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Von 1995 bis 1998 wurde die Cladoceren- und Ostracodenfauna des eutrophen Feldungelsees nördlich Osnabrück untersucht. Die Anzahl der Cladocerenarten ist gegenüber einer früheren Untersuchung auf 39 gestiegen. Ceriodaphnia laticaudata, Megafenestra aurita und Simocephalus exspinosus v. congener traten erstmalig im See auf. Die zwei Daphnia-Arten aus Nordamerika, D. ambigua und D. parvula, bewohnen weiterhin den See. 1995 wurden die jahreszeitliche Verbreitung und die Sexual perioden untersucht. Bei den meisten Cladocerenarten wurde Monozyklie beobachtet, 3 Arten traten dizyklisch auf. 17 Muschelkrebsarten wurden nachgewiesen, die für diese Gegend und diesen Lebensraum charakteristisch sind.
Resting egg banks of microcrustaceans have been used to reconstruct the evolutionary and ecological history of species. However, recent studies provided evidence for a discrepancy between dormant propagules in the sediment and the planktonic population. This pattern raises two questions: First, what is the value of data on resting egg banks for population dynamics over time and second, which component of the reproductive cycle causes the observed inconsistency? In our study we focussed on the second question by comparing the taxon composition of a resting egg bank with the reproductive success of ex-ephippial hatchlings. Species and interspecific hybrid identification of dormant and hatched stages was achieved through the application of restriction fragment length polymorphism analysis of an internal transcribed spacer region. We found no significant deviation between the proportion of hatched Daphnia galeata, D. galeata x hyalina and D. hyalina individuals and the observed taxon composition of the resting egg bank. However, species and hybrids differed in their mode and relative success of reproduction. We conclude that the components of reproductive success in Daphnia contribute differentially to the fitness of species and interspecific hybrids. The discrepancy between resting egg banks and "active" planktonic populations results not from differential hatching of species but from the reproductive success of ex-ephippial females and the timing and frequency of sexual reproduction of the different taxa.