Under-shelf ice foraging of Weddell seals

The Weddell seal (Leptonychotes weddellii) inhabits the Antarctic coastal ecosystem and aggregates in areas characterized by a stable fast ice layer. Due to their extreme diving capabilities, they are able to exploit both pelagic and benthic prey resources. They mainly feed on fishes but occasionally also take cephalopods and crustaceans. Weddell seals instrumented with still-picture camera loggers in the Drescher Inlet, eastern Weddell Sea, detected an unknown cryo-benthic community underneath the floating ice shelf. Images show dense aggregations of invertebrates that likely represent an attractive food horizon for Weddell seals. In this context, we conducted a retrospective analysis of dive profiles collected in the Drescher Inlet to identify favoured hunting depths of Weddell seals and correlate those to the local physical and biological environment. A total of 34 adult Weddell seals were instrumented with dive loggers in the course of six summer field campaigns between 1990 and 2016. An automated broken stick algorithm was used to separate each dive profile into different segments. Segments with a high sinuosity were considered to indicate hunting. Segments characterized by a straight dive trajectories (low sinuosity), were assumed to be transit phases with no hunting activities. A tri-modal distribution of mean hunting depths suggests that Weddell seals concentrated their foraging activities in three depth strata. A peak in hunting depths below 370 m corresponds to the sea floor of the Drescher Inlet, indicating demersal foraging. A second peak between 110 and 160 m matches with the depth of the underside of the floating ice shelf, which suggests shelf ice associated foraging. The third peak probably represents hunting in the pelagic realm. Our investigation highlights the importance of the shelf ice underside as an attractive food horizon for Weddell seals suggesting a re-evaluation of trophic interactions and bentho-pelagic processes in the coastal Antarctic ecosystem

Shelf ice-associated cryo-benthos and environmental features

Incidences of cryo-benthic communities beneath ice shelves are rare and recent discoveries. Combined seal- and ROV-borne imagery and novel sampling technologies allowed for a re-assessment and augmentation of earlier findings on a cryo-benthic isopod community (Antarcturus cf. spinacoronatus), being attached head-down to the underside of floating shelf ice at depths of around 80-150m. The shelf ice-associated cryo-benthos was discovered at Drescher Inlet (-72.83667 -19.15300), Riiser-Larsen Ice Shelf (eastern Weddell Sea). The inlet constitutes a 25km long and between 2 and 4km wide crack in the surrounding shelf ice, which is associated with certain environmental features. Here we compile all available local physical, biological, and biogeochemical data and discuss their relevance in the wider regional context for this faunal hotspot. These include data on shelf, sea and platelet ice, seafloor topography, hydrography and water chemistry, as well as associated pelagic and benthic marine life, in particular affinities of the cryo-benthic isopod community to related fauna occurring in nearby seabed communities using molecular barcoding

Exploration of an ice-cliff grounding zone in Antarctica reveals frozen-on meltwater and high productivity

Ice fluxes across the grounding zone affect global ice-sheet mass loss and sea level rise. Although recent changes in ice fluxes are well constrained by remote sensing, future projections remain uncertain, because key environments affecting ice-sheet dynamics – the ice-sheet bed and grounding zone – are largely unknown. Here, we used a remotely operated vehicle to explore the grounding zone of a Weddell Sea tidewater ice cliff. At 148 m we found a 0.3–0.5 m gap between the ice and the seafloor and a 0.4 m clear facies of debris- and bubble-free basal ice, suggesting freeze-on of meltwater in the distal marine portion of the ice sheet over the last 400 yr. Ploughmarks and low epifauna cover reveal recent grounding line retreat, as corroborated by satellite remote sensing. We found dense algal tufts on the ice cliff and high phytoplankton pigment concentrations, suggesting high productivity fuelled by nutrients from ice melt. As grounded tidewater ice cliffs rim 38% of the Antarctic continent, sinking and downwelling of organic matter along with low benthic turnover may contribute to enhanced carbon sequestration, providing a potentially important feedback to climate.Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (Alfred-Wegener- Institute, Helmholtz Centre for Polar and Marine Research) https://doi.org/10.13039/501100003207https://doi.org/10.1594/PANGAEA.909526https://doi.org/10.1594/PANGAEA.92482

Glass sponge environments in the Weddell Sea, Antarctica

Glass sponges (Porifera, Hexactinellida) are one of the oldest existing animal groups. These marine sessile filter feeders were highly abundant throughout the ocean in the Jurassic period, when they formed vast reefs. Today, however, they mainly inhabit the deep sea, the Antarctic shelves and the shallow fjords along the coast of British Columbia, Canada. In the Antarctic, glass sponges are of particular ecological relevance, as they dominate the benthic biomass in many areas. The richly structured sponge beds harbor a diverse associated fauna and play a significant role in silicon cycling and bentho-pelagic coupling. Unfortunately, it is still unknown which factors determine glass sponge distribution and, therefore, how environmental changes might affect these important Antarctic communities. The aim of our study was to characterize environments with and without glass sponges to identify the determining parameters. We conducted video transects with a remotely operated vehicle (ROV) and measured various environmental parameters at corresponding stations in the Weddell Sea during expedition PS82 with RV Polarstern in January/February 2014. Our poster shows some preliminary results on glass sponge abundance and community composition, as well as profiles of environmental parameters at different stations. Glass sponge environments seem to differ from areas devoid of sponges for example in bottom water temperatures and dissolved silicate concentrations. Both parameters were found to be lower at ROV stations with glass sponges compared to those without. While the distribution of sponges might be affected by temperature and dissolved silicate, the animals might, on the other hand, reduce silicate concentrations themselves. However, it is not yet clear whether the observed differences are really related to the presence or absence of glass sponges. These first results provide ample opportunity for discussion of the relationships between glass sponge communities and their environment in the eastern Weddell Sea