Daniel J. Hornbach, Mark Hove, Sonya Clarkson, Liz Gilles, Ian Harmon, Erik Nelson, Kelly Paulson, Cristina Salazar, Emily Peters and Jessica Lynch
Dept. Biology, Macalester College, St. Paul, MN.

In the summer of 2000 we quantitatively assessed 4 mussel communities in the St. Croix River downstream of St. Croix Falls. We had sampled these populations
at least once before: 1992 at Bayport, 1993 at Osceola, 1995 at Lakeland and 1992, 1995 and 1998 at Interstate Park. Population density was greatest at Interstate Park (average 29-39 mussels/m²), followed by Lakeland (14-18 mussels/m²), Bayport (5-10 mussels/m²) and Osceola (7-9 mussels/m²). Twenty-eight species of mussels were found at Interstate Park, with 25, 24 and 16 species at Lakeland, Osceola and Bayport, respectively. The endangered Lampsilis higginsii was found at all locations except Bayport and the endangered Quadrula fragosa was found at Interstate Park. The Lakeland location had the highest species diversity (H'=2.0) followed by Osceola (H'=1.88), Interstate Park (H'= 1.87) and Bayport (H'=1.44). Lakeland had 5 dominant species while Bayport and Osceola had 2 dominant species and Interstate Park only had 1 dominant species. Zebra mussels were found at Lakeland in 2000.

At all 4 locations mussel density was lower in 2000 compared to earlier periods. The most dramatic decline, 51% was at Bayport; other populations had declines of 22-26%. None of the changes were statistically significant. However, juvenile mussel density (mussels < 30mm), at all locations except Osceola showed a significant decline over the period. Two possible reasons for the decline in juvenile density could be high juvenile mortality, or lack of recruitment. Shell-length frequency diagrams for dominant species show a general lack of recruitment during this time period. Since mussels require approximately 6 years to mature, long periods of low recruitment may be possible.

An increase in fine sediments was noted at all locations. Previous studies found neither increased sedimentation nor lack or recruitment above the dam at St. Croix Falls. This suggests that increased sedimentation below the dam may be influencing mussel recruitment or juvenile mussel survival.