Doctor of Philosophy (PhD)
Renewable Natural Resources
Summer hypoxia in the Northern Gulf of Mexico has been attributed to large nutrient inputs, especially nitrate-nitrogen, from the Mississippi-Atchafalaya River system. The 2008 Gulf Hypoxia Action Plan calls for river corridor wetland restoration to reduce nitrate loads, but it is largely unknown how effective riverine wetland systems in the lower Mississippi River (MR) are for nitrate removal. This dissertation research examined nitrate and carbon export from the Atchafalaya River (AR) to: (1) determine nitrate processing by a river swamp basin under varied seasons, (2) investigate nitrate retention and processing in the AR during a major flood event, and (3) assess the relationship of nitrate with organic and inorganic carbon in the AR and MR. I investigated changes in nitrate, δ15NNO3, and δ18ONO3 for water samples collected biweekly to monthly from April 2007 to April 2009 at the AR input- (Simmesport) and outlets (Morgan City and Wax Lake) and on the MR at Baton Rouge. Water samples were also collected weekly during the 2011 major MR spring flood (May to July) and analyzed for nitrate isotopes and concentrations. AR outflow had significantly, but only slightly lower mean nitrate concentrations (1.1 mg L-1) and δ15NNO3 (7.0o/oo) than the MR (1.5 mg L-1, 7.7o/oo); with no difference in δ18ONO3 (4.6o/oo). Limited differences in both isotope values between the two rivers reflect limited nitrate processing in the Atchafalaya. During the 2011 spring flood a total nitrate-nitrogen mass load of 89,600 megagrams (Mg) entered the basin and 83,200 Mg exited the basin, resulting in a low 7% retention of NO3N. There was little variation in δ15NNO3 and δ18ONO3 values between the input and two outlets, further indicating little nitrate processing in this system. The AR appears to have an additional and potentially higher quality organic carbon source from the Red River. The findings in this dissertation research show that as currently designed, dissolved nutrients like nitrate and DOC in the Atchafalaya are transported with little processing. This suggests the Atchafalaya and potentially other similar systems may be ineffective in reducing riverine nitrate because of limited residence time necessary for the biochemical reactions to occur.
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BryantMason, April Elizabeth, "Nitrogen and Carbon Export to the Gulf of Mexico by the Atchafalaya River, a Major Distributary of the Mississippi River" (2012). LSU Doctoral Dissertations. 900.
Xu, Yi Jun