Enhanced nutrient fluxes at the shelf sea seasonal thermocline caused by stratified flow over a bank

•Observations were carried out as part of a multidisciplinary cruise to the Celtic Sea.•Nutrient supply into the thermocline is increased over banks, compared to flat seabed.•Increased fluxes are forced by locally generated internal mixing.•Temporal variability in mixing leads to patches of nutrient enhanced thermocline.•Nutrient supply over bank is greater than observed at tidal fronts or the shelf edge.

Patches of enhanced chlorophyll a (Chl) concentrations within the thermocline were observed over the slopes of several banks in the Celtic Sea. The turbulent mixing of nutrients from the bottom water into the thermocline was found to be greatly enhanced over the slope of a bank (up to 52 mmol nitrate m−2 day−1), compared to over nearby flat seafloor (∼2 mmol nitrate m−2 day−1). This increased nutrient supply, forced by locally generated lee waves and internal mixing, is greater than nitrate supplies to the productive tidal mixing fronts or to the shelf edge. We hypothesize this nutrient flux promotes an increase in phytoplankton growth in the thermocline over and downstream of shelf sea banks, contributing to the horizontal patchiness in the thermocline Chl signal. The persistence of the strong biological response to mixing at the bank, combined with the ubiquity of shelf sea banks, suggests these bathymetric features have wide importance for “new” primary production in shelf seas.