Lake acidification in Norway - present and predicted fish status

The Thousand Lake Survey was conducted in Norway in the autumn of 1986 to determine both the chemical and the fish status of lakes in areas sensitive to acidic deposition. The survey was also designed to detect possible changes in water quality and fish status as a follow-up to the extensive regional surveys which were conducted in 1974-1975. Lakes and watersheds in widespread areas of southern Norway are affected by acidic precipitation. The greatest loss of fish populations is to be found in southernmost Norway. The number of barren lakes in southernmost and southwestern Norway has doubled since 1971-1975. The chemical changes in these lakes are characterized by a decrease in calcium and sulfate and an increase in aluminum and nitrate concentrations. There has been little change in pH levels. The total land area affected by acidification has increased from$33\ 000\ {\rm km}^{2}$in 1974-1979 to$36\ 000\ {\rm km}^{2}$in 1986. In 1986, over$18\ 000\ {\rm km}^{2}$were almost totally damaged compared to$13\ 000\ {\rm km}^{2}$in 1974-1979. The greatest increase was registered in eastern Norway. At present 52% of the lakes surveyed are endangered. An empirical model predicts that a 30% reduction in loadings of sulfur compounds would lead to the recovery of 28% of these. A further reduction of 50% would ensure viable conditions for fish in 40% of these lakes. These prognoses assume that content of nitrate and humic compounds in these lakes remains constant. A further increase in nitrate concentrations might counteract effect of reduced sulfur deposition.