Changes in the dreissenid community in the lower Great Lakes with emphasison southern Lake Ontario

A field study was conducted in the lower Great Lakes to assess changes in s patial distribution and population structure of dreissenid mussel populatio ns. More specifically, the westward range expansion of quagga mussel into w estern Lake Erie and toward Lake Huron was investigated and the shell size, density, and biomass of zebra and quagga mussel with depth in southern Lak e Ontario in 1992 and 1995 were compared. In Lake Erie, quagga mussel domin ated the dreissenid community in the eastern basin and zebra mussel dominat ed in the western basin. In southern Lake Ontario, an east to west gradient was observed with the quagga mussel dominant at western sites and zebra mu ssel dominant at eastern locations. Mean shell size of quagga mussel was ge nerally larger than that of zebra mussel except in western Lake Erie and on e site in eastern Lake Erie. Although mean shell size and our index of numb ers and biomass of both dreissenid species increased sharply in southern La ke Ontario between 1992 and 1995, the increase in density and biomass was m uch greater for quagga mussels over the 3-year period. In 1995, zebra musse ls were most abundant at 15 to 25 m whereas the highest numbers and biomass of quagga mussel were at 35 to 45 m. The quagga mussel is now the most abu ndant dreissenid in areas of southern Lake Ontario where the zebra mussel w as once the most abundant dreissenid; this trend parallels that observed fa r dreissenid populations in the Dneiper River basin in the Ukraine.