Doctor of Philosophy (PhD)


Oceanography and Coastal Sciences

Document Type



Extensive wetland loss and nutrient-enhanced eutrophication occur across the Mississippi River delta and include newly emergent landscapes, in the early stages of ecological succession, and older landscape formations, with fully developed ecological communities. Here I tested how the anthropogenic effects of a climate-induced vegetation shift, an oil spill, and nitrate-enrichment regulate the principal environmental factors controlling nutrient biogeochemistry in wetland soils at different stages of development throughout the Mississippi River delta. In the older, transgressing Barataria basin, there was no clear effect of the climate-induced species shift from Spartina alterniflora Loisel to Avicennia germinans L. on soil nutrient chemistry. Observed soil development patterns were attributed to allochthonous sediment deposition from disturbances rather than autochthonous soil development. Throughout the salt marsh-mangrove ecotone, gross denitrification (mean net N2 flux 81.4 µmol N m-2 h-1) was the dominant N2 pathway and low nitrate concentrations (< 10 µM) likely limited direct denitrification. The oiling of Avicennia and Spartina habitats, during the Deepwater Horizon oil spill, doubled soil organic matter stimulating net N2 production and nitrate/nitrite uptake. In the actively regressing Wax Lake delta (WLD), soil nutrient chemistry exhibited patterns characteristic of primary substrate development; total nitrogen and organic matter increased, while total phosphorus remained relatively constant. Under ambient nitrate concentrations (> 60 µM), gross denitrification dominated the mean net N2 flux (163.2 µmol N m-2 h-1). However, under low nitrate concentrations (< 2 µM), soils switched from net denitrification to net nitrogen fixation. As soils in the WLD aged, the subsequent increase in organic matter stimulated fluxes of N2 and nitrate/nitrite in more mature soils. In conclusion, patterns of soil nitrogen biogeochemistry were linked to the distinct stages of delta formation. Low nitrate availability in the older, transgressive regions limited direct denitrification yielding a net N2 flux dominated by coupled nitrification-denitrification fueled by organic matter mineralization. In contrast, young, regressive regions demonstrated a high capacity for direct denitrification of riverine nitrate that was regulated by substrate age and organic matter accumulation. Throughout the delta cycle, nitrate availability and soil organic matter were the principal factors regulating nitrogen biogeochemistry, and thus the anthropogenic impact of nitrate-enrichment had a marked influence on the observed patterns.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Twilley, Robert R.