SHORT-TERM CRAYFISH EFFECTS ON STREAM ALGAE AND INVERTEBRATES

Short-term caging experiments were conducted to examine the direct eff ects that the crayfish Orconectes propinquus has on benthic communitie s. Stream-colonized tiles (precolonized within the stream) and uncolon ized clay tiles were placed in eight elevated, hardware-cloth metal ca ges. Four cages had crayfish (similar to 8 crayfish m(-2)); the other four were empty controls. Chl a measured on pre-colonized tiles five d ays after the commencement of the experiment indicated that crayfish h ad no significant effect on algal biomass. O. propinquus also had no m easurable impact on snail abundance from tiles sampled after five days . Samples taken after seventeen days from initially uncolonized tiles showed that crayfish had a direct negative effect on diatom abundance. Tiles with crayfish had 50% fewer diatoms than control cages, althoug h there were no differences in the relative abundance of the 6 most ab undant diatom genera. Insects collected from these tiles (day 17) did not differ in density, relative abundance (by order), or size between crayfish treatments. The results indicate that crayfish can have direc t negative effects on the diatom portion of the algal community.