Didymosphenia geminata is a diatom native to the northern hemisphere. It recently changed its growth pattern circum-globally, forming dense mats of mucilaginous material.  The conspicuousness of the previously cryptic algae has caused concern among ecologists and recreationalists. Concern is probably warranted, as most benthos depend either directly (i.e., scrapers, shredders) or indirectly (i.e., predators, collectors) on the composition of biofilms. The growth of this diatom physically alters the habitat and may reduce the success of certain invertebrate groups.  We sampled five locations (each on three separate months) on the Kootenai River below Libby Dam in northwestern Montana. Three Slack Sampler samples were collected along a transect perpendicular to the shoreline (n=45). From each sample, three cobbles were removed and a quantitative periphyton sample was collected and analyzed for AFDM. A variety of habitat variables were also measured within each slack sampler. We report the relationship among several biological metrics, functional feeding groups, and invertebrate orders using multiple and non-linear regressions. AFDM was a stronger predictor of many metrics than any other habitat variable, including flow. Relative abundance of Shredders and Scrapers maximized at 3-5mg/cm² AFDM and were absent from biofilms with masses >8mg/cm², where they were replaced by burrowers.