Patterns of algal abundance and community composition were assessed in related to both regional and local environmental variables under low flow conditions in the Raisin River during the summer of 1996.  Algal abundance, as measured by chlorophyll a concentrations and cell abundance, was strongly negatively related to canopy cover.  Current was positively related to algal abundance, presumably by increasing diffusion gradients (15-50 cm/s).  Nutrient concentrations were determined to be above typical limiting levels (N-NO3/NO2=0.5mg/l, P-PO4=0.035 mg/l) and were unrelated to algal abundance.  Regional variables in the form of agricultural land use and fine textured surface geology were correlated with increased sediments in the streams, which may have had a deleterious affect on live algae.  However, ash-free dry mass (AFDM) was positively correlated with higher percentages of these regional variables.  AFDM overestimated algal abundance due to the presence of depositional organic materials and algal senescence.

Algal community composition was strongly related to agriculture, fine textured surface geology, shade, and soluble reactive phosphorous (P-PO4).  Blue-green and green algae were most abundant at sites with high phosphorous.  Diatom guilds, based on morphological characteristics, showed that the larger and/or motile guilds (Navicula spp. And Nitzschia spp.) dominated at sites with high nutrients, agriculture, and fine textured surface geology.  These taxa appeared to be the most tolerant of human modification to the stream.  The Achnanthes spp., stalked, and erect guilds were associated with sites with low agriculture, fine textured geology, phosphorus and shade.  This study suggests that sedimentation has a strong impact on the abundance and composition of algae in nutrient enriched streams.