The U.S. EPA has developed a freshwater ecosystem simulation model, AQUATOX, which predicts the fate of various pollutants, such as nutrients and organic chemicals, and their effects on the ecosystem, including fish, invertebrates, and aquatic plants. AQUATOX is a valuable tool for ecologists, biologists, water quality modelers, and anyone involved in performing ecological risk assessments for aquatic ecosystems. AQUATOX has a myriad of potential applications to water management issues and programs, including water quality criteria and standards, TMDLs (Total Maximum Daily Loads), and ecological risk assessments of aquatic systems. AQUATOX can be used to predict ecological responses to proposed management alternatives. It may help to determine the most important of several environmental stressors, e.g. where there are both nutrients and toxic pollutants. The program can be downloaded from this site