Mark Hove, Katie Dietrich, Carlos Gonzalez, and Daniel Hornbach
Dept. Biology, Macalester College, St. Paul, MN 55105
 

Diverse and dense mussel communities can be found at a number of locations in the St. Croix River. Between June-August 2003 we quantitatively assessed 4 mussel communities and their habitat. The study was conducted using standard methods to quantitatively describe the mussel communities at: Riverside, WI; Wild River State Park, MN; Franconia, MN; and Prescott, WI. We sampled these populations at least twice before: Riverside in 1992 and 2001, Wild River in 1993, 1996, 1999, and 2001, Franconia in 1991, 1995, 1999, and 2001, and Prescott in 1994, 1999, and 2001. In 2003 we observed 18 species at Wild River, 17 species at Franconia, 16 species at Prescott, and 10 species at Riverside. Over the last 12 years two species numerically dominate the mussel communities at Riverside, Wild River, and Franconia, and three species at Prescott. The federally endangered Higgins’ eye was collected at Prescott as were invasive zebra mussels. Average mussel density was highest at Wild River (17.7 mussels/m2), followed by Prescott (9 mussels/m2), Franconia (5.4 mussels/m2), and Riverside (2.5 mussels/m2). Over time mussel density has decreased at all sites except Prescott. The decline has been up to 50% at select sites, but due to the variability in mussel density these changes were statistically significant only at Riverside. Shell-length frequency diagrams suggest there has been recent recruitment among some dominant species at select sites. The low level of recruitment at some sites in the river has been of concern in the past and despite the new recruitment, juvenile density is lower at all of the sites except Riverside compared to earlier sampling periods (significantly so at Franconia). Sediment particle size did not change significantly over time at these sites.

The St. Croix National Scenic Riverway and the NRPP-Threatened and Endangered Species Fund provided administrative, financial, and field support.

SUGGESTED READING:

Hornbach, D. 2000. Macrohabitat factors influencing the distribution of naiads in the St. Croix River, Minnesota and Wisconsin, USA. Pages 213-230 in Ecology and evolution of the freshwater Unionoida. Bauer, G, and K. Wachtler (Eds.). Springer-Verlag, Berlin, Germany. 394 pp.