High current event at an Arctic deep-sea sponge ground Version 4

Deep-sea sponge grounds are hotspots of benthic biomass and diversity comparable to tropical coral reefs. So far very limited data exists on the range of environmental conditions deep-sea sponges and their accompanying fauna thrive in or which factors are driving their distribution and sustenance. Therefore we investigated oceanographic conditions at a deep-sea sponge ground located on the summit of an Arctic Mid-Ocean Ridge seamount. Hydrodynamic measurements were performed along CTD transects and a bottom lander was deployed within the sponge ground that recorded near-bottom physical properties as well as vertical fluxes of organic material over an annual cycle. The water column data demonstrated that the sponge ground is situated at the interface between two water masses. Internal waves on the interface interact with the seamount topography, and produce turbulent mixing and resuspension of organic material with temporarily very high current speeds up to 0.72 m s-1 near the seafloor. At the same time vertical movement of the water column delivers food and nutrients from water layers above and below towards the sponge ground. Only one major organic carbon flux event was observed during the year, providing fresh organic matter from the surface layers during the summer phytoplankton bloom period. The amount of organic matter sinking from the surface is likely not able to sustain the carbon demand of this ecosystem. Therefore the availability of dissolved matter, bacteria and nutrients and their delivery by tidal forced internal waves might play an important role in meeting their food demand.