According to a 1996 report to congress, the Environmental Protection Agency stated that nutrients, primarily from agriculture, are the most common pollutants affecting lakes and constitute 49% of all the water quality problems that have been identified in lakes. Therefore, there is a strong need to evaluate the agricultural and other anthropogenic factors that contribute to the degradation of aquatic ecosystems-especially in areas that are dominated by agriculture. This project takes a watershed approach to analyzing water quality issues by evaluating agricultural land use impacts on indicators of aquatic ecosystem integrity. This approach incorporates the diverse aspects of agricultural and aquatic ecosystems through the use ofbiophysical models and a geographic information system (GIS).;An integrated modeling environment was developed by incorporating the agricultural non-point source pollution model (AGNPS), the enhanced stream water quality model (QUAL2E), and the ArcView geographic information system. Analyses of land management data have shown that excess fertilizer application to agricultural land in a watershed will increase the tendency of a receiving water body to be eutrophic or hypereutrophic. Computer simulation results have also shown that changing a crop from corn to soybean production can be beneficial to a receiving water body. This integrated approach enables resource managers to predict how fertilizer application levels, and changes in land management practices, will affect the concentrations of water quality constituents that are used to measure lake trophic condition. The approach adopted in this research, and the tools developed, facilitate the analyses of "what if" scenarios, cost-effective evaluation of impacts of alternative land management on indicators of the structure and function of aquatic ecosystems, and the determination of "best case scenarios" for natural resource decision-making.