Recent changes in the offshore crustacean zooplankton community of Lake Ontario

Crustacean zooplankton communities in the offshore of Lake Ontario have undergone substantial changes between 1997 and 2011. A shift was apparent in 2004 from an initial assemblage dominated by cyclopoid copepods (mostly Diacyclops thomasi), Daphnia retrocurva and bosminids, and with Cercopagis pengoi (which invaded the lake in 1998) the dominant predatory cladoceran, to one characterized by reduced numbers of cyclopoids, a more varied predatory cladoceran community, and increased biomass of calanoid copepods. These changes represented a shift from a community which has been relatively stable in the offshore of the lake for at least 40 years. A further change was seen in 2008, marked by increased biomass of Leptodiaptomus sicilis, Bythotrephes longimanus and Daphnia mendotae. Unlike the somewhat similar changes seen recently in the zooplankton communities of Lakes Huron and Michigan, the shifts in Lake Ontario have not been accompanied by a trend towards increased oligotrophy. A more likely explanation for the observed shifts in Lake Ontario is decreased vertebrate predation, likely due to alewife declines, and changes in the predatory invertebrate community.