Dreissenids in Lake St. Clair in 2001: Evidence for Population Regulation

Zebra mussel ( Dreissena polymorpha [Pallas]) density was surveyed at 12 stations in Lake St. Clair in September 2001. Lake-wide mean density was 1,824 individuals/m 2; whole wet biomass was 148 g/m 2; and dry tissue biomass was 1.23 g/m 2. Compared to historical data, density did not change significantly, whereas biomass showed a significant downward trend. Our data support the assertion that the zebra mussel population in Lake St. Clair has undergone important changes since the mid-1990s. Some areas of the lake are now juvenile-dominated, others are adult-dominated, and some have a balanced size distribution. These data are consistent with the hypothesis that zebra mussels have changed the lake ecosystem in two ways that have contributed to their own population limits in a density-dependent manner. First is the reduction of adult microhabitat due to the elimination of native mussels from the lake proper. Second is the massive redirection of larval settlement onto a greatly expanded aquatic macrophyte community which senesces and dies at the end of each season, thus decreasing survivorship of juvenile D. polymorpha. If sustained, these recent changes, especially biomass reduction, suggest that the impact of dreissenids on the Lake St. Clair ecosystem will be more moderate in the future.