Examining Inputs of Biogenic and Oil-Derived Hydrocarbons in Surface Waters Following the Deepwater Horizon Oil Spill

Aliphatic hydrocarbons in the surface ocean are derived from phytoplankton and oil, but the coexistence and cycling of these two sources is not well-defined. Moreover, phytoplankton and oil can create thin layers of a nonmiscible phase that appear as sheens on surface water and are visually difficult to distinguish. Here, we examine the co-occurrence of hydrocarbon compounds in surface water samples to determine the inputs from phytoplankton and oil using pentadecane (C-15-n-alkane) and heptadecane (C-17-n-alkane) as molecular markers. Surface water sheens collected from a 2015 field survey in the Northern Gulf of Mexico contained hydrocarbons from natural oil seepage, phytoplankton blooms (i.e., biogenic), and mixtures of the two. Microbial communities examined in surface water sheen samples were dominated by cyanobacteria of the Genus Trichodesmium. The hydrocarbon content of the field-collected surface sheens was used to inform the categorization of 2171 samples collected in 2010 during the Deepwater Horizon (DWH) oil spill. Of the water samples categorized, a small fraction (<1%) contained only biogenic hydrocarbons, and similar to 10% contained a biogenic-hydrocarbon input mixed with oil. This study provides a method for identifying biogenic inputs to oil slicks and surface sheens, and highlights a molecular approach distinguish the two sources.