Semester of Graduation

May 2024

Degree

Master of Oceanography and Coastal Sciences (SOCS)

Department

Department of Oceanography and Coastal Sciences

Document Type

Thesis

Abstract

Coastal wetlands provide numerous ecological services, but high rates of wetland loss have been seen, especially within the Mississippi River Delta. The erodibility of the marsh, which depends on its strength, can be affected by waterlogging-impacted factors. This study focused on investigating factors influencing soil shear strength in brackish to intermediate salinity marshes along the northern Gulf of Mexico coast using a comparative field study and a controlled greenhouse experiment to test the hypothesis that soil waterlogging reduces soil strength. I predict species-specific responses such that marshes dominated by the Sporobolus pumilus (formerly, Spartina patens), which is less flood tolerant and forms hummock-hollow micro topography, will have lower soil strength than marshes dominated by the more flood-tolerant Sagittaria lancifolia. The comparative field study tested differences in strength and stability between stable and unstable Sporobolus marshes spanning three locations and four sites (Chenier Plain and Barataria Bay, Louisiana and Grand Bay, Mississippi) and a stable Sagittaria marsh (Barataria Bay, LA). Hummock-hollow topography was evident in both the Chenier Plain and Barataria marshes. These hollows had significantly weaker shear strength than the hummocks. The stable marshes had 3.5 times greater soil shear strength on average when compared to the unstable marsh. Sporobolus and Sagittaria marshes in Barataria Bay had lower elevation and higher inundation depths than the two stable Sporobolus sites in the Chenier Plain and Grand Bay. The greenhouse study tested the effects of flooding (5, 45, and 90% time flooded) and nutrients (control and 2 mgN/L + 0.1 mgP/L) on growth and strength of Sporobolus pumilus and Sagittaria lancifolia. Sporobolus at higher elevation had higher aboveground biomass and higher shear strength than at low elevation. Overall, in both field and in a greenhouse setting, soil strength and biomass production of Sporobolus is negatively affected by highly flooded conditions, while Sagittaria is able to remain consistent throughout various flooding regimes in terms of above and belowground biomass and shear strength at low salinity.

Date

4-3-2024

Committee Chair

Quirk, Tracy

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