Graduation Year


Document Type




Degree Name

Master of Science (M.S.)

Degree Granting Department

Marine Science

Major Professor

John Walsh, Ph.D.

Committee Member

Kent Fanning, Ph.D.

Committee Member

Robert Masserini, Ph.D.


biodegradation, denitrification, marine snow, dissolved organic nitrogen, ammonium


The Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM) in the spring of 2010 introduced 4.4 million of barrels of oil to the ecosystem. Some biodegradation of the oil occurs when microorganisms, particularly bacteria, metabolize the oil as a carbon source. During this process, the microbes also require nutrients for energy. An introduction of oil at this magnitude has the ability to induce large blooms of microbes, which in turn can affect nutrient concentrations. Microbial petroleum degradation decreases nutrient concentrations, whereas the microbial assimilation and decay of organic matter increase nutrient concentrations. This study assessed whether any changes in nitrate, ammonium, and/or phosphate concentrations from historical levels could be attributed to the oil spill as a result of biodegradation, and how those changes can impact the GoM ecosystem. Nutrient samples were collected at discrete depths throughout the water column, in a cross-shelf transect inland from the spill site outside the DeSoto Canyon, in August 2010, February 2011, and May 2011 (four months to one year after the spill). In August 2010, a subsurface oil plume was found at depths of 1000 m to 1200 m. At the same depth of ~1000 m, a significant decrease in nitrate was observed, indicating the biodegradation of oil by heterotrophic bacteria of the aphotic zone, compared to earlier data during August 2000, when no known oils were present. Overall temporal increases in ammonium and dissolved organic nitrogen (DON) were observed both in near-surface waters and at an intermediate depth of ~400 m next to the walls of the DeSoto Canyon, suggesting an incremental die-off of both plankton and benthic organisms during accelerated recycling of nutrients. Continued decreases of phosphate were observed into February 2011, supporting ongoing biodegradation then as well. By May 2011, however, there were more increases in near-surface ammonium concentrations, compared to April 2000, with the implication that continued interseasonal recycled nitrogen accumulations may have been due to a decadal ecological regime shift, after a combination of top-down overfishing, petroleum perturbations, and/or increases of toxic harmful algal blooms (HABs).