OLIGOTROPHICATION AS A RESULT OF PLANKTIVOROUS FISH REMOVAL WITH ROTENONE IN THE SMALL, EUTROPHIC, LAKE MOSVATN, NORWAY

In September 1987 the shallow, eutrophic, Lake Mosvatn was treated with rotenone to eliminate planktivorous fish (mainly whitefish, Coregonus lavaretus, L.), and the effects were studied. The first summer after treatment the zooplankton community changed markedly from rotifer dominance and few grazers, to a community with few rotifers and many grazers. Accordingly there was a fivefold increase in the biomass of Daphnia galeata. Adult females of D. galeata approximately doubled in weight. The decrease in rotifer biomass was probably mainly due to a loss of food by competition with the daphnids. The phytoplankton community was also markedly affected. Prior to treatment Secchi depth was 1.7 m and Chl-a 23 μg l-1 in the summer. After treatment there was an increase in the proportion of small and gelatinous algae and the mean chlorophyll concentration fell to 7 μg Chl-a l-1. Secchi depth increased to>2.3 m (bottom-sight most of the season). After the treatment there were also fewer cyanobacterial blooms. This seems to be related to oligotrophication caused indirectly by increased grazing by the zooplankton. Total nutrient concentrations were affected. Prior to treatment the mean summer concentration of total phosphate was 44 μg P l-1. This decreased to 29 μg P l-1 in the first summer and 23 μg P l-1 the second summer after the treatment. Total nitrogen decreased from 0.68 mg N l-1 before treatment to 0.32 mg N l-1 the first summer after the treatment. The phosphate loading was not reduced, therefor it can be concluded that the fish removal provided a biomanipulation which caused the more oligotrophic conditions. © 1990 Kluwer Academic Publishers.