenvironment that may underlie the difference in the strength of phylosymbiosis. Our study highlights the relevance of considering host microenvironment and intraspecific levels in testing the phylosymbiosis hypothesis to better understand the intricate eco-evolutionary relationships between hosts and their microbiomes. [ABSTRACT FROM AUTHOR]

We tracked adult emperor penguins from Rothschild Island, west Antarctic Peninsula in 2015/2016 during a summer with extensive sea ice of long duration, contrasting with past years of reduced sea ice extent associated with the recent, rapid, warming trend across the region. We fitted ARGOS PTT devices to penguins of unknown breeding status. Of 33 penguins tracked, nine returned to the colony, presumably to provision offspring. Their foraging trips lasted 9.6 ± 3.7 days, with maximum distances of 75 ± 45 km from the colony within coastal waters. Also, 18 instruments transmitted until the initiation of the annual moult. Penguins travelled at ~ 2.3 km h−1 before slowing for moult. Post-moult, some devices continued to transmit, with speeds of ~ 0.8 km h−1, plausibly due to ice drift, which is rapid in this region. Penguins remained within the seasonal sea ice throughout, staying within 100 km of land, and generally within 5 to 10 km of features (open water, polynyas, leads, icebergs) that offered potential access to the ocean. Penguins were unlikely to have been constrained by the extensive sea ice habitat in 2015/2016. Similar habitats would also have been available in most years of the satellite record (since 1979); however, the moult locations in 2015/2016 would not have been available in many years, and penguins would have needed to find alternative moult locations during some years. Despite uncertainties, the moult period is a critical time for emperor penguins, particularly as sea ice declines, potentially affecting adult survival. [ABSTRACT FROM AUTHOR]

Killer whales (KWs) are apex marine predators. In Antarctica, they have diverse trophic interactions and belong to five ecotypes that have differences in morphology, ecology, and acoustic repertoire. Around the Antarctic Peninsula (AP), two sympatric forms of Type B KWs are the most common. They have similar pigmentation patterns, but a larger form has been observed to feed mainly on seals (B1), and a smaller form feed on Pygoscelis penguins (B2). The waters of the West AP are experiencing ecosystem transformations caused by climate change at one of the fastest rates on Earth. Little is known about the austral winter distribution and ecology of KWs in this changing environment. The purpose of the study was to reveal the winter distribution and ecology patterns of KWs in the waters of the Wilhelm Archipelago. Boat-based visual observations and photo-identification were performed. The boat cruises were conducted within 14 nm of the Ukrainian Akademik Vernadsky Research Station, Galindez Island. In 2019, 10 surveys of a total of 194 nm were conducted, and in 2021 – 37 surveys of 605 nm. During both winters, three groups of KWs were recorded. A group of 7 KWs was encountered on 27 June 2019 near Hovgaard and Pleneau islands. It consisted of adults of both sexes and juveniles. In the same area, on 13 July 2021, another group of KWs was observed, consisting of at least 5 individuals (up to 7): 1 adult male, 2 adult females, a calf, and a juvenile of unknown sex. According to the external morphology patterns, it was determined that both groups belong to Type B, most likely of a smaller form (B1). Large gatherings of Pygoscelis penguins were observed in this area during both winters. On 22 July 2021, a group of KWs consisting of 2 adult males was observed in the Penola Strait. Observations of their joint feeding on a crabeater seal were performed. According to morphology and behaviour, they belong to the ecotype B2. As Antarctic krill move inshore during winter, penguins and crabeater seals likely took advantage of ice-free areas to feed on it, and their gatherings could attract KWs of both Type B ecotypes. Future work should include collecting photogrammetry data, skin biopsy samples, behaviour observations, and acoustic recordings to clearly differentiate between KW ecotypes and better understand their winter distribution and ecological patterns.

When an organism makes a long-distance transition to a new habitat, the associated environmental change is often marked and requires physiological plasticity of larvae, juveniles, or other migrant stages. Exposing shallow-water marine bivalves (Aequiyoldia cf. eightsii) from southern South America (SSA) and the West Antarctic Peninsula (WAP) to changes in temperature and oxygen availability, we investigated changes in gene expression in a simulated colonization experiment of the shores of a new continent after crossing of the Drake Passage, and in a warming scenario in the WAP. Bivalves from SSA were cooled from 7°C (in situ) to 4°C and 2°C (future warmed WAP conditions), WAP bivalves were warmed from 1.5°C (current summer in situ) to 4°C (warmed WAP), gene expression patterns in response to thermal stress by itself and in combination with hypoxia were measured after 10 days. Our results confirm that molecular plasticity may play a vital role for local adaptation. Hypoxia had a greater effect on the transcriptome than temperature alone. The effect was further amplified when hypoxia and temperature acted as combined stressors. The WAP bivalves showed a remarkable ability to cope with short-term exposure to hypoxia by switching to a metabolic rate depression strategy and activating the alternative oxidation pathway, whilst the SSA population showed no comparable response. In SSA, the high prevalence of apoptosis-related differentially expressed genes especially under combined higher temperatures and hypoxia indicated that the SSA Aequiyoldia are operating near their physiological limits already. While the effect of temperature per se may not represent the single most effective barrier to Antarctic colonization by South American bivalves, the current distribution patterns as well as their resilience to future conditions can be better understood by looking at the synergistic effects of temperature in conjunction with short-term exposure to hypoxia.

High-resolution optical imaging systems are quickly becoming universal tools to characterize and quantify microbial diversity in marine ecosystems. Automated classification systems such as convolutional neural networks (CNNs) are often developed to identify species within the immense number of images (e.g., millions per month) collected. The goal of our study was to develop a CNN to classify phytoplankton images collected with an Imaging FlowCytobot for the Palmer Antarctica Long-Term Ecological Research project. A relatively small CNN (~2 million parameters) was developed and trained using a subset of manually identified images, resulting in an overall test accuracy, recall, and f1-score of 93.8, 93.7, and 93.7%, respectively, on a balanced dataset. However, the f1-score dropped to 46.5% when tested on a dataset of 10,269 new images drawn from the natural environment without balancing classes. This decrease is likely due to highly imbalanced class distributions dominated by smaller, less differentiable cells, high intraclass variance, and interclass morphological similarities of cells in naturally occurring phytoplankton assemblages. As a case study to illustrate the value of the model, it was used to predict taxonomic classifications (ranging from genus to class) of phytoplankton at Palmer Station, Antarctica, from late austral spring to early autumn in 2017-2018 and 2018-2019. The CNN was generally able to identify important seasonal dynamics such as the shift from large centric diatoms to small pennate diatoms in both years, which is thought to be driven by increases in glacial meltwater from January to March. This shift in particle size distribution has significant implications for the ecology and biogeochemistry of these waters. Moving forward, we hope to further increase the accuracy of our model to better characterize coastal phytoplankton communities threatened by rapidly changing environmental conditions. [ABSTRACT FROM AUTHOR]

The FjordPhyto Citizen Science project is designed to engage the International Association of Antarctic Tour Operators and their Guests in hands-on science as they journey along the fjords of the west Antarctic Peninsula. The Antarctic Peninsula is one of the fastest warming regions in the world. Melting glaciers bring an influx of freshwater and nutrients into the fjords potentially altering the biology at the phytoplankton level. Phytoplankton play a critical role in regulating the atmosphere, drawing carbon dioxide into the ocean and producing over half the Earth’s oxygen. These microscopic drifting plants make up the foundation of the food system supporting whales, seals, and penguins. FjordPhyto aims to understand how glacial meltwater impacts phytoplankton communities among various fjords throughout the austral summer. Visitors will collect phytoplankton samples and photograph images using simple-to-operate tools. Equipment and educational material will be provided by the FjordPhyto research team as outlined in this Capstone Project. Citizen Science is a powerful tool bringing travelers and scientists together to answer critical science questions. FjordPhyto provides a fun and easy way to involve visitors in the legacy of research in Antarctica, while providing scientists with data that greatly expands the current knowledge of Antarctic fjord ecosystems.

In the last three decades, several interdisciplinary studies investigated the marine ecosystems off the West Antarctic Peninsula (WAP), one of the most impacted areas of the Southern Ocean by the global warming. Although the extent of near-shore habitats along the WAP is wider than elsewhere in Antarctica, the coastal fish communities have been rarely studied. Complementing these previous studies, we provide new data on the species composition, population structure and relative abundance of the inshore fish community living off the Argentine Islands (Bellingshausen Sea). Fish samples were caught all the year round during four different periods spread over ten years (from 2006 to 2017). The fish fauna consisted of fourteen high-Antarctic and low-Antarctic species of notothenioids, most of them belonging to the Nototheniidae. Notothenia coriiceps was by far the most abundant species, followed in decreasing abundance by Chaenocephalus aceratus, Notothenia rossii, Trematomus newnesi and Trematomus bernacchii. Our findings provide context for future ecological studies as this area represents either a spawning and nursery area for multiple species in this study. More generally, the inshore waters off the Argentine Islands represent the southern limit of distribution for several low-Antarctic species, and our results provide critical baseline data for assessing possible disruptions in population dynamics driven by the ongoing climate change. [ABSTRACT FROM AUTHOR]

Circumpolar Deep Water (CDW) intrusion onto the continental shelf significantly affects basal melting of ice shelves, oceanographic and ecological processes in the West Antarctic Peninsula (WAP). Understanding the changes in CDW upwelling and its impacts on marine ecology during historical periods is urgent. Here, we presented a new reconstruction for the variability in CDW intrusion onto the WAP over the past 1500 years by using the cadmium (Cd) concentrations in southern elephant seal hairs collected from King George Island. In addition, multi-proxies including the abundance of seal hairs and TOC, TN, LOI, and bio-elements in the sediments were integrated to infer historical seal populations by using the Generalized Additive Models (GAMs). The overall trend of Cd concentrations was well consistent with those of primary productivity and sea ice, reflecting the contribution of CDW on regional oceanic environment and ecology. Notably, during a period of ∼900–500 yr BP, enhanced CDW intrusion allowed for increased regional marine productivity and decreased sea ice, and ultimately promoting an increase in seal populations. Enhanced CDW intrusion was possibly driven by southward shift and intensification of the Southern Hemisphere westerly wind. These results indicated that large-scale atmospheric circulation patterns were coupled with regional oceanic circulation and ecological processes of the Southern Ocean. • The Cd concentrations in seal hairs recorded variations in CDW inflow to the WAP region over the past 1500 years. • CDW intrusion could impact seal populations by affecting primary productivity and sea ice. • Enhanced CDW intrusion was associated with poleward shift and intensification of Southern Hemisphere westerly wind. [ABSTRACT FROM AUTHOR]

Marine pollution is becoming ubiquitous in the environment. Observations of pollution on beaches, in the coastal ocean, and in organisms in the Antarctic are becoming distressingly common. Increasing human activity, growing tourism, and an expanding krill fishing industry along the West Antarctic Peninsula all represent potential sources of plastic pollution and other debris (collectively referred to as debris) to the region. However, the sources of these pollutants from point (pollutants released from discrete sources) versus non-point (pollutants from a large area rather than a specific source) sources are poorly understood. We used buoyant simulated particles released in a high-resolution physical ocean model to quantify pollutant loads throughout the region. We considered non-point sources of debris from the Antarctic Circumpolar Current, Bellingshausen Sea, Weddell Sea, and point source pollution from human activities including tourism, research, and fishing. We also determined possible origins for observed debris based on data from the Southern Ocean Observing System and Palmer Long-Term Ecological Research program. Our results indicate that point source pollution released in the coastal Antarctic is more likely to serve as a source for observed debris than non-point sources, and that the dominant source of pollution is region-specific. Penguin colonies in the South Shetland and Elephant Islands had the greatest debris load from point sources whereas loads from non-point sources were greatest around the southernmost colonies. Penguin colonies at Cornwallis Island and Fort Point were exposed to the highest theoretical debris loads. While these results do not include physical processes such as windage and Stokes Drift that are known to impact debris distributions and transport in the coastal ocean, these results provide critical insights to building an effective stratified sampling and monitoring effort to better understand debris distributions, concentrations, and origins throughout the West Antarctic Peninsula. [Display omitted] • Marine debris are an emerging threat to Antarctic coastlines and local wildlife. • Simulated buoyant debris released in coastal areas remained in the coastal ocean. • Penguin colonies in the South Shetland Islands may be exposed to high debris loads. • Observed debris in coastal ocean likely originated from coastal human activities. • Model improvements and systematic surveys of marine debris will improve predictions. [ABSTRACT FROM AUTHOR]

The ocean coastal‐shelf‐slope ecosystem west of the Antarctic Peninsula (WAP) is a biologically productive region that could potentially act as a large sink of atmospheric carbon dioxide. The duration of the sea‐ice season in the WAP shows large interannual variability. However, quantifying the mechanisms by which sea ice impacts biological productivity and surface dissolved inorganic carbon (DIC) remains a challenge due to the lack of data early in the phytoplankton growth season. In this study, we implemented a circulation, sea‐ice, and biogeochemistry model (MITgcm‐REcoM2) to study the effect of sea ice on phytoplankton blooms and surface DIC. Results were compared with satellite sea‐ice and ocean color, and research ship surveys from the Palmer Long‐Term Ecological Research (LTER) program. The simulations suggest that the annual sea‐ice cycle has an important role in the seasonal DIC drawdown. In years of early sea‐ice retreat, there is a longer growth season leading to larger seasonally integrated net primary production (NPP). Part of the biological uptake of DIC by phytoplankton, however, is counteracted by increased oceanic uptake of atmospheric CO2. Despite lower seasonal NPP, years of late sea‐ice retreat show larger DIC drawdown, attributed to lower air‐sea CO2 fluxes and increased dilution by sea‐ice melt. The role of dissolved iron and iron limitation on WAP phytoplankton also remains a challenge due to the lack of data. The model results suggest sediments and glacial meltwater are the main sources in the coastal and shelf regions, with sediments being more influential in the northern coast. Plain Language Summary: Some coastal ocean areas of Antarctica, like the West Antarctic Peninsula (WAP), have high biological productivity, which indicates they could absorb more atmospheric CO2. Studying these regions is hard since weather, clouds and sea ice make satellite and cruise data collection challenging. Using models is an alternative to fill in the gaps in the data. In this study, we used an ocean model that simulates circulation, sea ice, biological productivity, and the nutrient cycle in the WAP to study how much sea ice and biology influence the carbon cycle. We find that in years when the ice season is long there is a smaller flux of carbon between ocean and atmosphere, and more dilution of the surface waters by sea ice melt. Although the amount of inorganic carbon in the surface ocean is low by the end of the productive season, this does not reflect more CO2 being taken up or more biological productivity. In years of shorter sea ice season, in contrast, the ocean takes up more atmospheric CO2 and has a longer productive season. This is because the surface ocean is exposed for longer, so gas transfer happens more easily and there is more light available for photosynthesis. Key Points: Longer growth season in years of early sea‐ice retreat show higher seasonally integrated NPP, despite lower chlorophyll in JanuarySea ice is important for DIC drawdown as it influences the duration of phytoplankton bloom, air‐sea CO2 fluxes, and dilution by meltwaterIn the WAP, sedimentary iron has a larger role in the northern coast and shelf, glacial meltwater likely the main source in the south [ABSTRACT FROM AUTHOR]

The Southern Ocean is characterized by high eddy activity and high particulate organic carbon (POC) content during summer, especially near Antarctica. Because it encircles the globe, it provides a pathway for inter‐basin exchange. Here, we use satellite observations and a high‐resolution ocean model to quantify offshore transport of coastal water rich in POC off the West Antarctic Peninsula. We show that nonlinear cyclonic eddies generated near the coast often trap coastal water rich in POC during formation before propagating offshore. As a result, cyclones found offshore that were generated near the coast have on average higher POC content in their interior than cyclones generated locally offshore. This results in a POC enrichment of 5.7 ± 3.0 Gg C year−1 in offshore waters off the Peninsula. Actual POC enrichment is likely substantially larger, since about half of the volume transport of coastal water is driven by small eddies that are missed by observations. Plain Language Summary: Coastal waters off Antarctica during summer are generally highly productive, being characterized by high concentrations of particulate organic carbon (POC). Mechanisms driving offshore transport of coastal water are important because they contribute to spreading that organic carbon over large areas of the Southern Ocean. Here, we combine satellite observations and model results to show that when cyclonic eddies are generated near the coast, they often trap coastal water rich in POC in their interior. As those eddies propagate offshore, they carry that signature with them, resulting in POC enrichment in offshore areas. Model results indicate that small eddies, which are generally missed by satellite altimetry, can account for half of the eddy‐driven volume transport of coastal water. Key Points: Cyclonic eddies generated near the coast off the West Antarctic Peninsula often propagate offshoreThose eddies can trap coastal water rich in particulate organic carbon (POC) in their interior, resulting in POC enrichment in offshore waters off the PeninsulaModel results suggest that half of the eddy‐driven volume transport of coastal water is due to small eddies that are missed by altimetry [ABSTRACT FROM AUTHOR]

Abstract Background Matching animal movement with the behaviors that shape life history requires a rigorous connection between the observed patterns of space use and inferred behavioral states. As animal-borne dataloggers capture a greater diversity and frequency of three dimensional movements, we can increase the complexity of movement models describing animal behavior. One challenge in combining data streams is the different spatial and temporal frequency of observations. Nested movement models provide a flexible framework for gleaning data from long-duration, but temporally sparse, data sources. Results Using a two-layer nested model, we combined geographic and vertical movement to infer traveling, foraging and resting behaviors of Humpback whales off the West Antarctic Peninsula. This approach refined previous work using only geographic data to delineate coarser behavioral states. Our results showed increased intensity in foraging activity in late season animals as the whales prepared to migrate north to tropical calving grounds. Our model also suggests strong diel variation in movement states, likely linked to daily changes in prey distribution. Conclusions Using a combination of two-dimensional and three-dimensional movement data, we highlight the connection between whale movement and krill availability, as well as the complex spatial pattern of whale foraging in productive polar waters.

This work investigates the dynamics of the ice front retreat of two tidewater glaciers in adjacent valleys, Drummond and Widdowson (66°43'S, 65°46'W), on the western coast of the Antarctic Peninsula over the past six decades. The Widdowson Glacier had a more significant area loss (36.37 km2 or 16.97%) and a higher snow line elevation (200 m above sea level (a.s.l.) in 2016) than the Drummond Glacier (19.11 km2, or 4.33%; snow line at 100 m a.s.l. in 2016). This difference in the retreat of the two glaciers is attributed slopes and accumulation areas values. The glacier with a smaller area, Widdowson, has been shown to be more sensitive to environmental changes Their steeper glacier front may have influenced the calving rate and generated a more efficient basal slip, increasing the ice flow rate and consequently increasing the retreat rate. [ABSTRACT FROM AUTHOR]

The Southern Ocean is chronically undersampled due to its remoteness, harsh environment, and sea ice cover. Ocean circulation models yield significant insight into key processes and to some extent obviate the dearth of data; however, they often underestimate surface mixed layer depth (MLD), with consequences for surface water‐column temperature, salinity, and nutrient concentration. In this study, a coupled circulation and sea ice model was implemented for the region adjacent to the West Antarctic Peninsula, a climatically sensitive region which has exhibited decadal trends towards higher ocean temperature, shorter sea ice season, and increasing glacial freshwater input, overlain by strong interannual variability. Hindcast simulations were conducted with different air‐ice drag coefficients and Langmuir circulation parameterizations to determine the impact of these factors on MLD. Including Langmuir circulation deepened the surface mixed layer, with the deepening being more pronounced in the shelf and slope regions. Optimal selection of an air‐ice drag coefficient also increased modeled MLD by similar amounts and had a larger impact in improving the reliability of the simulated MLD interannual variability. This study highlights the importance of sea ice volume and redistribution to correctly reproduce the physics of the underlying ocean, and the potential of appropriately parameterizing Langmuir circulation to help correct for biases towards shallow MLD in the Southern Ocean. The model also reproduces observed freshwater patterns in the West Antarctic Peninsula during late summer and suggests that areas of intense summertime sea ice melt can still show net annual freezing due to high sea ice formation during the winter. Key Points: Sea ice redistribution has a large impact on the mixed layer depth seasonality and interannual variability in the West Antarctic PeninsulaIn the West Antarctic Peninsula, areas of high summer sea ice melt can show net annual freezing due to high sea ice formation in winterIncluding a parameterization for Langmuir circulation based on entrainment below the mixed layer improves the simulated mixed layer depth [ABSTRACT FROM AUTHOR]

Abatus agassizii is an irregular sea urchin species that inhabits shallow waters of South Georgia and South Shetlands Islands. As a deposit-feeder, A. agassizii nutrition relies on the ingestion of the surrounding sediment in which it lives barely burrowed. Despite the low complexity of its feeding habit, it harbors a long and twice-looped digestive tract suggesting that it may host a complex bacterial community. Here, we characterized the gut microbiota of specimens from two A. agassizii populations at the south of the King George Island in the West Antarctic Peninsula. Using a metabarcoding approach targeting the 16S rRNA gene, we characterized the Abatus microbiota composition and putative functional capacity, evaluating its differentiation among the gut content and the gut tissue in comparison with the external sediment. Additionally, we aimed to define a core gut microbiota between A. agassizii populations to identify potential keystone bacterial taxa. Our results show that the diversity and the composition of the microbiota, at both genetic and predicted functional levels, were mostly driven by the sample type, and to a lesser extent by the population location. Specific bacterial taxa, belonging mostly to Planctomycetacia and Spirochaetia , were differently enriched in the gut content and the gut tissue, respectively. Predictive functional profiles revealed higher abundance of specific pathways, as the sulfur cycle in the gut content and the amino acid metabolism, in the gut tissue. Further, the definition of a core microbiota allowed to obtain evidence of specific localization of bacterial taxa and the identification of potential keystone taxa assigned to the Desulfobacula and Spirochaeta genera as potentially host selected. The ecological relevance of these keystone taxa in the host metabolism is discussed. [ABSTRACT FROM AUTHOR]

The amphipod Gondogeneia antarctica is among the most abundant benthic organisms, and a key food web species along the rapidly warming West Antarctic Peninsula (WAP). However, little is known about its trophic strategy for dealing with the extreme seasonality of Antarctic marine primary production. This study, using trophic markers, for the first time investigated seasonal dietary shifts of G. antarctica in a WAP fjord. We analyzed δ13C and δ15N in G. antarctica and its potential food sources. The isotopic signatures revealed a substantial contribution of red algae to the amphipod diet and also indicated a significant contribution of benthic diatoms. The isotope results were further supported by fatty acid (FA) analysis, which showed high similarities in FA composition (64% spring–summer, 58% fall–winter) between G. antarctica and the red algal species. G. antarctica δ13C showed a small shift seasonally (−18.9 to −21.4‰), suggesting that the main diets do not change much year-round. However, the relatively high δ15N values as for primary consumers indicated additional dietary sources such as animal parts. Interestingly, G. antarctica and its potential food sources were significantly enriched with δ15N during the fall–winter season, presumably through a degradation process, suggesting that G. antarctica consumes a substantial portion of its diets in the form of detritus. Overall, the results revealed that G. antarctica relies primarily on food sources derived from benthic primary producers throughout much of the year. Thus, G. antarctica is unlikely very affected by seasonal Antarctic primary production, and this strategy seems to have allowed them to adapt to shallow Antarctic nearshore waters.

Fjords on the West Antarctic Peninsula (WAP) serve as sediment traps, preserving histories of glacial sediment supply. Regional warming trends are expected to change sediment supplies, altering water quality, depositional history, and ecosystem drivers. Our ability to assess magnitudes of these changes is limited by sparse data on modern sediment accumulation. Twelve new cores and four existing cores from Andvord Bay were used to characterize variability in sediment accumulation rates. These range from 1.5 to 7.9 mm/year (0.12 to 0.56 g·cm−2·year−1). Spatial differences and a weak down‐fjord gradient in rates suggest diverse sediment sources, including from outside the fjord. This data set provides a comprehensive assessment of sedimentation during the past century, indicating little change in rates due to recent WAP warming, and sets a benchmark for assessing climate‐related changes in sediment delivery and ecosystem drivers (e.g., burial disturbance) in the fjord over coming decades. Plain Language Summary: Tidewater glaciers commonly carve deep fjords, which fill with sediment as the glaciers retreat. Rates of fjord sediment accumulation increase from polar to temperate latitudes because glaciers become more erosive due to increases in air (and water) temperatures and more abundant subglacial meltwater. Presently, subpolar glaciers on the West Antarctic Peninsula (WAP) are weakly erosive, and adjacent fjords host high levels of biological diversity and abundance. Sediment delivery and seabed disturbance are expected to increase with regional climate trends; unfortunately, we have sparse information on present sediment accumulation rates. In this study, we collected a large set of cores and constructed a detailed picture of sediment properties in Andvord Bay on the WAP. Sediment accumulates at slow rates of millimeters per year, conditions which foster great biologic productivity in the WAP coastal zone. These data provide a critical foundation for assessing the sedimentation disturbance regime in Andvord Bay. Key Points: Sediment accumulation rates in Andvord Bay (Antarctica) are only millimeters per year and are spatially variable, reflecting slow delivery from diverse sourcesLimited evidence exists of sporadic gravity flows adjacent to the two primary tidewater glaciersSeabed radioisotope profiles do not indicate any substantial changes in sediment accumulation rates over the past century [ABSTRACT FROM AUTHOR]

In the Western Antarctic Peninsula, increased biological activity at many levels of the food web are spatially coherent with submarine canyons. One possible mechanism that links the presence of these canyons to increased biological productivity is through the local upwelling of nutrient‐rich modified Upper Circumpolar Deep Water (mUCDW) to the surface, which supports high phytoplankton stocks, krill, penguins, and whales. In the austral summer of 2015, we investigated this hypothesis by deploying three autonomous Slocum gliders over Palmer Deep Canyon, near Palmer Station, Antarctica. Although we observed the shallowing of mUCDW consistent with canyon‐driven isopycnal uplift, these deep waters did not penetrate the phytoplankton rich surface mixed layer. Waters below the mixed layer, however, were strongly coherent with bathymetry, suggesting the strong influence of the canyon. The decoupling of the surface mixed layer from the mUCDW suggests that local upwelling may not be the mechanism that supports the biological hotspot. New physical mechanisms that could support the biological hotspot at Palmer Deep Canyon are suggested. Plain Language Summary: It has been suggested that the association of deep submarine canyons and the biological hotspot at Palmer Deep Canyon in the Western Antarctic Peninsula is due to the local upwelling of deep, nutrient‐rich water to the surface, increasing phytoplankton growth and thus attracting local krill and, in turn, supporting upper trophic levels. However, we found that during the season of peak biological activity, summer stratification isolates the productive surface waters from the nutrient‐rich water. Therefore, we suggest that upwelling of nutrient‐rich water is not the mechanism driving this canyon associated biological hotspot. We suggest alternative mechanisms related to horizontal transport. Key Points: Summer stratification isolates the surface mixed layer from deep water masses, limiting the surface influence of the canyonLocal upwelling of modified Upper Circumpolar Deep Water to the surface mixed layer was not observed in Palmer Deep CanyonHorizontal transport may be important in maintaining the biological hot spot rather than local upwelling [ABSTRACT FROM AUTHOR]

• The WAP shelf ecosystem is regionally important for krill and higher trophic levels. • Long-term climate change and short-term natural variability control sea ice dynamics. • Physical changes regulate ecosystem functioning, biogeochemistry and air-sea exchange. • The WAP is a model system for how sea ice changes can restructure polar ecosystems. • Future research requires international cooperation and interdisciplinary integration. The west Antarctic Peninsula (WAP) region has undergone significant changes in temperature and seasonal ice dynamics since the mid-twentieth century, with strong impacts on the regional ecosystem, ocean chemistry and hydrographic properties. Changes to these long-term trends of warming and sea ice decline have been observed in the 21st century, but their consequences for ocean physics, chemistry and the ecology of the high-productivity shelf ecosystem are yet to be fully established. The WAP shelf is important for regional krill stocks and higher trophic levels, whilst the degree of variability and change in the physical environment and documented biological and biogeochemical responses make this a model system for how climate and sea ice changes might restructure high-latitude ecosystems. Although this region is arguably the best-measured and best-understood shelf region around Antarctica, significant gaps remain in spatial and temporal data capable of resolving the atmosphere-ice-ocean-ecosystem feedbacks that control the dynamics and evolution of this complex polar system. Here we summarise the current state of knowledge regarding the key mechanisms and interactions regulating the physical, biogeochemical and biological processes at work, the ways in which the shelf environment is changing, and the ecosystem response to the changes underway. We outline the overarching cross-disciplinary priorities for future research, as well as the most important discipline-specific objectives. Underpinning these priorities and objectives is the need to better define the causes, magnitude and timescales of variability and change at all levels of the system. A combination of traditional and innovative approaches will be critical to addressing these priorities and developing a co-ordinated observing system for the WAP shelf, which is required to detect and elucidate change into the future. [ABSTRACT FROM AUTHOR]

Diatoms are one of the dominant groups in phytoplankton communities of the western Antarctic Peninsula (WAP). Although generally well‐studied, little is known about size dependent photophysiological responses in diatom bloom formation and succession. To increase this understanding, four Antarctic diatom species covering two orders of magnitude in cell size were isolated in northern Marguerite Bay (WAP). Fragilariopsis sp., Pseudo‐nitzschia cf. subcurvata, Thalassiosira cf. antarctica, and Proboscia cf. alata were acclimated to three different irradiances after which photophysiology, electron transport, carbon fixation, and growth were assessed. The small species Fragilariopsis sp., Pseudo‐nitzschia cf. subcurvata, and large species Proboscia cf. alata showed similar photoacclimation to higher irradiances with a decrease in cellular chlorophyll a and an increase in chlorophyll a specific absorption and xanthophyll cycle pigments and activity. In contrast, pigment concentrations and absorption remained unaffected by higher irradiances in the large species Thalassiosira cf. antarctica. Overall, the small species showed significantly higher growth rates compared to the large species, which was related to relatively high light harvesting capacity and electron transport rates in the smaller species. However, photophysiological responses related to photoinhibition and photoprotection and carbon fixation showed no relationship with cell size. This study supports the dominance of small diatoms at low irradiances during winter and early spring, but does not provide photophysiological evidence for the dominance of large diatoms during the phytoplankton bloom in the WAP. This suggests that other factors such as grazing and nutrient availability are likely to play a major role in diatom bloom formation. [ABSTRACT FROM AUTHOR]

Intrusions of warm circumpolar deep water onto the Antarctic continental shelf are thought to drive accelerated loss of Antarctic glacial ice mass by triggering melt at the ice shelf grounding line. However, the mechanisms responsible for driving on‐shelf circumpolar deep water intrusions are not well understood. Here we examine how sea surface height (SSH) anomalies propagating around the Antarctic coastline as coastal‐trapped waves can drive warm water intrusions through changes in bottom Ekman transport. A wind perturbation motivated by the recent intensification and poleward shift of the southern annular mode during its positive phase is applied over Eastern Antarctica between 20°E and 120°E in two global ocean sea‐ice models (1/4° and 1/10°) and a single‐layer shallow water model. The changes in winds generate a drop in coastal SSH that propagates around Antarctica as a barotropic Kelvin wave. The SSH drop is accompanied by a barotropic flow, which alters the bottom stress, generating an onshore transport of warm water wherever thermal gradients are favorable. We estimate the resulting anomalous bottom Ekman flow and use temperature gradients calculated from the Southern Ocean State Estimate, along with the 1/4° and 1/10° models, to evaluate the resultant heat advection. We find that this mechanism can drive warming of up to 0.7 °C along the West Antarctic Peninsula within a year, depending on the mean state of the cross‐shelf temperature gradient and the barotropic flow strength. Over longer time scales, warming eventually ceases due to saturation of the SSH field and arrest of the Ekman transport by buoyancy forces. Plain Language Summary: Antarctic glacial ice melt has accelerated in recent years, yet we still do not fully understand the mechanisms driving this increased melt. Part of the answer is thought to lie in intrusions of warm subsurface waters that come into contact with the base of Antarctica's ice shelves. However, the processes that drive these warm water intrusions are not yet well understood. Here we examine a mechanism for wind‐driven changes in coastal sea level that propagate around Antarctica as coastal‐trapped waves and consequently drive onshore heat transport via changes in bottom layer flow. We calculate an estimate for the warming rate associated with this mechanism using theory; model simulations; and, where available, observations. We further test the sensitivity of heat transport to local stratification as well as a parameter that determines the bottom flow strength. Each model simulation is forced with winds based on projections of an intensified westerly wind belt over the East Antarctic sector of the Southern Ocean. In each case we find substantial subsurface warming to develop along the West Antarctic Peninsula within 1 year, with the strongest warming seen where the net coastal flow, and the temperature gradients over the Antarctic continental shelf are greatest. Key Points: We examine the response of the Antarctic margin to East Antarctic wind anomalies based on an intensified southern annular modeBarotropic Kelvin waves propagate the signal around Antarctica, leading to changes in bottom stress, bottom Ekman transport, and near‐bottom temperatureWe quantify model‐simulated warming rates along the West Antarctic Peninsula that can be attributed to Kelvin wave‐induced bottom Ekman flow under an idealized wind forcing scenario [ABSTRACT FROM AUTHOR]

The keystone role of Antarctic krill, Euphausia superba Dana, in Southern Ocean ecosystems, means it is essential to understand the factors controlling their abundance and secondary production. One such factor that remains poorly known is the role of parasites. A recent study of krill diet using DNA analysis of gut contents provided a snapshot of the parasites present within 170 E. superba guts in a small area along the West Antarctic Peninsula. These parasites included Metschnikowia spp. fungi, Haptoglossa sp. peronosporomycetes, Lankesteria and Paralecudina spp. apicomplexa, Stegophorus sp. nematodes, and Pseudocollinia spp. ciliates. Of these parasites, Metschnikowia spp. fungi and Pseudocollinia spp. ciliates had previously been observed in E. superba , as had other genera of apicomplexans, though not Lankesteria and Paralecudina. In contrast, nematodes had previously only been observed in eggs of E. superba , and there are no literature reports of peronosporomycetes in euphausiids. Pseudocollinia spp., parasitoids which obligately kill their host, were the most frequently observed infection, with a prevalence of 12%. The wide range of observed parasites and the relatively high frequency of infections suggest parasites may play a more important role than previously acknowledged in E. superba ecology and population dynamics. [ABSTRACT FROM AUTHOR]

Paleoceanographic changes in response to Holocene climate variability in Bigo Bay, west Antarctic Peninsula (WAP) were reconstructed through geochemical, isotopic, sedimentological, and microfossil analysis. Core WAP13-GC47 is composed of 4 lithologic units. Unit 4 was deposited under ice shelf settings. Unit 3 represents the mid-Holocene open marine conditions. Unit 2 indicates lateral sediment transport by a glacier advance during the Neoglacial period. The chronological contrast between the timing of open marine conditions at core WAP13-GC47 ( ca . 7060 cal. yr BP at 540 cm) and the ages of calcareous shell fragments ( ca . 8500 cal. yr BP) in Unit 2b suggests sediment reworking during the Neoglacial period. Unit 1 was deposited during the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Surface water productivity, represented by biogenic opal and total organic carbon (TOC) concentrations, increased and bulk δ 15 N (nitrate utilization) decreased during the warmer early to middle Holocene and the MWP. In contrast, surface water productivity decreased with increased bulk δ 15 N during the colder Neoglacial period and LIA in Bigo Bay. The nitrate utilization was enhanced during cold periods in association with strong surface water stratification resulting from increased sea ice meltwater discharge or proximity to an ice shelf calving front in Bigo Bay. Reduced nitrate utilization during warm periods is related to weak stratification induced by less sea ice meltwater input and stronger Circumpolar Deep Water influence. [ABSTRACT FROM AUTHOR]

The coasts of the West Antarctic Peninsula are strongly influenced by glacier meltwater discharge. The spatial structure and biogeochemical composition of inshore habitats are shaped by large quantities of terrigenous particulate material deposited in the vicinity of the coast, which impacts the pelagic and benthic ecosystems. We used a multitude of geochemical and environmental variables to identify the radius extension of the meltwater impact from the Fourcade Glacier into the fjord system of Potter Cove, King George Island. The k-means cluster algorithm, canonical correspondence analysis, variance analysis and Tukey's post hoc multiple comparison tests were applied to define and cluster coastal meltwater habitats. A minimum of 10 clusters were needed to classify the 8 km² study area into meltwater fjord habitats (MFHs), fjord habitats and marine habitats. Strontium content in surface sediments is the main geochemical indicator for lithogenic creek discharge in Potter Cove. Furthermore, bathymetry, glacier distance and geomorphic positioning are the essential habitats explaining variables. The mean and maximum MFH extent amounted to 1km and 2 km, respectively. Extrapolation of the identified meltwater impact ranges to King George Island coastlines, which are presently ice-covered bays and fjord areas, indicated an overall coverage of 200-400km² MFH, underpinning the importance of better understanding the biology and biogeochemistry in terrestrial marine transition zones. 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 West Antarctic Peninsula (WAP) is a climatically sensitive region where periods of strong warming have caused significant changes in the marine ecosystem and food-web processes. Tight coupling between phytoplankton and higher trophic levels implies that the coastal WAP is a bottom-up controlled system, where changes in phytoplankton dynamics may largely impact other food-web components. Here, we analysed the inter-decadal time series of year-round chlorophyll-a (Chl) collected from three stations along the coastal WAP: Carlini Station at Potter Cove (PC) on King George Island, Palmer Station on Anvers Island and Rothera Station on Adelaide Island. There were trends towards increased phytoplankton biomass at Carlini Station (PC) and Palmer Station, while phytoplankton biomass declined significantly at Rothera Station over the studied period. The impacts of two relevant climate modes to the WAP, the El Niño-Southern Oscillation and the Southern Annular Mode, on winter and spring phytoplankton biomass appear to be different among the three sampling stations, suggesting an important role of local-scale forcing than large-scale forcing on phytoplankton dynamics at each station. The inter-annual variability of seasonal bloom progression derived from considering all three stations together captured ecologically meaningful, seasonally co-occurring bloom patterns which were primarily constrained by water-column stability strength. Our findings highlight a coupled link between phytoplankton and physical and climate dynamics along the coastal WAP, which may improve our understanding of overall WAP food-web responses to climate change and variability. 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 West Antarctic Peninsula (WAP) has experienced significant change over the last 50 years. Using a 24 year spatial time series collected by the Palmer Long Term Ecological Research programme, we assessed long-term patterns in the sea ice, upper mixed layer depth (MLD) and phytoplankton productivity. The number of sea ice days steadily declined from the 1980s until a recent reversal that began in 2008. Results show regional differences between the northern and southern regions sampled during regional ship surveys conducted each austral summer. In the southern WAP, upper ocean MLD has shallowed by a factor of 2. Associated with the shallower mixed layer is enhanced phytoplankton carbon fixation. In the north, significant interannual variability resulted in the mixed layer showing no trended change over time and there was no significant increase in the phytoplankton productivity. Associated with the recent increases in sea ice there has been an increase in the photosynthetic efficiency (chlorophyll a-normalized carbon fixation) in the northern and southern regions of the WAP. We hypothesize the increase in sea ice results in increased micronutrient delivery to the continental shelf which in turn leads to enhanced photosynthetic performance. 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 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]

Palmer Deep canyon along the central West Antarctic Peninsula is known to have higher phytoplankton biomass than the surrounding noncanyon regions, but the circulation mechanisms that transport and locally concentrate phytoplankton and Antarctic krill, potentially increasing prey availability to upper-trophic-level predators such as penguins and cetaceans, are currently unknown. We deployed a three-site high-frequency radar network that provided hourly surface circulation maps Palmer Deep canyon along the central West Antarctic Peninsula is known to have higher phytoplankton biomass than the surrounding non-canyon regions, but the circulation mechanisms that transport and locally concentrate phytoplankton and Antarctic krill, potentially increasing prey availability to upper-trophic-level predators such as penguins and cetaceans, are currently unknown. We deployed a three-site high-frequency radar network that provided hourly surface circulation maps over the Palmer Deep hotspot. A series of particle release experiments were used to estimate surface residence time and connectivity across the canyon. The majority of residence times fell between 1.0 and 3.5 days, with a mean of 2 days and a maximum of 5 days. We found a highly significant negative relationship between wind speed and residence time. Our residence time analysis indicates that the elevated phytoplankton biomass over the central canyon is transported into and out of the hotspot on time scales much shorter than the observed phytoplankton growth rate, suggesting that the canyon may not act as an incubator of phytoplankton productivity as previously suggested. It may instead serve more as a conveyor belt of phytoplankton biomass produced elsewhere, continually replenishing the phytoplankton biomass for the local Antarctic krill community, which in turn supports numerous top predators. 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 West Antarctic Peninsula (WAP) is a highly productive marine ecosystem where extended periods of change have been observed in the form of glacier retreat, reduction of sea-ice cover and shifts in marine populations, among others. The physical environment on the shelf is known to be strongly influenced by the Antarctic Circumpolar Current flowing along the shelf slope and carrying warm, nutrient-rich water, by cold waters flooding into the northern Bransfield Strait from the Weddell Sea, by an extensive network of glaciers and ice shelves, and by strong seasonal to inter-annual variability in sea-ice formation and air-sea interactions, with significant modulation by climate modes like El Niño-Southern Oscillation and the Southern Annular Mode. However, significant gaps have remained in understanding the exchange processes between the open ocean and the shelf, the pathways and fate of oceanic water intrusions, the shelf heat and salt budgets, and the long-term evolution of the shelf properties and circulation. Here, we review how recent advances in long-term monitoring programmes, process studies and newly developed numerical models have helped bridge these gaps and set future research challenges for the WAP 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 deep basins of the Bransfield Strait (BS) are ventilated by Weddell Sea (WS) waters from different origins. Depending on the source and density, these water masses follow different routes across the complex topography near the tip of the Antarctic Peninsula and thus into the Bransfield Strait abyss. Using a global setup of the Finite Element Sea-ice Ocean Model (FESOM) we show that the WS waters found at the western WS continental shelf break have a higher influence on the short period variability of BS bottom waters than the waters present over the continental shelf. Adding passive tracers to the glacial melt water (GMW) from two different origins, Larsen Ice Shelf (LIS) and Filchner-Ronne Ice Shelf (FRIS), we show that the GMW from FRIS has a larger influence on BS bottom waters than the GMW from LIS. FRIS GMW has a higher concentration in the BS eastern basin, while LIS GMW is more abundant in the BS central basin. This duality mainly leads to the difference between BS central and eastern basins seen on the observations. This is a novel result and we believe is a significant contribution to the understanding of the BS-WS circulation and interactions. [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]

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]

Circumpolar Deep Water (CDW) intrudes from the mid-layers of the Antarctic Circumpolar Current onto the shelf of the western Antarctic Peninsula, providing a source of heat and nutrients to the regional ocean. It is well known that CDW is modified as it flows across the shelf, but the mechanisms responsible for this are not fully known. Here, data from underwater gliders with high spatial resolution are used to demonstrate the importance of detailed bathymetry in inducing multiple local mixing events. Clear evidence for overflows is observed in the glider data as water flows along a deep channel with multiple transverse ridges. The ridges block the densest waters, with overflowing water descending several hundred metres to fill subsequent basins. This vertical flow leads to entrainment of overlying colder and fresher water in localised mixing events. Initially this process leads to an increase in bottom temperatures due to the temperature maximum waters descending to greater depths. After several ridges, however, the mixing is sufficient to remove the temperature maximum completely and the entrainment of colder thermocline waters to depth reduces the bottom temperature, to approximately the same as in the source region of Marguerite Trough. Similarly, it is shown that deep waters of Palmer Deep are warmer than at the same depth at the shelf break. The exact details of the transformations observed are heavily dependent on the local bathymetry and water column structure, but glacially-carved troughs and shallow sills are a common feature of the bathymetry of polar shelves, and these types of processes may be a factor in determining the hydrographic conditions close to the coast across a wider area. [ABSTRACT FROM AUTHOR]

Barium cycling in the ocean is associated with a number of processes, including the production and recycling of organic matter, freshwater fluxes, and phenomena that affect alkalinity. As a result, the biogeochemical cycle of barium offers insights into past and present oceanic conditions, with barium currently used in various forms as a palaeoproxy for components of organic and inorganic carbon storage, and as a quasi-conservative water mass tracer. However, the nature of the oceanic barium cycle is not fully understood, particularly in cases where multiple processes may be interacting simultaneously with the dissolved and particulate barium pools. This is particularly the case in coastal polar regions such as the West Antarctic Peninsula, where biological drawdown and remineralisation occur in tandem with sea ice formation and melting, glacial meltwater input, and potential fluxes from shelf sediments. Here, we use a high-precision dataset of dissolved barium (Ba d ) from a grid of stations adjacent to the West Antarctic Peninsula in conjunction with silicic acid (Si(OH) 4 ), the oxygen isotope composition of water, and salinity measurements, to determine the relative control of various coastal processes on the barium cycle throughout the water column. There is a strong correlation between Ba d and Si(OH) 4 present in deeper samples, but nevertheless persists significantly in surface waters. This indicates that the link between biogenic opal and barium is not solely due to barite precipitation and dissolution at depth, but is supplemented by an association between Ba d and diatom tests in surface waters, possibly due to barite formation within diatom-dominated phytodetritus present in the photic zone. Sea-ice meltwater appears to exert a significant secondary control on barium concentrations, likely due to non-conservative biotic or abiotic processes acting as a sink for Ba d within the sea ice itself, or sea-ice meltwater stimulating non-siliceous productivity that acts as a Ba d sink. Meteoric water input, conversely, exerts little or no control on local barium levels, indicating that glacial meltwater is not a significant coastal source of barium to the West Antarctic Peninsula shelf waters. [ABSTRACT FROM AUTHOR]

At the West Antarctic Peninsula (WAP), continued atmospheric and oceanic warming is causing significant physical and biogeochemical changes to glaciers and the marine environment. We compare sediment sources and drivers of macronutrient distributions at two bays along the WAP during austral summer 2020, using radioactive radium and stable oxygen isotopes to trace sedimentary influences and quantify different freshwater inputs. In the Ryder Bay, where the Sheldon Glacier is marine-terminating, radium activities at the sediment-water interface indicate considerable benthic mixing. Using radium isotope activity gradients to resolve radium and macronutrient fluxes, we find buoyant meltwater proximal to the glacier drives vigorous mixing of sediment and entrainment of macronutrient deep waters, on the order of 2.0 × 105 mol d−1 for nitrate. Conversely, in the Marian Cove, where the Fourcade Glacier terminates on land, low salinities and oxygen isotopes indicate a meltwater-rich surface layer <1 m thick and rich in sediment, and strong vertical mixing to the seafloor. A continued shift to land-terminating glaciers along the WAP may have a significant impact upon nutrient and sediment supply to the euphotic zone, with impacts upon primary productivity and carbon uptake efficiency. The future of primary production, carbon uptake, and food web dynamics is therefore linked to glacier retreat dynamics in the many fjords along the WAP. • Glacial melting along the West Antarctic Peninsula impacts marine nutrient cycling. • Radium isotopes are powerful tools to trace sediment fluxes. • Oxygen isotopes provide valuable analysis of freshwater contributions. • Surface runoff carries a high sediment load to surface waters. • Submarine glacial melt could entrain significant nutrient fluxes to the euphotic zone. [ABSTRACT FROM AUTHOR]

The extent, duration, and seasonality of sea ice and glacial discharge strongly influence Antarctic marine ecosystems. Most organisms' life cycles in this region are attuned to ice seasonality. The annual retreat and melting of sea ice in the austral spring stratifies the upper ocean, triggering large phytoplankton blooms. The magnitude of the blooms is proportional to the winter extent of ice cover, which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant metazoan populations on Earth, consume phytoplankton blooms dominated by large diatoms. Krill, in turn, support a large biomass of predators, including penguins, seals, and whales. Human activity has altered even these remote ecosystems. The western Antarctic Peninsula region has warmed by 7°C over the past 50 years, and sea ice duration has declined by almost 100 days since 1978, causing a decrease in phytoplankton productivity in the northern peninsula region. Besides climate change, Antarctic marine systems have been greatly altered by harvesting of the great whales and now krill. It is unclear to what extent the ecosystems we observe today differ from the pristine state.

The West Antarctic Peninsula (WAP) shelf is deep and detritus-based (i.e., it is fueled by organic material sinking from intense seasonal cycles of primary production in the water column), leading to pelagic-benthic coupling. The WAP is warming rapidly, yielding increases in seawater temperatures and reductions in sea ice that may fundamentally alter pelagic-benthic coupling and shelf benthic ecosystems. RVIB Nathaniel B. Palmer and ARSV Laurence M. Gould have provided year-round access to the WAP sea ice zone, facilitating studies of pelagic-benthic coupling and climate change. In the Food for Benthos along the Antarctic Continental Shelf (FOODBANCS) Project, we conducted a 15-month field program to evaluate benthic ecosystem function across the mid-WAP shelf, testing the hypothesis that "phytodetrital material deposited from the summer bloom provides a sustained source of food for benthic detritivores during winter months, when organic-matter flux from the water column is extremely low." We found that the intense seasonality in primary production and food availability in the WAP water column is heavily dampened at the shelf floor by the presence of a "food bank" that sustains benthic ecosystem functions (including sediment-community respiration, deposit feeding, vitellogenesis, spawning, and recruitment of benthos) over the winter; this food bank also influences community structure and life-history strategies of the WAP benthos. The persistence of the food bank may be mediated by low bottom-water temperatures, with the consequence that climate warming might reduce food availability in shelf communities. During the FOODBANCS2 Project, we studied the benthic ecosystem response to the strong latitudinal sea ice gradient along the WAP to explore the ecosystem consequences of sea ice loss from climate change. We found that some aspects of benthic ecosystem structure (e.g., macrofaunal dominance by the polychaete Aurospio foodbancsia) covaried with sea ice duration and are likely to be sensitive to sea ice loss. Other benthic parameters (e.g., the standing crop of macro- and megabenthos) exhibited nonlinear responses, with evidence of resilience along much of the sea ice gradient and abrupt change near one end. Still other benthic parameters (e.g., sediment community respiration) changed very little with sea ice duration. We also found that climate warming is facilitating invasion of the WAP shelf by predacious king crabs, with dramatic reduction in benthic biodiversity and altered ecosystem function. In summary, some important benthic ecosystem parameters along the WAP may be resilient to climate-induced changes in pelagic-benthic coupling, while many others may be highly sensitive, responding nonlinearly to sea ice loss. Incorporation of climate change effects into WAP benthic ecosystem models, including the effects of invasive species, will be challenging until mechanisms, nonlinearities, synergies, and tipping points of climate change effects are better understood.

Significant strides in our understanding of the marine pelagic ecosystem of the Western Antarctic Peninsula (WAP) region have been made over the past two decades, resulting from research conducted aboard ARSV Laurence M. Gould and RVIB Nathaniel B. Palmer. These advances range from an understanding of the physical forcing on biology, to food web ecology (from microbes to top predators), to biogeochemical cycling, often in the larger context of rapid climate warming in the region. The proximity of the WAP to the Antarctic Circumpolar Current and WAP continental shelf bathymetry affects the hydrography and helps structure the biological community. Seasonal, spatial, and interannual variability at all levels of the food web, as well as the mechanisms supporting their production, are now more clearly understood. New tools and technologies employed in the region were critical for making this research possible. As a result, our knowledge of the WAP pelagic ecosystem during a time of rapid climate change has vastly improved.