SMALL MUSSEL DENSITY STILL LOW AT INTERSTATE STATE PARK, ST. CROIX RIVER, MINNESOTA AND WISCONSIN

Daniel J. Hornbach, Mark C. Hove, Matthew J. Cox, Ben D. Dickinson, Elana G. Evans, and Emily A. Kushner, Macalester College, Department of Biology

The St. Croix River at Interstate State Park, Minnesota and Wisconsin, houses a dense and diverse freshwater mussel assemblage. Included in the assemblage are the federally endangered winged mapleleaf (Quadrula fragosa) and Higgins eye (Lampsilis higginsii), and twelve state-listed species. We have quantitatively sampled the mussel assemblage at this location 6 times since 1992 (about every 2-3 years). In 1992 the overall density of mussels at Interstate averaged 38.4 mussels/m2. The density declined to a low of 14.6 mussels/m2 in 2002, with a slight rebound to 18.5 mussels/m2 in 2005. We partitioned sampled mussels into large (= 30 mm shell length) and small (< 30 mm) size classes. Large mussel density did not change significantly over this period, from a high of 22.6 mussels/m2 in 1995 and 1998, to a low of 13.9 mussels/m2 in 2002, and 15.5 mussels/m2 in 2005. We observed significant differences in small mussel density from a high of 19.2 mussels/m2 in 1992 to a low of 0.8 mussels/m2 in 2002, with a slight rebound to 3.0 mussels/m2 in 2005. The densities of small mussels from 2002 and 2005 are significantly lower than earlier years. We further partitioned small mussels into 3 size classes: 0-10 mm, 10-20 mm and 20-30 mm. There were significant declines in the density of 10-20 mm and 20-30 mm mussels from 1992 through 2005. There were no significant differences in the density of the smallest mussels (0-10 mm) among years, with 1992 having 0.56 mussels/m2 and 2005 having approximately half that number (0.27 mussels/m2).

Several factors influence these trends. We collected relatively few of the smallest mussels and variance was large. Although density of the smallest mussels (= 1 year) did not change, there were significant declines in slightly older mussels suggesting there is little difference in juvenile recruitment but a difference in juvenile survivorship. Also, the decline in the number of small mussels is not equally distributed across species. Small Truncilla truncata declined from 2.2 mussels/m2 in 1992 to 0.25 mussels/m2 in 2005, a 10-fold decline in density. The densities of small Truncilla donaciformes and Fusconaia flava declined from 0.5 mussels/m2 in 1992 to 0 in 2002 and 2005, and 0.3 small mussels/m2 in 1992 to 0 in 2002 and 2005, respectively. During the same period, small Actinonaias ligamentina density remained relatively constant, ranging from 0.13 mussels/m2 in 2005 to 0.22 mussels/m2 in 1992. Finally, there was a significant increase in the amount of fine sediment at Interstate between 1992 and 2005. Average particle size (phi) dropped from 4.8 to 0.8, meaning the fraction of substrate comprised of sand increased from 38% in 1992 to 64% in 2005. The causes for increased sediment accumulation at Interstate State Park are unknown, but careful monitoring of this site and changes in discharge levels are needed.