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AimWe tested the energy and metabolic theories for explaining diversity patterns of crustacean zooplankton in Canadian lakes, and evaluated the influence of regional and local environments on community structure. LocationThe 1665 studied lakes are distributed across Canada in 47 ecoprovinces. MethodsOur database included the occurrence of 83 pelagic crustacean species. The regional species richness in each ecoprovince was estimated using the average local species richness per lake and the first-order jackknife diversity index. Using a principal component plot and forward selection in a multiple regression we identified the most important predictors of regional species richness estimates. We tested the predictions of the species richness-energy hypothesis using climatic variables at regional scale, and of the metabolic theory using the inverse of air temperature. To evaluate the influence of regional and local environmental drivers, we carried out a redundancy analysis between crustacean species occurrences and regional climate and lake environmental factors on a subset of 458 lakes. ResultsEstimates of pelagic crustacean species richness in Canadian ecoprovinces varied from 3 to 10 species per lake (average local species richness) or 8 to 52 species per ecoprovince (Jackknife diversity index). Our study fully supports the species richness-energy hypothesis and partially the metabolic theory. Mean daily global solar radiation was the most important regional predictor, explaining 51% of the variation in the regional species richness among ecoprovinces. Together, regional climate and local lake environment accounted for 31% of the total variation in community structure. Regional-scale energy variables accounted for 24% of the total explained variation, whereas local-scale lake conditions had less influence (2%). Main conclusionsThe richness-energy theory explains diversity patterns of freshwater crustacean zooplankton in Canadian ecoprovinces. Solar radiation is the best predictor explaining regional species richness in ecoprovinces and community structure of pelagic crustaceans in Canadian lakes.

Rivers draining different geological 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 planktonic 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 phosphorus loading, explained 97% of variance.

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was $0.18 mg P m )2 d )1 before pike addition, declined rapidly to approximately 0.03-0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6-14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.