A conceptual model of an aquatic ecosystem has been formulated. The formulation incorporates largely empirical deterministic relationships describing biological response to abiotic and biotic environmental parameters into a stochastic representation of birth and death events. The occurrence of these events may be described as a Poisson process. Mathematical system theory provides a methodology for organizing the available information on aquatic ecosystem processes into a coherent and logical structure. This organizational capability is demonstrated. The portion of the conceptual model describing primary productivity has been calibrated and tested on an independent data set. This model works well for the Lake Mead system but needs to be tested on other aquatic systems to evaluate its managerial utility. The modeling of the complex interactions of aquatic food web processes requires further investigation to define an acceptable set of model coefficients.