Benthic macroinvertebrate communities were quantified at natural cobble and artificial reef sites in Lake Ontario in 1983 (7 y pre-Dreissena invasion) and in 1991 to 1992 and 1999 to 2000 (1–2 and 9–10 y post-Dreissena invasion, respectively). Overall, the cobble community had higher diversity and abundance than the reef community. In both communities, diversity and abundance of non-Dreissena taxa rose sharply between 1983 and 1991 to 1992 and declined to 1983 levels by 1999 to 2000. However, taxonomic composition (excluding Dreissena and the recent invader Echinogammarus ischnus) was consistent across study years. Between 1983 and 1999 to 2000, Stagnicola catascopium and Physella spp. increased in abundance, several taxa (Musculium partumeium, Bithynia tentaculata, Elimia livescens, Trichoptera, and Chironomidae) decreased in abundance, and changes in the abundance of many taxa were correlated with Dreissena abundance. Dreissena continued to make up >40% of total macroinvertebrate abundance at the cobble site and >60% at the reef site in 1999 to 2000; however, numbers dropped sharply and the size of individual mussels increased as D. bugensis largely replaced D. polymorpha between 1991 to 1992 and 1999 to 2000. Dreissena biomass dropped sharply at the reef site between 1991 to 1992 and 1999 to 2000, but not at the cobble site. The exotic amphipod E. ischnus was abundant in 1999 to 2000, but any effect on the abundance of the related amphipod Gammarus fasciatus was unclear. At the time of this study, the round goby Neogobius melanostomus was not present. We conclude that the transition from D. polymorpha to D. bugensis and processes associated with the ongoing oligotrophication of Lake Ontario are responsible for reduced density of larger-bodied Dreissena in the nearshore region, and that changes in the Dreissena population are largely responsible for changes in the non-Dreissena benthic macroinvertebrate communities.
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