Plankton distribution during a coastal upwelling event off Hiiumaa, Baltic Sea: impact of short-term flow field variability

Abstract

The evolution of an upwelling event and the associated plankton distribution off Hiiumaa island, northeastern Baltic Sea, is traced through continuous registration of wind and currents, consecutive CTD and chlorophyll fluorescence surveys and underway shipboard measurements of near-surface temperature and particle concentration over a 10-day period in June 1986. The earlier mesoscale pattern of warmer (13°C) near-shore waters containing higher chlorophyll concentration was drastically changed as the wind turned from SW to NNW and increased up to 12 m s −1, which resulted in the offshore Ekman transport of the warmer coastal water and upwelling of cold (6–7°C) phytoplankton-poor deeper water along the coastal slope. A relatively fast biological response to the upwelling resulted in the form of enhanced primary production and 4–7-fold increase of the standing crop of some phytoplankton populations (mainly dinoflagellates) within 4 days (at the upwelling frontal boundary). It is shown that a persistent, moderate wind is favourable to sustain a local phytoplankton bloom while keeping the vertical transport of the deeper nutrient-rich water still going but being not powerful enough to stir away the growing phytoplankton. Our measurements confirm the importance of coastal upwellings for the productivity of the Baltic near-shore ecosystems in the summer stage.