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Keywords:

  • Archipelago Sea (SW Finland);
  • chlorophyll-a;
  • eutrophication;
  • periphyton;
  • Secchi depth;
  • nitrogen;
  • phosphorus;
  • zoobenthos

Abstract

Eutrophication in the Archipelago Sea, SW Finland, as in the whole Baltic Sea has proceeded in recent decades. In order to identify impacts of eutrophication different monitoring programs have been applied for several years. However, the usefulness and general value of different monitoring methods has seldom been evaluated. The purpose of this work was to study the applicability of different methods for monitoring environmental impacts of fish farming and other coastal nutrient effluents in the Archipelago Sea, SW Finland. Especially we study whether the nutrient loading by fish farming may have detectable impact on the total concentration of phosphorus and nitrogen of sea water, the concentration of planktonic chlorophyll-a, Secchi depth, the growth of periphyton and the zoobenthic communities. We analyzed the data collected between 1983 and 1997 in Rymättylä (SW Finland) comprising three fish farming areas. We show that distance from the fish farm neither explains the total phosphorus and nitrogen concentrations of sea water nor the abundance and composition of the zoobenthic communities. However, the concentration of plank-tonic chlorophyll-a and the growth of periphyton significantly increased in the closest study stations of the fish farms. Hence, the planktonic chlorophyll-a concentrations and the growth of periphyton seem to be better indicators of impacts of eutrophication than water chemistry and the abundance and composition of zoobenthic communities. Furthermore, the planktonic chlorophyll-a increased with the total nitrogen loading to the Archipelago Sea. Hence, the analysis of the planktonic chlorophyll-a concentration is a method which enables to detect and separate local as well as large scale changes in the sea water.