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Zusammenfassung:
We studied over two weeks the decay of a population of Daphnia pulex that had invaded the previously cladoceran-free epilimnion of Lake Ciso. The cladoceran was found at concentrations up to 1,900 ind l-1 in the oxygenated epilimnion of the lake while most of the available food was concentrated in the metalimnion. D. pulex was never found in the hypolimnion with sulfide concentrations up to 1.2 mM. Probably due to the food distribution, D. pulex was forced to move into the anoxic lower part of the metalimnion. The animals spent most of the day at the surface, and descended especially at dawn to the upper level of the sulfide distribution. Mainly animals of sizes between 1.3 and 1.9 mm migrated. This pattern of population movement was related only to the food distribution and the oxygen profiles, and not to other factors commonly implicated in zooplankton vertical migrations (fish predation was absent). There was strong variability in the horizontal distribution of the animals that we associated to swarming behavior. During our study the population was in decline as shown by the lower indices of fertility and the higher mean sizes of the animals in the later weeks of sampling as compared to the first week
