Anthropogenic impacts on an oligotrophic clear water lake in Halland, Sweden, assessed from two different data sets.

Lake Skärsjön is a dimictic, oligotrophic, clear water lake with bottom plant communities including the rare Nostoc zetterstedtii. The lake is located in western Sweden, south of Gothenburg. During the late 1970s to mid-1980s, there was fish cage farming located near the outlet of the lake. When the fish farming was first introduced, there was concern over the health of the late which prompted a monitoring study which was conducted from 1980 to 1990 by the County Administration Board (regional governing body of Sweden). Starting in 1983, another National Monitoring study commenced simultaneously with the 10 year study. This monitoring study had one site taking measurements from 0.2-2m and collected the same data as the 10 year study. There were notable changes starting in 1985, the year the fish farming was closed down. After these changes, there are patterns indicating the lake returning to similar conditions before 1985. The pH of the lake is increasing and the acidity decreasing which reflects the ongoing decrease in atmospheric sulphur deposition and concentration in the lake. Overall, the impacts from the fish farming may have been more intense if it was located farther from the lake outlet. This would have allowed nutrient emissions from the fish cages to influence the lake more severely before exiting through the outlet. The lake is slowly recovering from this anthropogenic event and the current monitoring program, the National Monitoring study, should remain intact.