A modeling analysis of lake ecosystem responses to inorganic fertilization

Abstract

Separate mathematical models for phytoplankton and fish were developed to describe nominal dynamics and responses to single applications of fertilizer in Postoak Lake, Texas. Models were based on data from three fertilization experiments and interim periods between 1966 and 1976. Simulations were used to identify factors regulating primary productivity and fish growth and to evaluate pathways through which fertilization affected biotic ecosystem components. Phytoplankton biomass and productivity were formulated as functions of nutrient concentrations, temperature, illumination, grazing, and sinking losses. Nominal simulations which treated the phytoplankton as a single homogeneous unit indicated that phytoplankton dynamics were primarily regulated by the feedback of chlorophyll on light attenuation. Following simulated fertilization of the lake, predicted carbon uptake rates were approximately equal to observed values, but chlorophyll concentrations were overestimated. These results, in combination with those from previous studies of Postoak Lake, suggested that the phytoplankton did not respond homogeneously to fertilization and that a significant portion of phytoplankton biomass was lost from the photic zone immediately after nutrient enrichment. An energetic model for bluegills, largemouth bass, and warmouth incorporated the influences of food availability, feeding selectivity, and temperature on growth of individual fish. Fertilization effects were evaluated by examining growth responses to simulated increases of zooplankton, benthos, and forage fish biomass within ranges suggested by previous studies on Postoak Lake..