HR: 16:15h
AN: H14B-04    [Abstracts]
TI: Simulated Changes in the Ratios of Nutrients Delivered to the Gulf of Mexico in Response to Changes in the Nutrient Sources of Inland Watersheds of the Mississippi River Basin
AU: * Alexander, R B
EM: ralex@usgs.gov
AF: U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192 United States
AU: Smith, R A
EM: rsmith1@usgs.gov
AF: U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192 United States
AU: Schwarz, G E
EM: schwarz@usgs.gov
AF: U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192 United States
AU: Boyer, E W
EM: boyer@nature.berkeley.edu
AF: University of California, Berkeley, Dept. of Environmental Science, Policy, Management 137 Mulford Hall #3114, Berkeley, CA 94720 United States
AB: Increases in riverine nitrogen loads to the northern Gulf of Mexico have contributed to increased hypoxia in the coastal waters of the Louisiana shelf during the past several decades. Nitrogen is the most limiting nutrient for algal production in these waters, however, Mississippi River nutrient loads entering the Gulf are closely balanced with seasonally shifting Redfield ratios. Moreover, state concerns over phosphorus, which is generally more limiting to primary production in inland waters, may contribute to an increased emphasis on future phosphorus reductions to meet designated use requirements in state waters. Currently, knowledge is lacking about how nutrient ratios in Mississippi River loads are likely to respond to future changes in nutrient sources in inland watersheds. An improved understanding is initially needed of the major sources and processes in the Mississippi River Basin (MRB) that control both phosphorus and nitrogen delivery to the Gulf. Earlier modeling studies of nutrients in the MRB have focused primarily on nitrogen with little attention to phosphorus. Here, we develop a Spatially Referenced Regression on Watershed Attributes (SPARROW) model of mean-annual total phosphorus (TP) loads for streams in the MRB. The SPARROW model links measurements of TP loads in streams with geographic data on phosphorus sources (e.g., fertilizer, livestock wastes, urban sources) and properties of the landscape that influence transport (e.g., climate, topography, vegetation, soils, water routing). The model employs mechanistic components and mass balance constraints within a formal parameter-estimation structure to empirically quantify the sources, attenuation rates, and transport of phosphorus in the terrestrial and aquatic ecosystems of the MRB. The model was used to quantify the interior watersheds and nutrient sources that contribute to phosphorus delivery to the Gulf. Using a previously estimated SPARROW nitrogen model for the MRB, we computed Redfield ratios of the predicted nutrient loads near the outlet of the Mississippi River. We evaluated the change in these nutrient ratios in response to simulated changes in agricultural and urban sources of phosphorus and nitrogen in the MRB. The results indicate that changes in nutrient ratios are sensitive to changes in both the type of source and the location of inland watersheds of the MRB that are targeted for reduction.
DE: 1803 Anthropogenic effects
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2005 Joint Assembly