goal of this project is to provide data in support management actions to reduce mercury concentrations in recreational and sports fish through understanding how methylmercury is formed in water and sediments. Understanding both synergistic and antagonistic effects of key processes is necessary to have a robust ability to forecast changes in the risk mercury poses to aquatic food webs due to management of reservoirs, wetlands, and lakes. The scientific community recognizes the importance of water quality conditions of dissolved organic matter chemistry and microbial processes on mercury methylation, but key knowledge gaps remain to forecast how mercury methylation will respond to management actions and how this will cascade to mercury levels in recreational and sports fish. This project aims to address these management needs using freshwater systems currently studied by the USGS, selected because they are representative of settings across the nation with urgent needs to decrease mercury levels in fish.This funding opportunity aims to develop a research effort between the USGS and a CESU partner that brings together expertise and capabilities, respectively, to address the overall needs of the management community. The results of the project on water and sediment from a CESU partner will be used with ecological data from the USGS to provide an understanding of the risk posed by mercury to diverse freshwater environments.