Glass sponge pump rates and their role in biogeochemical cycles in the Weddell Sea, Antarctica

Glass sponges (Porifera, Hexactinellida) are an important part of epibenthic communities in the shelf areas of Antarctica. They can grow to considerable sizes of up to 2m height and form vast sponge beds which provide habitat for various other animals. Despite continuous research on Antarctic benthic community structure and distribution and the obvious dominance of glass sponges in some shelf communities, very little is known about the physiology of these sessile filter feeders and their role in biogeochemical cycles. While past studies hypothesized that they have a significant impact on carbon and silicon cycling, this had so far not been investigated in situ. During a recent expedition with RV Polarstern to the south-eastern Weddell Sea in 2015/16, we conducted a first in-situ study of pump rates and metabolism in Antarctic glass sponges. Using a remotely operated vehicle (ROV), we investigated 27 sponges of various sizes (15-65cm height) belonging to several common species (Rossella sp. and Anoxycalyx joubini) at four stations between 210m and 420m water depth. We applied a fluorescent dye at the outside wall of the sponges to measure their pump rates and found 63% of them to be actively pumping. Furthermore, we sampled the exhalant current and the corresponding ambient water of 22 individuals with a new custom-made water sampling system. The samples were analysed for dissolved silicon, ammonium and picoplankton and the differences between ambient water and exhalant current were calculated for each sponge. In addition to that, we measured oxygen consumption in 10 sponges using an oxygen optode. Here, we relate the measured pump rates to sponge size, and present biomass-specific fluxes of carbon, nitrogen, and silicon, as well as clearance and respiration rates for these important foundation species. Extrapolation of our findings to larger spatial scales will help to better estimate the role of Antarctic glass sponges on carbon and silicon cycling.