The effect of Daphnia body size on filtering rate inhibition in the presence of a filamentous cyanobacterium.

We tested the hypothesis that shifts in the size structure of daphnid assemblages may be caused by differing sensitivities of particular species to interfering filamentous cyanobacteria. Three Daphnia species of different size were fed the green alga Scenedesmus and filtering rates were measured in the presence and absence of straight filaments of the cyanobacterium Cylindrospermopsis raciborskii. Cylindrospermopsis was not lethally toxic to Daphnia, but was inadequate as the sole food. The filtering rates of all three Daphnia species were reduced in the presence of the filaments. The larger Daphnia pulicaria and Daphnia hyalina were more affected than the smallest, Daphnia cucullata, and within the size range of each species larger individuals were more affected than smaller ones. Our results are consistent with field observations that body size is an important factor determining the competitive success of Daphnia species in the presence of cyanobacterial filaments.