Effects of three Orconectes crayfishes on epilithic microalgae: A laboratory experiment

We tested the implications of littoral zone food web changes for periphyton abundance by comparing algal removal rates of three Orconectes crayfishes and a grazing snail (Amnicola sp. or spp.) in a laboratory experiment. Peri phyton communities were established on unglazed clay tiles incubated in gra zer free enclosures in the littoral zone of Carrol Lake, Wisconsin. In the laboratory tiles were placed in individual arenas that were randomly assign ed to one of four grazing treatments or a control treatment. After 96 h, to tal algal biovolume was reduced except by O. rusticus. However, when algal removal rates were expressed as per unit shell-free dry weight, Amnicola re moval rates were at least 17x higher than that of any crayfish. These data are consistent with previous field observations that show snails remove (pr imarily grazing) periphyton more efficiently than crayfish. Combined with p revious work on crayfish predation on snails, our results suggest that cray fishes have a strong indirect positive effect on periphyton by reducing sna il abundances, whereas the direct effects due to crayfish grazing are minim al.