Size structure and species composition of plankton communities in deep Ontario lakes with and without Mysis relicta and planktivorous fish

Twenty-three deep oligotrophic lakes in the Huntsville-Bancroft region of Ontario and three locations in Lake Ontario were sampled to investigate the impact of size-selective planktivores on the size structure and species composition of plankton communities. The planktivores include Mysis relicta and planktivorous fishes, usually lake herring (Coregonus artedii) in the Huntsville-Bancroft region or alewife (Alosa pseudoharengus) in Lake Ontario. Previous work had related the presence of these planktivores to higher contaminant levels in lake trout (Salvelinus namaycush). We found that lakes with planktivorous fish had a lower proportion of biomass contributed by zooplankton over 1 mm than lakes without M. relicta and planktivorous fish, which suggests a direct effect of these visual planktivores on larger zooplankton. Lakes with both M. relicta and planktivorous fish had a lower biomass of both small and large Daphnia spp. than lakes with planktivorous fish only. This suggests that the effect of M. relicta extends to smaller cladocerans than those affected by planktivorous fish. Planktivory did not have a strong effect on the species composition of plankton. On the basis of the filter fractionation of particulate phosphorus, lakes with planktivores had a smaller proportion of large particles than lakes without planktivores.