Degree

Doctor of Oceanography and Coastal Sciences (POCS)

Department

Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

Increasing concentrations of nitrogenous fertilizer (NO3-) in rivers have potentially altered benthic nitrogen (N) cycling in coastal deltaic floodplains at the continental margins of major rivers. Here I evaluated the N removal capacity of a newly emergent coastal deltaic floodplain [Wax Lake Delta (WLD)] within the Mississippi River Delta Plain using continuous flow-through incubations. I investigated the response of benthic N dynamics in WLD to deltaic succession, anthropogenic fertilization and hurricane events. The estimated annual NO3- removal of 896 Mg N yr-1 in WLD accounts for 10 to 27% of total NO3- load to WLD, most of which is converted to N2 through denitrification. WLD is a continuously emerging ecosystem where the capacity of N removal increases by 0.2 to 2% per year prior to riverine NO3- is export to coastal ocean. Organic matter content increased from younger to older deltaic sediments from subtidal to supratidal hydrogeomorphic zones, which were coupled with increasing benthic NO3- uptake and N2 production. Direct denitrification, as the dominant N pathway in WLD, was one to two magnitudes larger than coupled nitrification-denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox). Increased NO3- loading due to anthropogenic fertilization enhanced denitrification rates but inhibited heterotrophic N2 fixation rates. N2 fixation was comparable to denitrification under historically lower NO3- concentrations (10 µM) and less than 32% of N removed via denitrification was from riverine NO3- loading. In contrast, currently higher overlying NO3- concentration (100 µM) makes denitrification the dominated benthic N pathway compared to N2 fixation and over 61% of the removed N via denitrification was from riverine NO3- loading. Hurricane Barry generated a decrease of denitrification, enhanced proportion of DNRA to total NO3- loss and increased NH4+ effluxes in most experimental sites in WLD. These hurricane effects lead to a decreased capacity of coastal deltaic floodplain to process eutrophic riverine water.

This research highlights the increasing capacity of an emerging coastal deltaic floodplain in processing elevated riverine NO3- and clarifies the contribution of denitrification, N2 fixation, DNRA and anammox to benthic N dynamics at different stages of deltaic development and anthropogenic NO3- loading.

Date

5-29-2020

Committee Chair

Twilley, Robert R.

Available for download on Wednesday, May 24, 2023

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