Size Structure of the Plankton Community Along the Trophic Gradient of Lake Memphremagog

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Abstract
The size distribution of epilimnetic plankton at four sites of differing trophic state was examined from May 1977 to May 1978 to examine the applicability to lakes of Sheldon's hypothesis that logarithmically ordered size classes contain approximately constant concentrations of biomass. The plankton was sized by serial filtration through a series of screens of mesh size 102, 75, 55, 35, 20, and 10 μm and a 0.45-μm Millipore filter. Biomass in each fraction was measured as particulate phosphorus concentration. I tested the hypothesis by examining the fit of regressions of particulate P concentration in each sequential fraction against the logarithm of the geometric mean of the meshes used to delimit that fraction. Sheldon's hypothesis leads one to expect a linear relationship. In most cases this was found but exceptions were common, and some size classes contained significantly more or less P than the linear model (Sheldon's hypothesis) would suggest. Comparison of the amount of P in the size classes I studied with previous estimates of the biomass of zooplankton and fish suggest that larger organisms may also fit the relation. The slope and intercept of these regressions between biomass and filter size increase with lake trophic state. This may allow an approximate prediction of community size structure in other lakes based on measured or predicted concentrations of total phosphorus.

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