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

Nonconsumptive mortality of mesozooplankton in Lake Constance was directly estimated by collecting dead zooplankton with sediment traps. Patterns of z ooplankton sedimentation observed in the sediment trap reflected the popula tion dynamics in the pelagic zone. The migration of Cyclops vicinus toward the sediment (where they spend their diapause) at the end of May resulted i n a pronounced occurrence in the traps (migration rate >10(6) individuals ( ind.) m(-2) d(-1)). Other copepods and Daphnia hyalina had only sedimentati on losses of 0.5 and 0.2% of the standing stock per day, respectively, demo nstrating the minor role of nonconsumptive mortality for these species. In contrast, nonconsumptive mortality had a high significance on the populatio n dynamics of nonmigrating epilimnetic Daphnia galeata. High sedimentation rates (up to 3 x 10(3) ind. m(-2) d(-1); in total, 38% of the total populat ion decline from 16 x 10(4) to 1 x 10(4) ind. m(-2) within 21 d) in June 19 93 were attributed to an unidentified infection. From April to November, lo sses 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 p roduction (10% of the standing stock per day).