Zooplankton community changes in Lake Ontario: species introductions and oligotrophication

Session: Great Lakes Lower Trophic Level Community Dynamics (1)

Kelly Bowen, Fisheries and Oceans Canada, [email protected]
Robin Rozon, Fisheries & Oceans Canada, [email protected]
Warren Currie, Fisheries and Oceans Canada, [email protected]

Abstract

Forces driving lower food web productivity of Lake Ontario have undergone many changes in the last 40 years, including phosphorus control, fluctuations in planktivores (Alewife), and invasion by dreissenid mussels and predatory cladocerans. We compare zooplankton population metrics in the Kingston Basin at Station 81 between two time stanzas (1981-1995 and 2007-2016). May to October mean densities, biomass and production of crustacean zooplankton have declined by 93%, 84% and 77%, respectively. Bosmina, Eubosmina, Daphnia retrocurva and Diacyclops thomasi have declined the most, whereas larger taxa, including calanoids, Daphnia galeata mendotae, Holopedium and predatory cladocerans have remained stable or increased. Dreissenid veliger larvae now numerically dominate the zooplankton and have replaced some of the lost crustacean production. Few zooplankton taxa showed changes in mean size or egg ratios among time stanzas, but there has been a shift to larger herbivorous crustacean species in the 2000s. There have also been changes to seasonal succession patterns of zooplankton, especially during the summer and early fall when Bosmina, Eubosmina and cyclopoid numbers have plummeted. We hypothesise that predation by the invasive cladocerans Cercopagis and Bythotrephes, in conjunction with resource competition by Dreissena are largely responsible for these changes.

Twitter handle of presenter
@rockinrobin03