EXAMINATION OF MUSSEL POPULATION AND COMMUNITY STABILITY AT INTERSTATE PARK, ST. CROIX RIVER, MN AND WI.

D.J. Hornbach, M. Kemperman, V. Kurth, A. Andersen, L. Cunningham, F. Hareldson, B. Miller,
and P. Vanselow. Department of Biology, Macalester College, St. Paul, MN 55105.

The Interstate Park area of the St. Croix River contains an abundant and diverse mussel assemblage. This area is subjected to daily variations in water level because of a hydroelectric dam. There is controversy concerning the most appropriate water release regime for this facility which would protect and enhance mussel habitat. We have quantitatively sampled (at least 100 0.25-m 2 quadrats) this location in 1992, 1995 and 1998. After removing the substrate and mussels from the quadrats, we sieved the material to assess substrate size and to collect all mussels. We were able to collect mussels <10 mm using this procedure. We identified and measured the shell length of all mussels. At each quadrat we also measured water depth and velocity. We found significant decreases in both average mussel density and average species richness over this 6 year period. We found that there appeared to be strong young age classes in 1992 and an absence of another large reproductive output during this period. The decrease in mussel density, thus may be due to higher juvenile mortality. It is unknown whether the presence/absence of strong recruitment peaks is a 'natural' event or a response to habitat alterations.