Composition and Horizontal Distribution of Crustacean Plankton in Lake Ontario

Abstract

The horizontal distribution of planktonic crustaceans was determined in Lake Ontario at monthly intervals from June to October 1967, based on net hauls from 50 to 0 m at 32–62 stations. A separate study conducted over a 24-hr period at one station showed that on the average 90% of the zooplankters occupied the 0–50-m stratum through which the net was hauled. Eleven species of copepods and 11 species of cladocerans were found. The most abundant forms were Cyclops bicuspidatus thomasi, Daphnia retrocurva, Bosmina longirostris, Bosmina coregoni coregoni, Tropocyclops prasinus mexicanus, and Ceriodaphnia lacustris. Most species appeared in the collections during June–July in the eastern part of the lake with abundance patterns later moving westward, contrary to the general movement of wind and water. At the time of maximal population density there was a strong positive correlation between zooplankton abundance and heat content of the water column from 0 to 25 m. The eastern part of the lake averaged 1.7 times more specimens/cm² than the western part of the lake. Upwelling was responsible for low numbers of zooplankters along the northwestern shore. Abundance patterns similar to generalized current flows were observed in the northern and southern parts of the lake. Flow from the Niagara River was associated with increased abundance of zooplankters in spring and summer, but decreased abundance in autumn. A detailed study of the abundance and distribution of zooplankton offshore from Toronto Harbour showed that harbour water does influence zooplanktonic populations in the vicinity. Temperature and depth of the epilimnion as influenced by the general direction of wind motion were key factors in understanding the zooplanktonic community of Lake Ontario.