Mark Hove, Dan Allen, Russel Derhak, Kristin Swenson, Jana Thomas, and Daniel Hornbach
Dept. Biology, Macalester College, St. Paul, MN

The St. Croix River is a nationally important ecological resource with a diverse mussel community that serves as a refuge for upper Mississippi River fauna. During the summer of 2001 we quantitatively assessed 4 mussel communities and searched for federally endangered winged mapleleaf in the St. Croix River. Standard methods were employed to quantitatively describe the mussel communities at: Riverside, WI; Wild River State Park, MN; Franconia, MN; and Prescott, WI. We had sampled these populations at least once before: at Riverside in 1992, at Wild River in 1993, 1996, and 1999, at Franconia in 1991, 1995, and 1999, and at Prescott in 1994 and 1999. We observed 17 species at Wild River and Franconia, 14 species at Prescott, and 9 at Riverside. Average mussel density was highest at Wild River (14.5 mussels/m²), followed by Prescott (5.3 mussels/m²), Franconia (4.4 mussels/m²), and Riverside (3.1 mussels/m²). The federally endangered Higgins eye was collected at Prescott as were zebra mussels. Over the last 10 years two species numerically dominate the mussel communities at Riverside, Wild River, and Franconia, and three species at Prescott. Over time mussel density has dropped 50-60% at Riverside, Wild River, and Franconia, and 32% at Franconia but due to the variability in mussel density none of these changes are statistically significant. Shell-length frequency diagrams suggest there has been recent recruitment among some dominant species at select sites.

Results from the summers winged mapleleaf survey reveals these animals once enjoyed a much broader distribution in the St. Croix River. We used SCUBA equipment to survey 21 locations for 15-90 min between Nevers Dam and Stillwater, MN. More time was spent at select sites. All live and dead mussels were recorded. Some empty valves were deposited as voucher specimens at the Bell Museum. Diverse, dense mussel beds were uncommon and were generally where we found winged mapleleaf valves. Although no live winged mapleleaf were observed empty valves were collected as far upstream as Wild River State Park and downstream as far as Arcola, MN. These results show the species once occupied at least an additional 42 km of river beyond its present 8 km range. Additional work is needed to describe the range of this species in the St. Croix River and determine if it is extant upstream of the dam.

Administrative and financial support provided by the St. Croix National Scenic Riverway and the NRPP-Threatened and Endangered Species Fund.

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.