| Summary: | Seasonal variations in the abundance of eight species of crustacean zooplankton in four small man-made lakes in southern Manitoba were studied from samples taken in a one year period from June of 1977 to June of 1978. Three stations on each lake were sampled at weekly intervals in summer, 1977 and the spring of 1978 and at approximately biweekly intervals in autumn. Winter samples were taken at one station per lake biweekly. Crustacean species shared by the four lakes were: Diaptomus siciloides, Cyclops bicuspidatus thomasi, Cyclops vernalis, Mesocyclops edax, Daphnia galeata mendotae, Daphnia parvula, Ceriodaphnia lacustris, and Bosmina longirostris. Lake I had the highest total numbers of animals per liter in all seasons as well as the greatest relative abundance of cyclopoids and cladocerans. In each lake, the number of dominant species ranged from one to five on various sampling dates. Lake IV generally had the least complex community in that in 21 of a total 32 sampling dates, no cladoceran was dominant. In the other lakes, combinations of one diaptomid, one to two cyclopoids and one or more cladocerans dominated more often than in Lake IV. A comparison of relative abundance and dominance in June of 1977 and June of 1978 showed a large shift in community composition in each of the four lakes in favour of cyclopoid copepods which comprised over 60 percent of the total abundance of crustacean zooplankton by the spring of 1978. The percent similarity of community index calculated for all possible lake pairs showed that the degree of similarity between the lakes changed in different seasons. Lake pairs I and IV and II and IV were least similar in autumn, 1`977 (PSC - 46.0 and 40.6 respectively) and most similar in spring, 1978 (96.1 and 96.5 percent). Discriminant function analysis was performed on species abundance data from summer and fall, 1977. The analysis achieved good separation among Lakes I, II, and IV and poor discrimination between Lakes III and IV. Possible effects of differing environmental conditions on the abundance and timing of the seasonal maxima of some of the crustacean species were discussed.
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