Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
The Mississippi River Delta is threatened by a growing pressure to support large human populations in the United States both with food production, navigation systems, and urban development in the Mississippi River Basin. Nitrate-nitrogen load in the Mississippi River, up to 100 Tg N yr-1 from agricultural and urban runoff, leads to phytoplankton blooms and hypoxia across the Louisiana continental shelf, creating dead zones of low dissolved oxygen threatening a significant commercial fishery. Along the coast and river corridors, floodplain ecosystems have the capacity to retain and remove nitrate. This dissertation explores the role of productive, actively growing coastal deltaic floodplains as nitrate removal hotspots in an active delta. A combination of field, laboratory, and numerical modeling tools are utilized in studying the physical and biological factors that control the nitrate removal potential of coastal deltaic floodplains. The overall impact of coastal deltaic floodplains in removing nitrate is contingent on connectivity or delivery of channel water to floodplains, which is spatially and temporally variable. Field studies indicate that denitrification removes nitrate on long-term scales and accounts for 28-71% of the nitrate fluxes across WLD floodplains. Water column assimilation, on the other hand, only temporarily removes nitrate as remineralization and decomposition recycles organic nitrogen back into bioavailable forms. Storage within sediment and vegetation compartments account for only 1-26% of nitrate fluxes, indicating slow transfer rates from surface water to the marsh sediment and vegetation. Longer water residence times within the floodplains of WLD facilitate greater water retention that promotes nitrate removal. However, residence time within floodplains reduces from 3.6 toSimilarly, areas with the greatest denitrification rates and longest residence times represent a relatively small area of the active delta. Water quality models of WLD indicate only 2.4-4.1% of nitrate loaded from Atchafalaya River is removed prior to export to Gulf of Mexico. Therefore, the overall contribution of WLD to long-term nitrate removal may be small considering the large nitrate load in this system. Results from this research highlight the need to consider both physical and biological conditions when designing restoration projects aimed at improving water quality along the coast of Louisiana.
Christensen, Alexandra, "Physical and Biological Factors Controlling the Fate of Nitrate in a Louisiana Coastal Deltaic Floodplain" (2020). LSU Doctoral Dissertations. 5229.