Sonya Clarkson, Mark Hove, and Kelly Paulson
Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105

The St. Croix River contains a diverse, abundant mussel community. Our goal was to quantitatively assess mussel assemblages at various locations and examine temporal variations associated with habitat changes. This summer we collected at least one hundred quadrat samples at each of four locations in the lower St. Croix River. Substrate samples were poured through a series of sieves, each of which was weighed to determine substrate coarseness. All mussels were identified and measured. Downstream of the St. Croix Falls dam fine sediment levels have increased significantly since 1995. Mussel density and species richness have decreased significantly over the same period of time. The increase in fine sediments below the dam may have negative effects on mussel recruitment. Studies designed to differentiate between natural cycles of recruitment due to population versus environmental factors leading to changes in mortality need to be conducted to identify mechanisms driving community dynamics.