The blow out of BP's Macondo well released enormous amounts of methane (CH4) to the deep Gulf of Mexico. Methane was the most abundant hydrocarbon in the oil/gas mixture. When early sampling cruises found large amounts of methane but didn't detect bacteria that oxidize methane its fate and its effect on the environment was very uncertain.
Later sampling cruises show that the apparent exponential growth of initially undetectable methanotrophic bacteria populations led to the destruction, by oxidation, of the methane and the reduction of dissolved oxygen levels to lower but apparently safe levels for marine life in the parts water column sampled. However, bottom dwelling organisms may have been smothered by a rain of dead bacteria and sediment that created a dead, anoxic zone on affected parts of the sea floor.
Persistent areas of oxygen depletion were found in Gulf of Mexico waters at a depth range of 800 - 1200 meters. Those areas were associated with fluorescence in water samples which indicated the presence of hydrocarbons. The depleted areas were primarily located on the continental shelf slope to the southwest of the blown out BP Macondo well.
Figure 1.
(A) Sampling stations overlaid on a Google Earth image highlighting the area of the intrusion. Blue plus, red diamond, and white triangle symbols indicate sampling stations for the 18 August – 2 September, 7 – 17 September, and 22 September – 4 October 2010, expeditions respectively. The yellow and green boundaries indicate the extent of the contouring bounds as determined from the extent of the DO and fluorescence anomalies and bathymetric restrictions.
(B) Contour plot within the yellow boundary of the vertically integrated DO anomaly at each station using data from the 18 August – 2 September 2010 expedition. Units are moles DO m-2.
(C) Profiles of DO (SBE-43, Sea-Bird Electronics Inc.; red line calibrated with Winkler titrations) and fluorescence (UV AquaTracka (Emission = 360nm), Cheslea Technologies Group; black line). The green circles represent Winkler titration samples. Station PC198 (26.7098ºN, 90.6286ºW).
One dimensional modeling of methane concentrations and levels of dissolved oxygen depletion indicates that exponential bacterial growth led to a rapid removal of methane and associated depletion of oxygen from days 70 to 100 of the spill. Because the methane had spread and dispersed significantly in that time the drop in dissolved oxygen levels was not large enough to be likely to have a significant effect on marine organisms in the water column.
Figure 3. Model results from a one-dimensional time dependent model of (●) average DO anomaly from CH4 respiration (μM reduction in the intrusion layer), (□) average CH4 concentration (μM), and (×) first-order CH4 oxidation rate constants (days-1) in the intrusion layers. Labels (A) – (D) on the figure represent measured values.
(A) Avg CH4 Concentration = 25 μM (range = 0.57 – 183 μM; n = 73), 11 - 20 June (11);
(B) Avg CH4 concentration = 1.43 ± 2.00 nM (n = 671), Avg CH4 oxidation rate constant = 0.0015 days-1 (range = 0.0005 – 0.0038 day-1, n = 10), 7 - 17 September;
(C) Avg CH4 oxidation rate constant = 0.001 day-1 (range = 0 – 0.0127 day-1, n = 22 minus one outlier), 11-20 June (11); (DM) Max Dissolved O2 Anomaly = 36.7 μM, (DA) Avg Dissolved O2 Anomaly = 5.6 ± 5.8 μM (n = 202), 18 August – 4 October, 2010.
However, bottom dwellers may not have been so lucky. They may have been suffocated by a rain of "sea snot" which was observed on the surface, then later observed layering the sea floor.