CHANGES IN THE FRESH-WATER MUSSEL COMMUNITY OF LAKE ST-CLAIR - FROM UNIONIDAE TO DREISSENA-POLYMORPHA IN 8 YEARS

To determine density changes in both the zebra mussel, Dreissena polym orpha, 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 sam ples at 29 sites located throughout the lake. The total number of unio nids 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 fo ur respective years was 18, 17, 12, and 5. The decline in the unionid community occurred gradually over this time period as the D. polymorph a population expanded from the southeast region of the lake to the nor thwest region. Mean density and biomass of D. polymorpha throughout th e 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 dens ity increase can be attributed to the expansion of the population into the northwest region, while the decrease in biomass was mostly a resu lt 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 r ates, the filtering capacity of the D. polymorpha population in 1994 w as 12 times greater than the filtering capacity of the unionid communi ty in 1986. This increase has likely led to reported changes in the La ke St. Clair ecosystem (increased water clarity, increased plant growt h, and shifts in fish communities).