The research described in this chapter represents part of a larger program whose aim is to provide the knowledge and tools to better understand the dynamics of natural and managed ecosystems. More specifically, we aim to produce mathematical models that can translate the effects of environmental stress on individual aquatic organisms to the dynamics of populations. One very practical concern motivates this work: while environmental management demands understanding of long-term effects of stress on populations of plants and animals, much experimental information relates only to short-term effects on individuals. We are addressing this concern by establishing how to develop testable, individual-based models (DeAngelis and Gross 1992) capable of predicting population responses to environmental change. For example, the physiological response of many animals to certain forms of environmental stress (e.g., eutrophication, toxicants, lake acidification) involves changes in the rates of assimilation and utilization of food. Our overall aim is to predict the consequences for the dynamics of natural populations of these changes in individual energy acquisition and use.