Organic Nitrogen Retention in the Atchafalaya River Swamp |
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Authors: | Y Jun Xu |
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Institution: | (1) School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, Louisiana, USA |
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Abstract: | Freshwater diversions from the lower Mississippi River into the region’s wetlands have been considered an alternative means
for reducing nitrogen loading. The Atchafalaya River Swamp, the largest freshwater swamp in North America, carries the entire
discharge of the Red River and 30% of the discharge of the Mississippi River, but it is largely unknown how much nitrogen
actually can be retained from the overflowing waters of the Mississippi–Atchafalaya River system. Nitrogen discharge from
the upper Mississippi River Basin has been implicated as the major cause for the hypoxia in the Northern Gulf of Mexico, which
threatens not only the aquatic ecosystem health, but also Louisiana’s fishery industry, among other problems. This study was
conducted to determine the change in organic nitrogen mass as water flows through the Atchafalaya River Swamp and into the
Gulf of Mexico. By utilizing the river’s long-term discharge and water quality data (1978–2002), monthly and annual organic
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 large swamp. The results showed
that on average, TKN input into the Atchafalaya was 200 323 tons year−1 and TKN output leaving the basin was 145 917 tons year−1, resulting in a 27% removal rate of organic nitrogen. Monthly TKN input and output in the basin were highest from March to
June (input vs. output: 25 000 vs. 18 000 tons month−1) and lowest from August to November (8000 vs. 6000 tons month−1). There was a large variation in both annual and inter-annual organic nitrogen removals. The variability was positively correlated
with the amount of inflow water at Simmesport, suggesting that regulating the river’s inflow at the Old River flood control
structures may help reduce nitrogen loading of the Mississippi River to the Gulf of Mexico. Furthermore, the in-stream loss
of organic nitrogen indicates that previous studies may have overestimated nitrogen discharge from the Mississippi–Atchafalaya
River system. |
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Keywords: | eutrophication hypoxia nitrogen retention total Kjeldahl nitrogen (TKN) fish kill Mississippi-Atchafalaya Gulf of Mexico |
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