Direct and Indirect Effects of Crayfish Grazing in a Stream Community

In surveys and experiments in a Michigan stream I found that large crayfish (Orconectes propinquus) can indirectly facilitate epilithic diatoms and sessile, grazing insects by virtually excluding a filamentous alga (Cladophora glomerata) from deep water habitats. In shallow (<20 cm) habitats, Cladophora cover and ash—free dry biomass usually exceeded 75% and 50% g/m², respectively. In deep habitats (20—50 cm), Cladophora cover rarely exceeded 10% and an ash—free dry biomass was usually <3 g/m². Mean densities of large crayfish in deep water (3—11 individuals/m²) were 3—20 x greater than their densities in shallow water. Two field experiments were conducted to determine if large crayfish were responsible for the reduced abundance of Cladophora in deep water. In the first experiment, depth and current velocity were held constant for all substrata, and crayfish were excluded from half of the substrata. As physical factors such as current velocity and light may vary with water depth, a second experiment was conducted that compared the effect of substrata depth vs. crayfish grazing on Cladopohora abundance. In both experiments, Cladophora biomass was at least 10 x greater on substrata which were not grazed by large crayfish. In the second experiment, substratum depth did not have a significant effect on Cladophora abundance. The results of both experiments demonstrated that grazing by large crayfish was the primary mechanism generating the pattern of Cladophora distribution observed in Augusta creek. The virtual exclusion of Cladophora from deep water habitats by large crayfish can indirectly facilitate epilithic diatoms and sessile, grazing insects that consume these diatoms. Diatoms and sessile, insect grazers were found to be less abundant beneath Cladophora mats in field experiments. Total diatom abundance on substrata without a Cladopohora canopy was 20 x that of substrata with a Cladophora canopy. Two sessile, grazing insects (Psychomyia flavida and Leucotrichia pictipes) were 2—3 x more abundant on substrata which did not have heavy Cladophora cover. These results suggest that crayfish are a keystone consumer in this stream community, but the effect is habitat specific and related to crayfish body size.