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

The diverse mussel community in the St. Croix River is an important national 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. During 2002 we observed 21 species at Interstate State Park, 19 at Lakeland, 13 at Osceola, and 9 at Bayport. 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/m2), followed by Lakeland (9.3 mussels/m2), Bayport (5.2 mussels/m2), and Osceola (2.6 mussels/m2). 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.