EXPERIMENTAL STUDY OF PHYTOPLANKTON SPECIES COMPOSITION, ALTERNATE STATES, AND COMMUNITY BREAKPOINTS ALONG A PHOSPHORUS GRADIENT

Abstract

A model combining algal mortality due to selective zooplankton grazing with the Monod model of phytoplankton growth was used to predict phytoplankton species composition along a gradient of phosphorus (P) concentration and zooplankton grazing pressure. Model predictions were compared to the results of a mesocosm experiment of phytoplankton species composition along a range of P concentrations of five to 115 μg L−1 and potential grazing loss rates of 0.001 to 0.27 day−1. The effect of the P gradient was measured by monitoring phytoplankton and zooplankton species composition and biomass, and physical and chemical parameters for seven weeks in 12 fiberglass tanks (5500 L) filled with lake water and associated plankton. P concentrations were manipulated so that tanks were evenly distributed along the gradient. Using laboratory derived Monod growth constants for P and literature values of phytoplankton selectivity coefficients, zooplankton filtering rates, and zooplankton assimilation efficiencies, the model correctly predicted (1) the existence of alternate community states, (2) breakpoints between the alternate states which are similar to the P concentrations defining Vollenweider's lake trophic states, and (3) phytoplankton species composition along the P gradient in the mesocosm study. These results suggest that the trade‐off between competitive ability for P and edibility is a primary factor structuring phytoplankton communities, and that multiple states exist which may be consistent with the long‐standing P‐based lake trophic classification.