An introduction to the limnology of Finnish Integrated Monitoring lakes

The limnological monitoring of the lakes in the Finnish Integrated Monitoring catchment areas was started in 1990 in order to study the effects of airborne pollutants on ecosystems. The four pristine headwater lakes made up a wide gradient in terms of location, pH, alkalinity, water colour and total phosphorus concentration. The productivity of the lakes decreased from south to north, as was implied by data on phytoplankton biomass, chlorophyll concentration, primary production of phytoplankton and abundance of rotiferan zooplankton. However, the test fishing catches were not related to the productivity of the lakes. Perch, Perca fluviatilis, was the only fish species present in all four lakes. The appearance of new year-classes of roach, Rutilus rutilus, in one lake was interpreted to be a consequence of the slight decrease in the water acidity during 1988–1996.