Benthic Nitrogen Dynamics in Response to Deltaic Succession, Anthropogenic Fertilization and Hurricane Events in an Emerging Coastal Deltaic Floodplain within the Mississippi River Delta Plain

Author

Song Li, Louisiana State University and Agricultural and Mechanical CollegeFollow

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

Recommended Citation

Li, Song, "Benthic Nitrogen Dynamics in Response to Deltaic Succession, Anthropogenic Fertilization and Hurricane Events in an Emerging Coastal Deltaic Floodplain within the Mississippi River Delta Plain" (2020). LSU Doctoral Dissertations. 5281.
https://repository.lsu.edu/gradschool_dissertations/5281

Committee Chair

Twilley, Robert R.

DOI

10.31390/gradschool_dissertations.5281