Lake Trophic Status and the Development of the Clear-Water Phase in Lake Geneva

Abstract

A brief period of very clear water following the spring phytoplankton bloom has been observed in many mesotrophic and eutrophic lakes. For Lake Geneva, data on water transparency from Forel (1895) demonstrate that the phenomenon of a clear-water phase did not exist when the lake was still oligotrophic in the late nineteenth century. Comparable Secchi measurements over the last three decades indicate that the clear-water phase developed along with increasing eutrophication as a result—not of clearer water in June—but of significantly lower transparencies at the time of the spring bloom, which is usually dominated by fast-growing, nanoplanktonic algae. Generally in Lake Geneva, following this spring bloom, the numbers of cyclopoid copepods decline strongly, and the Daphnia populations explode. Severe Si limitation in May was observed in 9 out of the last 14 years. Although the sharp reduction in algal standing crop at this time of year may begin by the loss of diatoms due to nutrient stress, the development of a dramatic clear-water phase is mediated primarily by grazing, and in particular, by the grazing of Daphnia.