Sediment Biogeochemistry of Dredge Pits in the Northern Gulf of Mexico Shelf

Author

Laura J. Thompson, Louisiana State University and Agricultural and Mechanical CollegeFollow

Semester of Graduation

Spring 2020

Degree

Master of Science (MS)

Department

Oceanography & Coastal Sciences

Document Type

Thesis

Abstract

Barrier islands provide valuable coastal protection for essential inland ecosystems by providing protection from wind, waves, and storm surges, however, are subjected to immense land loss due to natural and human-induced processes. In particular, Louisiana has experienced the highest land loss in United States, due a combination of sea level rise, land subsidence, hurricane surges, dam construction, and oil/gas extraction. In order to combat this land loss, Louisiana has introduced a barrier island management plan that implements sand and mud dredging from nearby areas of the outer continental shelf. However, the post dredging implications on water quality and benthic nutrient cycling is essentially unknown in this region. This study investigates the seasonal biogeochemical cycling of O₂, N₂, NO₃^-, NH₄⁺, PO₄^3-, and Si, in two recently excavated dredge pits in coastal Louisiana. Sediment cores were collected from two dredge pits, Caminada (CA) and Sandy Point (SP), as well as four stations on the continental shelf in spring and summer seasons. Temperature controlled ex-situ continuous flow sediment core incubations were conducted to determine benthic fluxes. Caminada experienced significant differences between inside and outside stations with respect to SOC (6.27 to 51.8 mmol O₂ m^-2 d^-1), NH₄⁺ flux (-0.83 to 0.16 mmol N m^-2 d^-1), and PO₄^3- release (0.02 to 0.59 mmol P m^-2 d^-1). In contrast, SP experienced minor inside versus outside pit differences, but experienced significant seasonal differences most notably in NO₃^- (-4.55 to -0.02 mmol N m^-2 d^-1), PO₄^3- (0.01 to 0.84 mmol P m^-2 d^-1), and Si (-0.54 to 6.93 mmol Si m^-2 d^-1) fluxes. In comparison, continental shelf fluxes for sediment oxygen consumption (SOC) ranged 25.7 to 59.0 mmol m^-2 d^-1, N₂ fluxes ranged -0.85 to 1.80 mmol m^-2 d^-1, NO₃^- fluxes ranged -3.62 to -0.55 mmol m^-2 d^-1, NH₄⁺ fluxes ranged -2.29 to -0.13 mmol m^-2 d^-1, PO₄^3- fluxes ranged -0.26 to 0.23 mmol m^-2 d^-1, and Si fluxes ranged 1.72 to 5.55 mmol m^-2 d^-1. This study provides a better understanding of the sediment biogeochemical processes during the evolution of recently excavated dredge pits, especially in a region that experiences eutrophication and prolonged hypoxia every year.

Recommended Citation

Thompson, Laura J., "Sediment Biogeochemistry of Dredge Pits in the Northern Gulf of Mexico Shelf" (2020). LSU Master's Theses. 5120.
https://repository.lsu.edu/gradschool_theses/5120

Committee Chair

Maiti, Kanchan

DOI

10.31390/gradschool_theses.5120