Recovery of Lake Geneva from eutrophication: quantitative response of phytoplankton.

Abstract

During the seventies Lake Geneva (Switzerland-France) became eutrophie. Measures to reduce phosphorus inputs into the lake have been successful as the concen- tration of phosphorus has progressively decreased since 1981. This paper describes the long-term quantitative response of phytoplankton (primary production, total phytoplankton biomass, and chlorophyll-a) with regard to the decline in soluble reactive phosphorus (SRP) concentration. The annuaI means of phytoplankton parameters do not show the expected decrease. The long-term trends of annual means appeared to be the resultant of compensating trends which occurred in each season. SRP concentrations indicate that phytoplankton may be phosphorus limited only during summer which appears to be the most appropriate season to study the quantitative response of phytoplankton. From 1981 to 1992, the summer algal biomass and chlorophyll-a have been correlated to the late winter concentration of SRP. However, in the recent years, algal biomass and chlorophyll-a increased while phosphorus remained at low concentrations. This paradoxal increase in total biomass was due to the accumulation of inedible filamentous algae. The observed resilience of phytoplankton is a problem in terms of lake management. These results stress the importance of an appropriate time scale study and bring back into question the use of parameters such as primary production, total phytoplankton biomass, and chlorophyll-a for lake management.