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The hypothesis that aquatic biomass is uniformly distributed over logarithmic size classes was evaluated with samples from epilimnetic plankton communities. Although the hypothesis is true for oligotrophic lakes, biomass increases between successive size classes in mesotrophic and eutrophic lakes. The abundance of organisms in logarithmic size intervals is strongly negatively correlated with size. The relationship between organism size and physiological performance in mixed communities was examined by testing the hypothesis that limnoplankton respiration rates are predictable functions of mean body size. The equation governing this relationship was found to be similar to those obtained with laboratory cultures. The total epilimnetic phosphorus concentration is correlated with both biovolume and respiration rate. Many limnological relationships, including those established in this thesis, are based on linear regressions between log-transformed variables. The rules for the correct use of backtransformed predictions are elucidated. A theorem is proven, which sets limits to the relationship between the coefficients of determination on the original and transformed scales. Simulated data and empirical results are used to illustrate the applications and limitations of the theoretical results.