Long-term changes in the copepod community of Lake Geneva

Lake Geneva is a deep, peri-alpine lake where there has been a continuous monitoring program since the 1970s. The lake has long suffered from eutrophication. In the early 1980s, a restoration plan resulted in a progressive reduction in the phosphorus concentration. During the same period, the lake responded to large-scale climate changes by overall warming of the water, and changes in the timing of stratification. Both the composition and the phenology of the plankton have been affected. This paper focuses on the long-term changes observed in the copepod community. Our findings highlight a switch towards a copepod community dominated by calanoids. In addition we have detected a period (1986 to 1991, 1988 excluded) characterized by an unusual seasonal pattern of species succession. It is suggested that these changes have been induced by qualitative changes in the phytoplankton community, and underline fluctuations in trophic interactions between planktonic species. Re-oligotrophication and warming, therefore, emerge as indirect forcing factors, and these findings contribute to the debate about whether copepods can be used as indicators of environmental changes.