Daphnia populations in three interconnected lakes with roach as the principal planktivore

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Abstract
Data on two co-existing Daphnia species, D.cucullata (G.O.Sars) and D.hyalina (Leydig), from three neighboring and interconnected lakes in northern Poland, revealed an extremely confined range of population density fluctuations throughout summer in each of the two species in each of the three lakes, with D.cucullata being an order of magnitude more abundant than D.hyalina (mean of 11.5 ind. l–1 for D.cucullata and 1.1 ind. l–1 for D.hyalina with 95% confidence limits of ±2.5 and ±0.5 ind. l–1, respectively), in spite of markedly changing fecundity. There was no apparent phase of decline or increase that could be related to the distinct phases of low or high fecundity resulting from different food levels. Analysis of the gut contents of roach (Rutilus rutilus), the dominant planktivore in the three lakes, revealed identical selectivity for each of the two Daphnia species, suggesting that the lower density of the D.hyalina population was compensated for by the greater conspicuousness of individuals of this species. It is concluded that the population density of these Daphnia species remains far below the carrying capacity of the habitat and does not depend on food levels. Food availability merely sets the rate of population increase, while the actual population density in the lakes studied reflects the species' vulnerability to predation by planktivorous fish.

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