DECREASING PHOSPHORUS CONCENTRATIONS AND UNCHANGED OLIGOCHAETE COMMUNITIES IN LAKE GENEVA - HOW TO MONITOR RECOVERY

Total phosphorus concentrations have strongly decreased from 1976 to 1989 in Lake Geneva (Switzerland). However, relative abundance of oligotrophic species in worm communities was unchanged because primary production by planktonic algae was unabated. About 200 samples per survey are necessary to detect significant changes in the relative abundance of oligotrophic species. Furthermore, the choice of the sampling season and the location of samples within the lake affect the validity of comparisons between years.