Nutrients (nitrogen and phosphorus) are leading causes of water quality impairment in the Nation's rivers, lakes and estuaries. To address this problem, states need the technical resources to establish nutrient criteria, adopt them into their water quality standards, and implement them in regulatory programs. In recent years EPA developed and finalized a series of nutrient criteria documents to assist the states in adopting nutrient standards. Unlike most water quality criteria, the nutrient criteria were not based on finding cause and effect relations between pollutant levels and adverse water conditions. Rather, the criteria were based on assessing natural background and cultural eutrophication in 14 ecoregions in the country. However, as specified in the documents, states and tribes have the option of developing nutrient criteria using other scientifically defensible methods and data. This paper discusses one aspect of a demonstration project that uses the watershed model HSPF and the aquatic ecosystem model AQUATOX, which are both part of EPA's BASINS package. AQUATOX is used to link aquatic nutrient concentrations with concentrations of "response variables" (chlorophyll-a, algal composition, water clarity), and HSPF is used in turn to link land use practices with nutrient concentrations. The demonstration project, developed in partnership between EPA and the Minnesota Pollution Control Agency (MPCA), is the first of what may be several geographically diverse projects developed to illustrate the utility of models for developing nutrient criteria in different parts of the country. This paper reports on calibration of AQUATOX across a nutrient gradient in order to develop an ecoregional implementation of the model. By developing a robust parameter set for organisms that are adapted to either nutrientrich or nutrient-poor conditions, the model is more likely to represent changing conditions and to not require extensive site calibration in future applications.AQUATOX can model periphyton, phytoplankton, macrophytes, invertebrates, and fish as well as nutrients, sediments, and pesticides. It has been used to simulate high-nutrient rivers; however, it had not been previously calibrated for low-nutrient riverine conditions. This project was intended to represent the development of criteria and assessment of use attainability for the nutrient-rich, turbid Blue Earth River and the nutrient-poor, clear Crow Wing River. In a companion paper AQUATOX was used to predict the response to various permit and land-use changes that could affect nutrients, total suspended solids, and herbicides. Application of the model to the Crow Wing River site required the addition of algal species that are adapted to low-nutrient conditions. The high-nutrient species were calibrated so they would be at a competitive disadvantage and would decline in the simulation. Likewise, the low-nutrient 885 species were added to the Blue Earth River simulation and were calibrated so they would be at a competitive disadvantage in that system. By this me...
