The study examines the apparent bifurcation phenomena and associated multiple stable states among several of the variables in an aquatic ecosystem. The study and the proposed catastrophe model should be viewed as limited to shallow aquatic ecosystems with no thermal stratification. At certain nutrient loading thresholds, algal dynamics in such lakes are found to undergo major changes over short periods of time. The study focuses on a theoretical model comprising 3 nonlinear, coupled ordinary differential equations with 11 parameters. The equations for the biomass components of the system take into consideration biological interactions, but make no assumptions for the conservation of mass. Thus, the model differs from well-established lake ecosystem simulation models. (Copyright (c) 1989 Pergamon Press plc.)