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

Data on two co-existing Daphnia species, D.cucullata (G.O.Sars) and D.hyali na (Leydig), from three neighboring and interconnected lakes in northern Po land, revealed an extremely confined range of population density fluctuatio ns 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 w ith 95% confidence limits of +/-2.5 and +/-0.5 ind. l(-1), respectively), i n spite of markedly changing fecundity. There was no apparent phase of decl ine or increase that could br related to the distinct phases of low or high fecundity resulting from different food levels. Analysis of the gut conten ts of roach (Rutilus rutilus), the dominant planktivore in the three lakes, revealed identical selectivity for each of the two Daphnia species, sugges ting that the lower density of the D.hyalina population was compensated for by the greater conspicuousness of individuals of this species. It is concl uded that the population density of these Daphnia species remains far below the carrying capacity of the habitat and does not depend on food levels. F ood availability merely sets the rate of population increase, while the act ual population density in the lakes studied reflects the species' vulnerabi lity to predation by planktivorous fish.