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Consensus-building univariate and multivariate data analyses were used to identify patterns in space and time over seven years among 12 sampling stations in a 720 km2 hardwater Canadian lake (Lake Simcoe, Ontario, Canada). There were 15 copepods and 26 cladoceran species identified in samples collected throughout the May-October periods of 1986-1992 from Lake Simcoe. Eleven crustacean zooplankters accounted for 88% of the total average density of all recorded species in the lake. Most of these (the main exceptions being Eubosmina coregoni and Daphnia pulicaria) are ubiquitous taxa with wide environmental tolerances. Multivariate analyses of these data identified spatial differences and a temporal trend in community composition. The use of Cao et al.’s (1997a) "CY-dissimilarity" measure combined with Ward's Linkage clustering algorithm and non-metric multidimensional scaling ordination resulted in several clearly defined groups of sampling units (SUs), which apparently were separated predominantly on the basis of variables related to space (sampling station) and time (year). The 7-year record suggested several lines of evidence for trends in community structure. Averaged over all stations, the yearly intra-station dissimilarities steadily increased 78% by 1992. The greatest rates of increase in yearly intra-station dissimilarity with 1986 baselines were found for the deeper sampling stations. These whole community-level trends were the net results of a many different patterns of change identified in species-level analyses. Such patterns included increasing densities over the 7-year period of Daphnia longiremis and Diaphanosoma birgei and declining densities of Daphnia pulicaria. Many other taxa showed highest values in the middle years of the study. Community-level temporal trends were least evident at sampling locations in the shallower and warmer Cook’s Bay, despite clear trends in several individual species. A significantly greater representation by large-bodied zooplankton species in the latter years of the study was consistent with declining stocks of rainbow smelt (Osmerus mordax) and lake herring (Coregonus artedii), suggesting the associated relaxation of predation pressure as a possible cause. Several species indicative of eutrophic states (Acanthocyclops vernalis, Chydorus sphaericus and Bosmina longirostris) were common in the samples. The scarcity of the calanoid Senecella calanoides and the absence of another classic oligotrophic indicator calanoid, Limnocalanus macrurus in the Lake Simcoe samples suggests impairment of their deep, cold-water habitat and is consistent with the modelled historical loss of hypolimnetic dissolved oxygen in Lake Simcoe.
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