HR: 0800h
AN: H31C-0516    [Abstracts]
TI: Using Isotopic Tracers to Determine Denitrification of a Large River Swamp Basin
AU: * BryantMason, A
EM: amason5@lsu.edu
AF: School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70815, United States
AU: Xu, J
AF: School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70815, United States
AU: Altabet, M
AF: School for Marine Science and Technology, University of Massachusetts at Dartmouth, New Bedford, MA 02747, United States
AB: As the hypoxic dead zone in the Gulf of Mexico grows due to nitrogen enrichment from the Mississippi River, alternative means to reduce nutrient loads to the Gulf are evolving. One such approach is to divert river water into wetland areas, which allows for infiltration, sedimentation, and denitrification. It is, however, often uncertain how much nitrogen can be retained through these natural systems. Isotopic tracing provides a useful technique for analyzing flow pathways and residence times of nitrogen at watershed scales. This project analyzes isotopic composition of 15N and 18O of nitrate, nitrite and dissolved organic nitrogen at five locations along the 220-km Atchafalaya, the largest distributary of the Mississippi River. Changes in the isotopic ratios between these locations and over time will be investigated. This paper reports preliminary results from two sampling dates in April and May 2007. In all samples we found high concentrations of nitrate (107.6-162.4 uM), but no nitrite and very little ammonium. The d15N isotope ranged from 5.49 to 6.60 and d18O was 4.46-5.59 at all sites, indicating that some nitrate processing may have occurred in the water since this signal differs from that of pure fertilizer nitrate. Delta 15N was similar at all sites in the April sampling date, however, there was a slight trend of decreasing d15N from the upstream sites to the outlets. Although this change in the May sampling date was not significant, it 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 may yield crucial insights into riverine denitrification occurring as the water moves downstream.
DE: 1806 Chemistry of fresh water
SC: Hydrology [H]
MN: 2007 Fall Meeting