PRESENTATION
April BryantMason(1), Y. Jun Xu(2), and Mark Altabet amason5@lsu.edu
(1) School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70803
(2) School for Marine Science and Technology, University of Massachusetts at Dartmouth, New Bedford, MA 02747
Abstract
Isotopic tracing provides a useful technique for analyzing flow pathways and residence times of nitrogen at watershed scales and in different reservoirs. Few recent studies have shown that isotope ratios of riverine nitrogen species could offer an effective means to assess denitrification and nitrification processes. In this study we have analyzed isotopic composition of 15N and 18O of nitrate, nitrite and dissolved organic nitrogen at five locations along the Atchafalaya – Simmesport, Melville, Butte La Rose, Wax Lake Outlet, and Morgan City. The primary goal of the study is to assess denitrification process in the Atchafalaya through monitoring changes in the isotopic ratios between these locations and over time. This paper reports preliminary results from two sampling dates in April and May 2007. All samples had high concentrations of nitrate (107.6‐162.4 uM), but no nitrite and very little ammonium. Nitrate d15N and d18O isotope ranged from 5.49 to 6.60 per mil and 4.46‐5.59 per mil, respectively, at all sites, indicating that some nitrate processing may have occurred prior to infiltration into this system since these values differ from those for pure fertilizer nitrate, particularly with respect to d18O. Delta 15N was similar at all sites in the April sampling date but with a slight trend of decreasing d15N from the upstream sites to the outlets. Although this change in the May sampling date was not significant, the general trend may reflect the importance of higher water temperature, lower flow and increased biological activities of the swamp basin in denitrification. The preliminary results suggest that intense sampling in the summer, when this system has high water temperature and lower flow, may yield crucial insights into riverine denitrification occurring as the water moves downstream.
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