Mark Hove, Dan Allen, Katie Dietrich, Carlos Gonzalez, Kristin Swenson, and Daniel Hornbach
Dept. Biology, Macalester College, St. Paul, MN

The diverse and unique mussel community in the St. Croix River is a nationally recognized resource. We quantitatively assessed mussel communities during the summer of 2002 at: Interstate State Park, MN; Osceola, WI; Lakeland, MN; and Bayport, WI. These communities have been sampled at various times during the last 10 years: at Interstate State Park in 1992, 1995, 1998, and 2000, at Osceola in 1993 and 2000, at Lakeland in 1995 and 2000, and at Bayport in 1992 and 2000. During 2002 we observed 21 species at Interstate State Park, 19 at Lakeland, 13 at Osceola, and 9 at Bayport. The federally endangered Higgins eye was collected at Lakeland. Zebra mussels were also found at Lakeland. During the last 10 years 5 mussel species have numerically dominated the mussel community at Lakeland, 2 species at Bayport and Osceola, and 1 species at Interstate State Park. Average mussel density during 2002 was highest at Interstate State Park (14.6 mussels/m^2), followed by Lakeland (9.3 mussels/m^2), Bayport (5.2 mussels/m^2), and Osceola (2.6 mussels/m^2). Since 1992 total mussel density has declined significantly at Interstate State Park. Also,  declines in juvenile mussel density have occurred at many sites over the ten-year period.  Shell-length frequency diagrams suggest there has been little recruitment or there is low juvenile survival among most dominant species at all four sites. The decline in juvenile density at these four sites is consistent with data from four other sites in the St. Croix River, which suggests that a lack of recruitment or low juvenile survivorship is a system-wide issue. The reasons for the apparent decline in recruitment are unknown but at Interstate State Park there has been sediment deposition over the past 10 years. Declines at Interstate State Park are of particular concern due to its valuable mussel assemblage including the federally endangered winged mapleleaf.

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

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.