For nearly thirty years, research has proceeded at Castle Lake, California. Throughout this period, a strict protocol of monitoring of physical and chemical conditions and plankton dynamics has been maintained, producing an extensive long-germ data set that has recently been analyzing using statistical techniques of time-series and multivariate analysis. These studies have led to major insights into the processes which contribute to ecological variability in this system. Variability in system behavior in Castle Lake has been successfully decomposed into components which arise from external processes, such as unusual climatic conditions like El Nino, and those associated with internal, predatory interactions (effects induced by coupled predatory interactions from fish to zooplankton to phytoplankton). In this proposal, a long-term approach which will allow a better understanding and the distinguishing of the ways in which these processes occur and interact in Castle Lake is outlined. This proposal is centered around a cycle of whole-lake manipulations of predatory interactions, involving alteration of the stocking regime of the dominant planktivore in the lake, rainbow trout. Coupled with this manipulation and monitoring of Castle Lake, a second lake (Cliff Lake) to serve as a "reference" system during the initial years of the project will be monitored and manipulated in a reciprocal fashion in future years. The cycle of manipulations is to be completed within the 5 years of this award, after which it is planned to repeat the cycle of manipulations to test the repeatability of system responses to food web manipulation in years of contrasting climatic conditions. The second component of the program is designed to improve the understanding of how strong climatic signals enter the Castle Lake system during the crucial spring period of ice-melt, hydraulic loading, and establishment of thermal stratification. In each year of the study, an intensive series of measures of physical, chemical, and biological conditions beginning with the break-up of the ice cover will be conducted. By contrasting the physical and chemical conditions during spring in years which differ in climatic severity, greater insights into the mechanisms by which climatic variation contributes so strongly to interannual variability in this system should be obtained.
