Estimates of the nonconsumptive mortality of mesozooplankton by measurement of sedimentation losses

Abstract

Nonconsumptive mortality of mesozooplankton in Lake Constance was directly estimated by collecting dead zooplankton with sediment traps. Patterns of zooplankton sedimentation observed in the sediment trap reflected the population dynamics in the pelagic zone. The migration of Cyclops vicinus toward the sediment (where they spend their diapause) at the end of May resulted in a pronounced occurrence in the traps (migration rate >106 individuals (ind.) m-2 d-1). Other copepods and Daphnia hyalina had only sedimentation losses of 0.5 and 0.2% of the standing stock per day, respectively, demonstrating the minor role of nonconsumptive mortality for these species. In contrast, nonconsumptive mortality had a high significance on the population dynamics of nonmigrating epilimnetic Daphnia galeata. High sedimentation rates (up to 3 X 103 ind. m-2 d-1; in total, 38% of the total population decline from 16 x 104 to 1 x 104 ind. m-2 within 21 d) in June 1993 were attributed to an unidentified infection. From April to November, losses of D. galeata as a result of nonconsumptive mortality (average of 2.3% of standing stock per day) accounted on average for 23% of the estimated production (10% of the standing stock per day).