Changes in the Freshwater Mussel Community of Lake St. Clair: from Unionidae to \u3ci\u3eDreissena polymorpha\u3c/i\u3e in Eight Years

To determine density changes in both the zebra mussel, Dreissena polymorpha, and native mussels, Unionidae, in Lake St. Clair, surveys were conducted in 1990, 1992, and 1994 and compared to a similar survey in 1986 when no D. polymorpha was found. Collection methods were the same each year; divers used the quadrat method to collect 10 replicate samples at 29 sites located throughout the lake. The total number of unionids collected declined from 281 in 1986, to 248 in 1990, 99 in 1992, and 6 in 1994, while the number of species collected in each of the four respective years was 18, 17, 12, and 5. The decline in the unionid community occurred gradually over this time period as the D. polymorpha population expanded from the southeast region of the lake to the northwest region. Mean density and biomass of D. polymorpha throughout the lake was 1,700 m-2 and 4.7 gDW m-2 in 1990, 1,500 m-2 and 3.5 gDW m-2 in 1992, and 3,200 m-2 and 3.1 gDW m-2 in 1994. The density increase can be attributed to the expansion of the population into the northwest region, while the decrease in biomass was mostly a result of a decline in the weight per unit length. Mean biomass of the D. polymorpha population in 1994 was actually lower than the mean biomass of unionids in 1986; however, based on literature-derived filtering rates, the filtering capacity of the D. polymorpha population in 1994 was 12 times greater than the filtering capacity of the unionid community in 1986. This increase has likely led to reported changes in the Lake St. Clair ecosystem (increased water clarity, increased plant growth, and shifts in fish communities)