Rivers draining different geologic basins have the most important impact on the formation of the planktonic community in Lake Winnipeg. Very diverse patterns of distribution of individual species reflected the complexity of the water masses structured by lake morphology and the configuration of river inflows. Of the 34 species identified (15 copepods and 19 cladocerans), 12 were found exclusively in the South Basin, 7 exclusively in the North Basin, and 15 were common to both basins. A "core" group of 12 species was distributed over the whole lake, but the remaining 22 species ("unsuccessful invaders") were present only in restricted areas, mostly adjacent to river inflows. Plankton species composition has not exhibited major changes after 40 yr, but abundance has at least doubled, probably due to eutrophication. Several times more plankton crustaceans were found in the western part of the lake, affected by sedimentary drainage, than in the eastern part, influenced by the Precambrian Shield. The plankton community in a lake as large as Lake Winnipeg can be affected by differences in climate within its shores. Midsummer epilimnion temperature was the single best parameter predicting crustacean abundance in Lake Winnipeg and other North American great lakes and, combined with phosphorous loading, explaining 97% of variance.