HR: 16:00h
AN: H14B-03    [Abstracts]
TI: A 25-Year Retrospective Analysis of River Nitrogen Fluxes in the Atchafalaya
AU: * Xu, Y
EM: yjxu@lsu.edu
AF: Louisiana State University, 227 RNR Bldg. School of Renewable Natural Resources Louisiana State University, Baton Rouge, LA 70803 United States
AB: Nitrogen enrichment from the upper Mississippi River Basin has been attributed to be the major cause for the hypoxia in the Northern Gulf of Mexico. The hypoxia threatens not only the aquatic ecosystem health but Louisiana's fishery industry directly among other problems. Although fresh water diversion from the lower Mississippi River into the region's wetlands has been considered an alternative means for reducing nitrogen loading, it is largely uncertain how much nitrogen can actually be retained from the overflowing waters in these natural wetlands. Generally, there is a knowledge gap in what tools are available for accurate assessment of nitrogen inflow, outflow and removal potential for the complex and diverse coastal floodplain systems. This study is to seek answers to three critical questions: (1) Does the Atchafalaya River Swamp remove a significant amount of nitrogen from the overflowing water or release more nitrogen into the Gulf than removing it? (2) How seasonally and annually do the nitrogen removal or release rates fluctuate? (3) What are the relationships between the nitrogen removal capacity and the basin's hydrologic conditions such as river stage and discharge? By utilizing river's long-term discharge and water quality data (1978-2002), monthly and annual nitrogen fluxes were quantified, and their relationships with the basin's hydrologic conditions were investigated. A total Kjeldahl nitrogen (TKN) mass input-output balance between the upstream (Simmesport) and downstream (Morgan City and Wax Lake Outlet) locations was established to examine the organic nitrogen removal potential for this largest freshwater swamp basin in North America. The results showed that on average, TKN input into the Atchafalaya was 200,323 Mg yr^-1 and TKN output leaving the basin was 145,917 Mg yr^-1, resulting in a 27% removal rate of nitrogen. Monthly nitrogen input and output in the basin were highest from March to June (input vs. output: 25,000 vs. 18,000 Mg mon^-1) and lowest from August to November (8,000 vs. 6,000 Mg mon^-1). There was a large variation in both annual and inter-annual nitrogen removals, and the variability was positively correlated with the amount of inflow water at Simmesport. However, no close relationship between the river inflow and percentage nitrogen removal rate was found. The results gained from this study suggest that regulating the river's inflow will help reduce nitrogen loading of the Mississippi River to the Gulf of Mexico. The in-stream loss of nitrogen indicates that previous studies may have overestimated nitrogen discharge from the Mississippi-Atchafalaya River system. Furthermore, the study found that knowledge on spatial hydrological conditions in the basin is needed to understand nitrogen dynamics in the Atchafalaya River Swamp.
DE: 1719 Hydrology
DE: 1871 Surface water quality
DE: 4834 Hypoxic environments
DE: 4845 Nutrients and nutrient cycling
DE: 9350 North America
SC: Hydrology [H]
MN: 2005 Joint Assembly