Water mass characteristics and sill dynamics in a subpolar cold-water coral reef setting at Stjernsund, northern Norway

The Stjernsund, located in subpolar setting at 70.5°N off northern Norway, hosts a thriving cold-water coral reef community on a morainic sill. Dives with manned submersible JAGO identified the different reef zones and sedimentary facies on top and on the slopes of the sill. Hydrographic investigations indicate different water mass distribution east and west of the sill. Winter Mode Water and Norwegian Coastal Water variability depends on the runoff and freshwater discharge into the fjord. Atlantic Water dynamics are almost entirely tidally driven. High-resolution CTD time series covering a full tidal cycle demonstrate mixing processes occurring east of the sill. Additionally, the different bathymetric distribution of living corals on the western and eastern slope of the sill portrays the dependence on these tidal dynamics. The living corals thrive just below the isopycnal of 27.5 kg m− 3, which marks the boundary between Norwegian Coastal Water and Atlantic Water.