Decreased sedimentation efficiency of petro- and non-petro-carbon caused by a dispersant for Macondo surrogate oil in a mesocosm simulating a coastal microbial community.

Abstract Large amounts of mucous-rich oil-containing marine snow formed in surface waters adjacent to the Deepwater Horizon spill. This marine oil snow (MOS) was implicated in oil delivery to the seafloor. Whether the use of chemical dispersants increased or decreased MOS sedimentation remains controversial. We conducted mesocosm experiments testing the effects of oil and oil plus a dispersant on MOS formation and sedimentation, in coastal seawater. The four treatments used were a surrogate Macondo oil water accommodated fraction ("WAF"), two concentrations of chemically-enhanced WAF ("CEWAF" and diluted CEWAF, "DCEWAF") containing a dispersant and oil, and Controls (no additions). Based on radiocarbon and 13C NMR results, the presence of dispersants enhanced the amounts of petro-carbon being incorporated into the sinking oil-carrying aggregates (aka MOS). However, most of the chemically-dispersed oil preferentially partitioned into the colloidal and suspended particulate fractions rather than into the rapidly forming MOS. Thus the oil and non-petro-carbon sedimentation efficiency in treatments with a dispersant was much lower, compared to those in the Control and WAF treatments, during the four-day mesocosm experiment. Formation of MOS and its subsequent sinking sequestered the oil in two stages: first via terrestrial-derived detritus containing humic compounds, and subsequently via freshly produced material, such as exopolymeric substances produced by phytoplankton and bacteria. Graphical abstract Unlabelled Image Highlights • Δ14C and NMR molecular mixing model to study petrocarbon incorporation to MOS • Corexit enhanced amounts of petro-carbon incorporated into the sinking MOS • Terrestrial-derived detritus played a role in MOS formation and sedimentation • Exopolymeric substances produced by algae and bacteria also influenced this process • Corexit retarded petro- and non-petrocarbon sedimentation in the 4-day mesocosm [ABSTRACT FROM AUTHOR]