Harmful algal blooms in the Lower St. Johns River Basin resulted in initiation of an interagency effort by the Florida Department of Environmental Protection, St. Johns River Water Management District, and U.S. Army Corps of Engineers Waterways Experiment Station to develop a Total Maximum Daily Load (TMDL) to define appropriate ambient nutrient levels. ATM was retained by FCMA to represent industrial dischargers in the basin and provide third party review of the TMDL during development. As part of this effort, ATM provided oversight during development of computer models (e.g., PLSM and SJR-WAM) to simulate nonpoint sources of nutrients, development and calibration of a hydrodynamic and water quality model (EFDC & CE-QUAL-ICM). Oversight activities included statistical data analyses and conducting independent model runs to assure that the best data available was utilized and that modeling efforts were appropriate and properly calibrated. ATM’s oversight of the TMDL development process helped to assure that the TMDL determination was conducted in an equitable and defensible manner.