Title page for ETD etd-08232012-123106


Type of Document Master's Thesis
Author Branoff, Benjamin Lee
Author's Email Address bbrano1@tigers.lsu.edu
URN etd-08232012-123106
Title Nitrogen Biogeochemistry in a Restored Mississippi River Delta: A Modeling Approach
Degree Master of Science (M.S.)
Department Oceanography & Coastal Sciences
Advisory Committee
Advisor Name Title
Robert Twilley Committee Chair
Clinton Willson Committee Member
Dubravko Justic Committee Member
Keywords
  • Systems ecology
  • numerical analyses
  • estuary
  • soil
  • environment
Date of Defense 2012-07-27
Availability unrestricted
Abstract
There is evidence that significant reductions (about 50%) in surface water nitrate concentrations within coastal deltaic wetlands receiving diverted Mississippi River water can be contributed to denitrification. Yet there is also contrasting evidence that other processes could be responsible for this nitrate reduction. As Louisiana plans the implementation of major Mississippi River sediment diversions, a thorough understanding of nitrogen dynamics is necessary to reduce risks of coastal eutrophication and offshore hypoxia. A mechanistic numerical computer model has been developed to simulate nitrogen biogeochemistry within the wetlands of the prograding Wax Lake Delta. This model is calibrated to observed fluxes within laboratory experiments and validated against observed gradients in field observations, as well as against literature reports of other estuarine systems. Calibration of biogeochemical rate constants to the extremes of their bounds set by literature values, as well as the differences in effective rates exhibited between core incubation simulations and ecosystem simulations, suggests that laboratory experiments alone cannot account for full ecosystem biogeochemistry. Sensitivity analysis showed that, within soil core incubation simulations, nitrification had the greatest influence on nutrient fluxes. Dissimilatory nitrate reduction to ammonium (DNRA) had a similar influence on nitrate flux as denitrification and neither of these processes affected ammonium flux. In ecosystem simulations, denitrification exhibited the largest biogeochemical rate at 50 Ámol/m▓h, with vegetation uptake, DNRA, and nitrification at 27, 17, and 0.6 Ámol/m▓h, respectively. Retention efficiency of the study site fluctuated between 4% of loaded nitrogen in December and 16% in May. Temperature was found to have little effect on this efficiency, however loading rates and residence times were found to influence the nitrogen retention efficiency according to the same relationships of other wetland systems. Understanding the observed differences of nitrogen biogeochemistry operating at the laboratory and landscape scales, will aid in the interpretation of measured results. Further, consideration of DNRA as a significant influence on surface water nitrate, and understanding the influences of residence time and nitrogen loading rate, will help in determining the fate of nitrogen in similar systems.
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