Spatial heterogeneity of zooplankton biomass and size structure in southern Quebec lakes: variation among lakes and within lake among epi-, meta- and hypolimnion strata

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Abstract
Environmental control of zooplankton biomass size structure (53–100, 100–202, 202–500 and >500 μm) was investigated in the three limnetic strata of 25 southern Québec Shield lakes, Canada. Among-lake differences were the greatest source of variation of zooplankton biomass, whereas the strong lake–by–stratum interaction observed indicated that the vertical variations of zooplankton biomass and its size fractions were not constant from lake to lake. The analysis of spatial and local factors based on thermal strata is consistent with conceptual models of predation and nutrient control on the biomass and size structure of the zooplankton. Productivity of the aquatic systems, which was driven by lake depth, flushing rate and total phosphorus concentration, was the primary factor influencing total zooplankton biomass and size structure at among-lake scale in epilimnetic waters. The effects of the planktivorous fish on the large zooplankton biomass (>500 μm) was more clearly perceived when the effect of lake depth was removed by partial redundancy analysis. This study showed that although bottom-up and top-down forces are complementary in structuring of zooplankton communities, they can also act differently on the community attributes (e.g. biomass and size structure). Among-lake zooplankton biomass is predictable from lake trophy, but the size structure and vertical distribution of zooplankton communities appear to be controlled by lake stratification and by inference to interactions with size selective predation by fish. In metalimnetic waters, the 53–100 and 100–202 μm zooplankton biomass fractions were primarily dependent on abiotic factors, while the 202–500 and >500 μm fractions were related to planktivory and picophytoplankton concentrations. The well-oxygenated and cold hypolimnetic waters of some lakes offered a refuge from surface turbulence and planktivory to large zooplankton size fractions (202–500 and >500 μm).

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