Survey of macroalgal mats in the Gulf of Finland, Baltic Sea

Abstract

Abstract 1. The occurrence of macroalgal mats in shallow coastal waters was studied in the Gulf of Finland, Baltic Sea. 2. Altogether, 89 sites were visited and the near‐shore waters of the islands were investigated, mainly by scuba diver, to about 10 m depth. In 26 places, drifting algal mats were recorded. 3. The macroalgal mats occurred in patches of up to 10 m 2 in area, in a range of depths from 0.5 to 10 m, with the optimum depth being less than 5 m. The coverage of the mat varied between 10–70%, and the thickness of the mat was found to vary between 2 and 30 cm. The most frequently recorded, dominant species was Cladophora glomerata . 4. In some cases, anoxic conditions were observed as black sediment underneath the mat. This phenomenon could be a sign of very severe eutrophication of the Baltic Sea. Copyright © 2001 John Wiley & Sons, Ltd.