Mark C. Hove*, Thomas W. Hermanson, Leda A. Cunningham, Katie G. Esse, Jennifer L. Mann, Jensen C. Hegg, and Daniel J. Hornbach
Macalester College, Biology Department, 1600 Grand Avenue, St. Paul,

Minnesota 55105, USA

The St. Croix River holds one of the most diverse mussel communities in the upper Midwest. Population dynamics were assessed at seven locations since 1991. Mussels and substrate were collected from 100 0.25 m^2 quadrats at each location. Substrate was separated into 5 size classes and all mussels were identified and measured. From these measurements, population density and community diversity were calculated at each location and compared to past sampling years. At Franconia mussel density decreased from 10.4 mussels/m^2 in 1991 to 9.8 mussels/m^2 in 1995, and 4.5 mussels/m^2 in 1999. Species richness decreased from 26 species in 1991 to 19 species in 1995, and 15 species in 1999. At Wild River State Park, mussel density decreased from 37.4
mussels/m^2 in 1993 to 29.6 mussels/m^2 in 1996 continuing to decline to 21.1 mussels/m^2 in 1999. Species richness decreased from 21 species in 1993 to 18 species in 1996, and remained the same in 1999. At Prescott, Wisconsin mussel density decreased from 7.8 mussels/m^2 in 1994 to 5.6 mussels/m^2 in 1999, while species richness declined from 21 to 18 species. The remaining
locations will be studied in 2000 and data analyzed to determine if trends are significant and warrant management action.