U.S. Geological Survey Studies on Nutrient Criteria and Nutrient Load Allocation
Dale Robertson, U.S. Geological Survey, Wisconsin Water Science Center, Middleton, WI, dzrobert@usgs.gov
Excessive nutrients in surface waters is not a new problem, but it is among the most persistent water-quality problems in the Nation. Excessive concentrations can cause nuisance levels of algae and aquatic vegetation, and have been linked to eutrophication of downstream impoundments, outbreaks of Pfiesteria in several coastal states, and hypoxia in the Gulf of Mexico. Under recommendations of the Clean Water Action Plan released in 1998, USEPA is implementing a national strategy to develop waterbody-specific nutrient criteria for lakes and reservoirs, streams, wetlands, and estuaries. The intent of this strategy is to get all States and tribes to establish nutrient standards, that, if enforced, will reduce nutrient concentrations and improve the beneficial ecological uses of surface waters. To better understand the causes of the problems in the Gulf of Mexico, it is important to quantify the loads (flux) of nutrients transported from the Mississippi–Atchafalaya River Basin and determine where these loads originate. Studies by the USGS have and are being conducted to help provide information to help the States and USEPA develop nutrient criteria and allocate the nutrient and sediment loads throughout the Upper Midwest and entire Mississippi River Basin.
Nutrient Criteria Regionalization Framework:
Omernik’s ecoregions has been used to guide the delineation of National nutrient ecoregions by the USEPA. Although nutrient ecoregions are supposed to represent differences in a full suite of environmental characteristics, they primarily subdivide the Midwest into areas of different land-use intensities. A new approach, SPARTA (SPAtial Regression-Tree Analysis), was developed which uses water-quality data and environmental characteristics, both adjusted for differences in land use, to delineate areas with similar background nutrient concentrations and similar responses to changes in land use.
State Refinements: Regionalization and Biological Response:
The USGS and Wisconsin DNR collected data from streams and rivers throughout Wisconsin to: 1) describe how nutrients and biotic-community structure vary throughout the State; 2) determine which environmental characteristics are most strongly related to nutrient concentrations; 3) determine reference water-quality and biotic conditions for different areas of the State; 4) determine how biotic communities respond to changes in nutrient concentrations; 5) determine the best regionalization scheme to describe patterns in reference conditions and responses in water quality and the biotic community; and 6) develop new indices to estimate nutrient concentrations from a combination of biotic indices.
Allocation of Nutrient and Sediment Loads in the Upper Midwest and Entire Mississippi River Basin through the use of the SPARROW model:
Water-quality, streamflow, and environmental data from sites across the U.S. were used to estimate nitrogen and phosphorus transport in streams across the country by use of national applications of the SPAtially Referenced Regressions On Watershed attributes (SPARROW) model. The USGS has recently begun an effort to develop more accurate and less biased SPARROW models for the Upper Midwest and entire Mississippi River Basins. These models will be based on more extensive data assembled just in these areas. Information from these models will be used to better define nutrient and sediment loadings and rank all of the individual basins based on their relative loadings.