On the productivity and ecology of zooplankton and its role as food for fish in some lakes in Central Finland

Abstract

The study deals with the influence of some physical, chemical, and biotic factors upon the species composition, biomass, and production of zooplankton in several Finnish lakes. Different groups of zooplankton species are separated according to their relation to eutrophication and pollution caused by sewage and wastes from pulp and paper mills. The zooplankton biomass, estimated with the mean volumes of different species and stages, is considered in relation to eutrophication and pollution, and also to the production of phytoplankton. Some comparisons are made between different methods of estimating production. The significance of vendace (Coregonus albula L.) and smelt (Osmerus eperlanus L.) as predators of zooplankton is discussed with reference to analyses of stomach contents and the gastric digestion rate. Eutrophication seemed to increase the number of zooplankton taxa. Several taxa correlated so well with eutrophication and/or pollution that they can be used as indicators. The production of zooplankton followed the changes in the production of phytoplankton algae. In polluted areas bacteria seemed to form a large part of the energy source of zooplankton. Vendace and smelt fed mainly on zooplankters measuring more than O.4 mm. Owing to the preference of for large zooplankton species, their production in oligotrophic waters was mainly consumed by fish. Consumption by fish thus affected the zooplankton composition.