Distribution of flora and fauna in an area receiving pulp mill effluents in the Baltic Sea

Abstract
The quantitative distribution of macroscopic plant and animal species in the area receiving effluents from the sulphate pulp mill in Norrsundet, Bothnian Sea, is described. SCUBA-techniques were used to sample the bottoms. Close to the outlet turbidity and the load of cellulose fibres restrict species distribution, while in the intermediate area toxic substances seem to inhibit the growth of many species found further out. Species composition changes, and the number of species and the biomass increase with the distance from the outlet and are highest at the reference station. Green algae, mainly Cladophora aegagrophila, have a very high biomass close to the pulp mill but this decreases further out and contributes only a few percent to total biomass at the outer stations. In the intermediate area phanerogams, mosses and red algae increase, while brown algae occur only in the outer area. Fucus vesiculosus and Characeae dominate at the unpolluted reference station. Animal biomass is dominated by Macoma balthica and Lymnaea spp. in the whole area. Hydrobia spp. and Gammarus spp. contribute much to total animal biomass in the outer area whereas insect larvae are important in the intermediate parts. There is a tendency for fish density to increase in the intermediate area. This may be due to the occurrence of some major food organisms in the littoral zone, with an even distribution of 4 species. Functional groups of animals, as reflected by trophic levels, are also evenly distributed in the intermediate area.