Zebra mussel, Dreissena polymorpha (Pallas), proliferation in Saginaw Bay, Lake Huron is associated with increased water clarity and increased light levels on benthic substrata in the littoral zone. We hypothesized that the filtering activities of Dreissena and associated increases in light penetration should affect the structure and function of benthic algae in the bay. Monthly quantitatve benthic algal samples were collected from natural substrata by SCUBA in the littoral zone of the bay through the growing seasons of 1991 (initial Dreissena colonization), 1992 and 1993 (post-Dreissena colonization). Algal community structure was examined microscopically and productivity rates were measured using carbon-14 in sealed acrylic chambers in situ. Our data demonstrate that, following Dreissena proliferation, light penetration, benthic algal biomass, chlorophyll concentrations and rates of benthic primary productivity have increased. These changes coincided with a shift from diatom domination of the benthic algal community to a flora dominated by filamentous green algae (Zygnematales). We suggest that these shifts have the potential to affect benthic food webs within littoral zones of the Great Lakes.