Long-term monitoring of near-bottom physical properties at an Arctic deep-sea sponge ground

Deep-sea sponge grounds are hotspots of benthic biomass and diversity. To date, very limited data exists on the range of environmental conditions in areas containing deep-sea sponge grounds and which factors are driving their distribution and sustenance. We investigated oceanographic conditions at a deep-sea sponge ground located on an Arctic Mid-Ocean Ridge seamount. Hydrodynamic measurements were performed along CTD transects and a lander was deployed within the sponge ground that recorded near-bottom physical properties as well as vertical fluxes of organic matter over an annual cycle. The data demonstrate that the sponge ground is found at water temperatures of -0.5 to 1 °C and is situated at the interface between two water masses at only 0.7° equatorward of the turning point latitude of semidiurnal lunar internal tides. Internal waves supported by vertical density stratification interact with the seamount topography and produce turbulent mixing as well as resuspension of organic matter with temporarily very high current speeds up to 0.72 m s-1. The vertical movement of the water column delivers food and nutrients from water layers above and below towards the sponge ground. Highest organic carbon flux was observed during the summer phytoplankton bloom period, providing fresh organic matter from the surface. The flux of fresh organic matter is unlikely to sustain the carbon demand of this ecosystem. Therefore, the availability of bacteria, nutrients and dissolved and particulate matter, delivered by tidally-forced internal wave turbulence and transport by horizontal mean flows, likely plays an important role in meeting ecosystem-level food requirements.