Assessment of the Spatial and Temporal Variability of Bulk Hydrocarbon Respiration Following the Deepwater Horizon Oil Spill

Following the Deepwater Horizon blowout, the respiration of hydrocarbons dissolved and trapped in the deep and intermediate waters of the Gulf of Mexico imparted a significant reduction in dissolved oxygen (DO) concentration and stimulated a bloom of bacteria biomass. The investigation of 1316 DO profiles measured from 11 May until 20 September 2010 revealed the spatial and temporal variability of bulk hydrocarbon respiration in these deep and intermediate plumes. These analyses suggest that while there were occasional reversals in direction, the general movement of these plumes was toward the southwest and that the cumulative loss of DO peaked from 14 August through 18 September at a value of 18.9 ± 3.8 Gmol. These oxygen-based analyses were extended to determine a first-order estimate of the total release of hydrocarbon mass to the environment that must be less than or equal to the true release based on the inherent assumptions; these analyses estimate a total environmental release of 0.47 ± 0.09 Tg of hydrocarbons. These analyses estimate a total mass of 0.18 ± 0.05 Tg hydrocarbons in the plume layers fully respired to CO(2), 0.10 ± 0.08 Tg hydrocarbons incorporated into biomass, and the biomass/hydrocarbon conversion efficiency of 0.36 ± 0.11 mg biomass/mg hydrocarbon. These analyses also suggest that methane was the dominant hydrocarbon controlling the bulk respiration rates, that the rates peaked around 11 July, and that the addition of dispersants to the wellhead effectively accelerated hydrocarbon respiration.