Degree
Doctor of Oceanography and Coastal Sciences (POCS)
Department
Oceanography and coastal sciences
Document Type
Dissertation
Abstract
The Mississippi River influenced northern Gulf of Mexico (nGOM) is a dynamic coastal system where high river discharge, increased wetland erosion, seasonal phytoplankton blooms, and tropical storms regulate organic carbon (OC) transport and their fate across the land-estuary-shelf continuum. This dissertation quantified OC cycling from the eroding Barataria Basin to the Louisiana shelf under contrasting seasonal and storm driven conditions by integrating microbial analyses, carbon measurements, sediment-core incubations, hydrodynamic modeling, stable carbon isotopes, and the naturally occurring radioisotopes (234Th and 7Be). In Barataria Bay, salinity reorganized microbial community composition, while carbohydrate-processing functional diversity remained stable, indicating taxonomic turnover without loss of metabolic capacity. Hurricane Francine generated excess export of 76.4 tons of dissolved OC and 60.7 tons of particulate OC within five days following landfall. On the shelf, benthic respiration driven inorganic carbon fluxes increased from 16.5 to 33.8 mmol/m2/d during bloom conditions to 25.1 to 68.6 mmol/m2/d during post-bloom. This increase was not directly related to bulk sediment OC and was associated with more 13C-depleted organic matter, indicating selective remineralization of reactive carbon. Particle residence times derived from 234Th-238U disequilibrium ranged from 0.22 to 10.40 days and reached maximum values at intermediate depths; their spatial and vertical distributions were associated with differences in particulate OC retention, water-column remineralization, lateral transport, and delivery to shelf sediments. Tropical storms also redistributed sedimentary carbon, altered benthic remineralization, weakened stratification, and increased surface water pCO2.
Together, these findings showed that carbon fate in the nGOM is controlled not by carbon supply alone but also by the combined effects of microbial processing, organic matter reactivity, particle residence time, seasonal bloom dynamics, sediment exposure, and episodic disturbance. These processes determine whether coastal carbon is stored in sediments, recycled within the water column, or returned to the atmosphere as CO2. These controls also affect bottom water oxygen demand, ocean acidification, and the capacity of coastal sediments to accumulate carbon. This dissertation therefore improves the scientific basis for predicting coastal carbon storage and release as wetland loss, changing river inputs, and tropical storms reshape the Louisiana coast, with relevance to coastal restoration planning and regional climate assessments.
Date
8-11-2026
Recommended Citation
Mahmood, Mukseet, "An Integrated Top-Down Bottom-Up Approach To Understanding Coastal Carbon Cycling In The Northern Gulf Of Mexico" (2026). LSU Doctoral Dissertations. 7193.
https://repository.lsu.edu/gradschool_dissertations/7193
Committee Chair
Maiti, Kanchan
LSU Acknowledgement
1
LSU Accessibility Acknowledgment
1
Included in
Biogeochemistry Commons, Geochemistry Commons, Oceanography Commons, Other Oceanography and Atmospheric Sciences and Meteorology Commons