Phytoplankton dynamics and the role of grazers in Castle Lake, California

During summer 1987, a series of experiments was performed in conjunction with monitoring of in situ plankton dynamics in nutrient-deficient Castle Lake, California. These factorial manipulations of nutrients and zooplankton were designed to separate the direct (grazing mortality) and indirect (stimulation of algal growth by nutrient recycling) effects of grazers on both species-specific phytoplankton dynamics, as well as aggregate properties such as total biomass, productivity, and biomass turnover. Grazing loss rates differed significantly among algal species with the mean grazing loss rate suffered by an algal species a negative function of cell or colony size. Stimulation of division rates in the presence of zooplankton (compared to an unenriched treatment lacking zooplankton) was commonly observed. Nutrient and grazer treatments also had significant impacts on algal biomass (indicated by chlorophyll concentration), productivity (PPR), and PPR/chlorophyll, and significant nutrient-grazer interactions were frequently observed. Species-specific algal responses to grazers were aggregated across the phytoplankton by weighting species-specific parameters by the contribution of each species to total community biomass in order to provide community-wide estimates of the importance of direct and indirect effects on algal dynamics. Finally, linkages between population-level responses and aggregate properties were demonstrated by examining relationships between total phytoplankton biovolume concentration and chlorophyll concentration in various treatments, by comparing community-wide grazing rates estimated from aggregated species-specific grazing rates and from changes in chlorophyll growth rates, and by relating the aggregated degree of species-specific growth stimulation to stimulation of biomass turnover. These results indicate that phytoplankton responses to grazers in Castle Lake reflect a mix of direct and indirect effects that varies considerably both between species and through time during the stratified season. Keywords: herbivory; indirect effects; mesocosms; nutrients; phytoplankton; zooplankton.