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Abstract
This dissertation identifies regulating factors that control methane (CH4) and nitrogen (N) cycling in anthropogenically-impacted oceanic and aquatic environments. Variations in microbial CH4 and N dynamics were evaluated in two Gulf of Mexico ecosystems, a coastal estuary, and a high latitude environment.The large reserves of oil and natural gas in the northern Gulf of Mexico make it the focus of extensive oil and gas exploration and production. Natural petroleum seeps in the northern Gulf of Mexico contribute the largest source of oil and gas into the marine environment on an annual basis, with the exception of the 3-month Deepwater Horizon (DWH) oil spill in 2010. CH4 oxidation rates post DWH blowout suggest that the methanotrophic active biomass was distributed by prevailing currents and was able to sustain increased activity for five years post blowout, even in regions with low CH4 concentrations. Additionally, The Gulf of Mexico is plagued by an annual low oxygen zone due to increasing N runoff from the Mississippi River and summertime stratification. Rate measurements of water column CH4 oxidation and denitrification reveal that water column denitrification and CH4 oxidation are not able to keep up with the significant amounts of N and CH4 in the low oxygen zone. An increase in N loading to the watersheds and coastal ecosystems comes with increasing population density across Georgia. These increases in N loading can result in the stimulation of denitrification, while also increasing nitrite and nitrous oxide production rates. These results suggest that enhanced N loading will lead to increases in nitrous oxide production in the sediments. Finally, the Arctic Ocean is undergoing increases in surface water temperatures. The temperature increases primary production- leading to enhancement of organic matter degradation and CH4 production in the sediments of the Chukchi Sea. Although, the depth integrated CH4 oxidation rates were less than the reported flux of CH4 to the atmosphere, CH4 oxidation still provides an ecosystem service by removing a fraction of the CH4 prior to release into the atmosphere. These studies advance the understanding of human impacts on specific aquatic and oceanic ecosystems.