Citation:

You, M., N. Detenbeck, D. Torre, D. Schwede, AND T. Plessel. Status and Trends of Nitrogen Loads to Estuaries of the Conterminous U.S. AWRA Spring Specialty Conference: GIS & Water Resources X, Orlando, Florida, April 22 - 25, 2018.

Impact/Purpose:

Nitrogen and phosphorus have beneficial uses as fertilizer but excess applications of nutrients in fertilizer (beyond crop needs) and manure can lead to eutrophication in downstream estuaries. Eutrophication can lead to excess algal growth and declines in dissolved oxygen when algae die and decay, contributing to fish kills. Other sources of nutrients include point source pollution from wastewater treatment plants and industries, as well as atmospheric pollutants emitted from industrial sources and vehicles, then deposited in dry or wet forms. Once eutrophication problems have been identified, coastal managers need to understand the type and magnitude of different sources in order to reduce loadings and to restore estuaries to a healthy condition. We have compiled data on nutrient sources to estuaries around the United States and applied existing SPARROW models with updated inputs to describe long-term trends in nutrient loads and sources to specific estuaries. Data are made publically available for coastal managers to use through EPA’s Estuary Data Mapper application, a virtual data portal.

Description:

We applied regional SPARROW (SPAtially Referenced Regressions On Watershed attributes) models to estimate status and trends of potential nitrogen loads to estuaries of the conterminous United States. The original SPARROW models predict average detrended loads by source based on average flow conditions and 2002 source terms. We have developed time series for nitrogen sources (1987 – 2016) and landscape features affecting land-to-water delivery ratios to enable an evaluation of recent trends in potential loads, factoring in both regional variation in land-to-water delivery ratios and instream- and reservoir attenuation between source and river mouth. Main data sources include: atmospheric N loads from Community Multiscale Air Quality (CMAQ) models and interpolated values of wet deposition from the National Atmospheric Deposition Program (NADP) adjusted for expected wet to total ratios, wastewater point sources from the U.S. Environmental Protection Agency (EPA) Discharge Monitoring Report (DMR) database, fertilizer and manure from the Nutrient Use Geographic Information System (NuGIS) database disaggregated by crop types in National Agricultural Statistics Service (NASS) crop layers, and urban sources based on land-use/land-cover in the National Land Cover Dataset (NLCD) and National Wall-to-Wall Anthropogenic Land Use Trends (NWALT). We will present results of nonparametric trend analyses by marine ecoregion. For estuaries in most marine ecoregions, delivered N loads from atmospheric deposition declined between 2000 and 2012, with the lowest median 2012/2000 ratio (0.60) observed for Floridian ecoregion estuaries. Interannual variability was greater for Gulf and West Coast models, which relied only on adjusted NADP inputs, as compared to the East Coast models, which used CMAQ inputs. Trends in delivered wastewater loads varied regionally, with slight decreases in values between 2007 and 2016 for estuaries in the Southern California Bight Ecoregion (median 2016/2007 = 0.86) but substantial increases for estuaries in the Northern Gulf of Mexico Ecoregion (median 2016/2007 = 2.21). Potential delivered loads from fertilizer have decreased (minimum median 2012/1987 value for Southern California Bight of 0.18) ,stayed relatively steady (Virginian, Puget Trough, Oregon/Washington Coast Ecoregions), or increased slightly (Gulf of Maine Ecoregion). In contrast, delivered loads from manure to Gulf of Maine estuaries have decreased substantially (median 2012/1987 = 0.29) while manure loads to Carolinian estuaries have increased (median 2012/1987 = 1.43). Potential delivered loads from urban runoff have consistently increased between 1982 and 2011 from 11% (Southern California Bight) to 36% (Carolinian Ecoregion). Information on loads to estuaries of the conterminous United States will be delivered to users through EPA’s Estuary Data Mapper application (www.epa.gov/edm), along with information on factors affecting estuarine sensitivity to nutrient loads.

Record Details: