Effects of Coastal Freshening on Porewater Salinity and Nitrogen Cycling in Brackish Marsh Soils: Implications for Biogeochemical Resilience in Barataria Basin, LA

Author

Robert C. Feder, Louisiana State University and Agricultural and Mechanical CollegeFollow

Semester of Graduation

Summer 2023

Degree

Master of Science (MS)

Department

Oceanography and Coastal Sciences

Document Type

Thesis

Abstract

The Mid-Barataria Sediment Diversion will divert sediments and fresh water from the Mississippi River into mid-Barataria Basin, LA for the purposes of building and sustaining coastal marshes, potentially reducing the estuarine salinity to 0 during operation. Increased freshwater inputs to the area of project influence may reduce porewater salinity and induce ammonium adsorption in brackish marsh soils of the region, potentially impacting the activity of ammonium-producing and denitrifying bacteria. However, brackish soils occur in zones of intermediate salinity and may, therefore, exhibit biogeochemical resilience to episodic changes in surface water salinity. I hypothesized that inundating brackish marsh soils with fresh (0 salinity) water would (i) reduce porewater salinity and (ii) increase exchangeable NH₄⁺concentrations with depth over time but (iii) not impact rates of potentially mineralizable nitrogen (PMN) or potential denitrification.

Intact soil cores from the mid-Barataria Basin were flooded with continuously flowing fresh water for 0, 7, or 28 d, sectioned in 2 cm intervals, and measured for porewater salinity, exchangeable and porewater NH₄⁺, and microbial biomass nitrogen. Porewater salinity of the top 4 cm decreased after 7 d of continuous freshwater inundation, but further freshening did not occur with depth after 28 d. The ratio of exchangeable NH₄⁺ to porewater NH₄⁺ increased at 0–2 cm between 0–7 d, likely due to the concomitant reduction of porewater salinity by 5.30 [4.22, 6.39], and similarly did not change further with depth over time. Microbial biomass N at 0–2 cm did not change between 0–28 d of the freshwater inundation.

The impact of extreme low- (0) and high-saline (35) pulses on nitrogen mineralization rates and denitrification was also quantified by measuring NH₄⁺ production and NO₃^– loss in shaken bottles over time. Salinity fluctuations did not significantly impact measured microbial activity in the brackish marsh soils. This thesis demonstrates that brackish marsh soils may exhibit biogeochemical resilience to episodic coastal freshening and salinization events.

Date

7-3-2023

Recommended Citation

Feder, Robert C., "Effects of Coastal Freshening on Porewater Salinity and Nitrogen Cycling in Brackish Marsh Soils: Implications for Biogeochemical Resilience in Barataria Basin, LA" (2023). LSU Master's Theses. 5807.
https://repository.lsu.edu/gradschool_theses/5807

Committee Chair

White, John R.