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要旨

Rotifers and Daphnia magna showed strongly alternating population fluctuations in Gro-beta-er Binnensee, a shallow, brackish water lake in Northern Germany. Daphnia appeared relatively late in the season and produced four pronounced biomass peaks between July and November. Three of the four peaks were associated with clear-water phases and reduced concentrations of sestonic carbon. The highest rotifer densities occurred before the daphnids appeared and, later in the season, the rotifer populations crashed when daphnids became abundant. Rotifer density was negatively correlated with Daphnia biomass but positively with POC levels. Most of the variation in rotifer abundance could be explained by fluctuations of POC levels, suggesting the existence of exploitative competition as a mechanism causing the inverse relationship between rotifers and daphnids. However, a closer inspection of the time course of events indicates that mechanical interference may also be a factor and may have increased in importance as the season progressed and as the average size of daphnid individuals increased.