MARINE EUTROPHICATION CASE-STUDIES IN SWEDEN

This article summarizes the eutrophication of two sea areas: 1) the Baltic Sea and in particular a coastal area which is affected primarily by effluents from a tertiary sewage plant; and 2) the Kattegat and in particular the Laholm Bay, which is affected primarily by nutrient inputs from agriculture and forest land. During this decade, inputs of N and P to these larger sea areas have increased by factors of approximately 4 to 6 and >8, respectively. Internal nutrient transport and atmospheric deposition is discussed. It is concluded that N availablility generally limits primary production, except in the Bothnian Bay, where P has this role. P is, however, the limiting factor at times in some coastal areas, and in the Baltic for N-fixing blue-green-algae, which bloom in warm summers when low N/P ratios prevail. Noted eutrophication effects are localized elevated primary production, decreased depth distribution and species richness of macroalgae. Eutrophication is most likely a stimulus to high benthic infaunal biomass and fish biomass in well ventilated sea areas. Other areas suffer today from almost permanent (the Baltic proper) or seasonal (the Kattegat) hypoxia with devastating effects on benthos and demersal fish. The N cycle and future possible consequences in relation to nutrient input are discussed. It is concluded that a reduction by at least half of external N and P inputs to the Baltic sea, and of N inputs to the Kattegat is needed to mitigate the negative effects of eutrophication The seas surrounding Sweden. > tniHef