There are several specific goals of the research presented in the following chapters: (1) to determine the effects of Orconectes propinquus on the algal and insect communities in caging experiments; (2) to expand this investigation to different areas in the Maple River to determine how the complex interactions between consumer control and resource enhancement affect algal abundance; (3) to determine how, in the context of the natural stream, nutrient enrichment and consumer control affect the algal species composition; and (4) to examine whether O. propinquus alters the abundance of benthic invertebrates. ... This dissertation examined how the crayfish, Orconectes propinquus, affects invertebrates, and periphyton in a low nutrient stream in the northern portion of the lower peninsula of Michigan, U.S.A. This study was conducted to examine suggestions that the crayfish O. propinquus functions as a keystone species in streams. While the results of this dissertation research cannot conclusively reject the hypothesis that crayfish are keystone species in all streams, the evidence presented here does not support that role for O. propinquus in the Maple River.