MUSSEL DENSITY AT INTERSTATE PARK, ST. CROIX RIVER, MN AND WI: A NEW EQUILIBRIUM?

Daniel J. Hornbach, Mark C. Hove and Kelly MacGregor
Departments of Biology, Environmental Studies and Geology, Macalester College. St. Paul, MN 55105.

Interstate Park on the St. Croix River contains a dense and diverse assemblage of mussels. We have established a long-term monitoring program at one of the most diverse mussel beds, which is located just downstream of a hydroelectric dam. Since 1992 we have quantitatively sampled the mussel assemblage and habitat characteristics of a five-km stretch of the river at Interstate Park every 2-3 years.

The density of adult mussels was fairly constant between 1992-2000 (approximately 20 mussels/m²). From 2002-2008 adult density was about 25% lower (approximately 15.5 mussels/m²). Despite these differences there was no statistically significant difference in adult density over this period.

There were significant changes in juvenile mussel density over this period. The juvenile density was 4.8 mussels/m² in 1992. Since that time the highest juvenile mussel density was 2.8 mussels/m² in 2000, with densities ranging from 0.07 to 1.7 mussels/m² from 2002-2008. This reduced juvenile mussel density may be responsible for the new lowered level of adults found at Interstate.

The change in mussel density is not uniform across species. The four dominant mussel species at Interstate are Truncilla truncata, Quadrula pustulosa, Actinonaias ligamentina and Fusconaia flava constituting 52, 7, 6, and 4% of the community, respectively. For T. truncata and F. flava there were significant decreases in juvenile density while there was no significant difference in density for Q. pustulosa and A. ligamentina.

Substrate was coarsest in 1992 (average particle size 3.6 mm) and became steadily finer through 2002 (1.2 mm). In 2005 and 2008 the substrate was coarser (1.5 and 1.6 mm respectively). There was a period from 1986 to spring 1995 when the annual peak water discharge in the river did not exceed 30,000 cfs. This was the longest stretch in the last 50 years without peak flows exceeding 30,000 cfs. We are examining the decrease in peak flow between 1986 and 1995 as a possible cause of decreased grain size. We are also examining whether a period with low peak discharge may be related to the higher than normal mussel recruitment resulting in high juvenile and adult density in 1992. With the possible return of "normal" conditions of finer sediment, it is also possible that the adult and juvenile densities found since 1995 are actually more "typical" of this are and the high densities in 1992 are actually an anomaly.