Forestry in catchments: effects on water quality, plankton, zoobenthos and fish in small lakes

Limnological responses of three small forest lakes in eastern Finland to forest clear cutting and soil scarification (in 15–33% of their catchments) were studied in 1991–1994, and compared with one reference lake. Increases in the concentrations of total phosphorus and iron indicated an increased inorganic load whereas the increase in water colour and chemical oxygen demand suggested an increased load of organic matter. No such changes were recorded in the reference lake. No clear response in phytoplankton biomass to catchment forestry was recorded. However, the changes in species composition, e.g. a bloom of cyanobacteria in one of the lakes in autumn 1993, indicated a slight eutrophication. Periphyton growth increased after the forestry operations resulting in increased concentrations of chlorophyll a. Zooplankton densities, both cladocerans and copepods, increased slightly in two of the lakes. Zoobenthos abundance increased in all the study lakes but increased biomass was recorded in only one. Population structure and growth of perch (Perca fluviatilis) remained unchanged in all the experimental lakes and in the reference lake, suggesting that there were no dramatic changes in the habitat of fish. Observed changes in zooplankton and zoobenthos community composition were also reflected in the diet of perch. The maximum Hg concentration in perch, 1.6 mg kg–1 (ww), was measured in a 19-year-old fish of 16.9 cm in total length. Otherwise, the mean Hg contents of 15 cm long perch were 0.4–0.8 mg kg–1 (ww), except in the reference lake where the mean concentrations were 0.1–0.2 mg kg–1. After the clear cutting and soil scarification a slightly decreasing trend in Hg concentrations in perch was recorded. Generally, the limnological responses to catchment forestry were modest, partly due to the protective zones of ca. 50 m in width left at the shores of the lakes according to the recent recommendations of forest management in Finland.