THE EFFECT OF HERBIVOROUS CRUSTACEAN ZOOPLANKTON ON EPILIMNETIC CARBON AND PHOSPHORUS CYCLING

Plastic limnocorrals (LCs; volume 70 m3) in oligo-mesotrophic Lake Lucerne, Switzerland were used to manipulate planktonic communities by (1) removing large zooplankton at the start of the experiment, and (2) adding phosphate during experiments of two weeks duration. Primary production (C-14-assimilation), chlorophyll, standing crops and sedimentation of particulate organic carbon (POC) and of particulate phosphorus, as well as plankton composition were assessed simultaneously in two to four differently treated LCs. Carbon and phosphorus mass balances were calculated from assimilation, temporal change in standing crop of particulate matter, and sedimentation. A quick elimination of crustaceans by screening hardly increased primary production, but decreased sedimentation and mineralisation of particulate organic carbon, and thus significantly enhanced the standing crop of POC, but decreased POC turnover. The exclusion of crustaceans increased the mean residence time of total phosphorus by a reduced P loss by sedimentation. Increased grazing pressure during the experiment showed little grazing induced effect on plankton composition and primary production except at the very end. We conclude that in an oligo-mesotrophic lake, buffering mechanisms attenuate the impact of changing grazing pressures on primary production.