Abstract
Concerns on the timing and processes associated with petroleum degradation were raised after the use of Corexit during the Deepwater Horizon oil spill. There is a lack of understanding of the removal of oil associated with flocculate materials to the sediment. Mesocosm studies employing coastal and open-ocean seawater from the Gulf of Mexico were undertaken to examine changes in oil concentration and composition with time. The water accommodated fractions (WAF) and chemically enhanced WAF (CEWAF) produced using Macondo surrogate oil and Corexit were followed over 3–4 days in controlled environmental conditions. Environmental half-lives of estimated oil equivalents (EOE), polycyclic aromatic hydrocarbons (PAH), n-alkanes (C10-C35), isoprenoids pristane and phytane, and total petroleum hydrocarbons (TPH) were determined. EOE and PAH concentrations decreased exponentially following first-order decay rate kinetics. WAF, CEWAF and DCEWAF (a 10X CEWAF dilution) treatments half-lives ranged from 0.9 to 3.2 days for EOE and 0.5 to 3.3 days for PAH, agreeing with estimates from previous mesocosm and field studies. The aliphatic half-lives for CEWAF and DECWAF treatments ranged from 0.8 to 2.0 days, but no half-life for WAF could be calculated as concentrations were below the detection limits. Biodegradation occurred in all treatments based on the temporal decrease of the nC17/pristane and nC18/phytane ratios. The heterogeneity observed in all treatments was likely due to the hydrophobicity of oil and weathering processes occurring at different rates and times. The presence of dispersant did not dramatically change the half-lives of oil. Comparing degradation of oil alone as well as with dispersant present is critical to determine the fate and transport of these materials in the ocean.
Highlights
The fate, transport, and transformation of oil components in the water column depend on complex interactions of processes called weathering which includes dissolution, dispersion, evaporation, photo-oxidation, sorption onto particulate matter, and biodegradation [1]
While the present study reported here focuses on oil chemistry, other related studies have reported on the preparation of the oil for the mesocosms [7], the role of bacteria [16], and phytoplankton [20] in addition to the various forms of marine oil snow (MOS) [21, 22] and its formation [23]
The objective of this research was to monitor temporal changes in oil composition in replicate mesocosms performed with open-ocean (M3) and coastal waters (M4) from the Gulf of Mexico, with either oil or oil plus dispersant introduced into the system
Summary
The fate, transport, and transformation of oil components in the water column depend on complex interactions of processes called weathering which includes dissolution, dispersion, evaporation, photo-oxidation, sorption onto particulate matter (e.g., association of oil with marine snow), and biodegradation [1]. One experimental approach for investigating the behavior of oil components in a controlled environment is the use of enclosed marine ecosystems, termed mesocosms [2, 3, 4]. Mesocosms consist of partially or fully enclosed containers providing an experimental system for researchers that partly mimic natural ecosystems [5] while allowing controlled replicate experiments that include biotic effects [3, 6, 7]. Vertical transport of petroleum hydrocarbons and their incorporation to bottom sediments [8, 4, 6] and the fate of the water-soluble fraction of oil and its effect on marine coastal organisms [9, 10] have been investigated using this approach
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