Trends and Gradients in Nutrient Concentrations and Loading in the Archipelago Sea, Northern Baltic, in 1970–1997

Abstract

Eutrophication has been a major environmental change affecting the water quality both in the coastal and open sea areas of the Baltic Sea since the 1960s. Although the total nutrient loading to open sea and also to the Finnish coastal waters is rather well known, the relative importance of various loading sources in different areas is still obscure. We found in the Archipelago Sea, SW Finland, that the great majority of nutrients come as non-point-source loading with the river discharges in the innermost archipelago zone. Fish farming in the middle archipelago zone exerts a remarkable influence on water quality there. The outer archipelago facing the Gulf of Finland and the Baltic proper is influenced by both depending on the season. In a time series study after the initial general rise since the 1970s the increase of nutrient concentrations culminated and levelled off or even turned to a decline in most of the locations studied. It is difficult to single out the cause of this culmination. A reasonable explanation is the general economical decline in Finland. However, same kind of culmination has been found in some other coastal areas of the Baltic Sea, too. The development of the N:P-ratio indicates that during the last two decades nitrogen has developed towards the limiting nutrient in the area studied. The general conclusion is that environmental care should be directed at different targets depending on the archipelago area in question.