WIND-INDUCED UPWELLING AS A POSSIBLE EXPLANATION FOR MASS OCCURRENCESOF EPIPHYTIC ETOCARPUS-SILICULOSUS (PHAEOPHYTA) IN THE NORTHERN BALTIC PROPER

The quantity of epiphytic filamentous algae varies considerably in mac roalgal vegetation along rocky shores of the northern Baltic Proper. T he main species responsible for irregular mass occurrences is the summ er annual brown alga Ectocarpus siliculosus (Dillw.) Lyngb. In this st udy, data collected over a 3-yr monitoring period are related to hydro graphical parameters. The tideless and brackish Baltic Sea is salinity stratified, and the salinity difference between surface and bottom wa ters can be used to indicate upwelling events. Mass occurrences develo ped when the salinity difference was low. These salinity differences e xplained 87% of the observed increase in E. siliculosus cover. We prop ose that the slight salinity changes themselves do not affect the grow th of E. siliculosus, but rather they can be used as indicators of sho rt nutrient pulses. These nutrient pulses are not detected in standard water sampling, as they are utilised by both pelagic and benthic orga nisms within a few hours or days. If the proposed mechanism is true, c hanges in wind speed and direction have an important effect on the nut rient dynamics of littoral algal communities. A higher frequency of SW winds may cause mass occurrences of filamentous algae, which are ofte n believed to indicate eutrophication of an anthropogenic origin.