Mixing and phytoplankton growth around an island in a stratified sea

An island in a stratified region of the shelf seas creates a local increase in tidal mixing. The influences of the enhanced mixing on both the physical structure and phytoplankton biomass distribution have been assessed in a detailed survey of the Scilly Isles region of the Celtic Sea. Marked asymmetries in the observed pattern of stratification and sea surface temperature are in accord with the h/u³ distribution which predicts low stability regions occurring on the sides of the island. Displacement of the low stability regions relative to the h/u³ minima is consistent with a northward mean flow. Levels of biomass and primary productivity were found to be increased by a factor ∼5 over a large region (∼20 × island area) surrounding the islands with particularly intense concentrations of phytoplankton in the pycnocline where chlorophyll levels up to 30 mg m^-3 were observed. The location of these maxima at some distance from the islands is suggestive of an intrusive flow of mixed water into the pycnocline. Estimates of nitrate flux, associated with the production of mixed water by stirring, are of the right order to sustain the observed levels of production.