ISOTOPIC EVIDENCE FOR MINIMAL FOOD WEB USE OF DEEPWATER HORIZON OIL IN LOUISIANA ESTUARINE FOOD WEBS

The 2010 Deepwater Horizon (DWH) well explosion led to the largest accidental oil spill in history. Such substantial oil inputs to the coastal zone might be expected to stimulate bacterial activity and provide a carbon source for estuarine food webs. We investigated uptake of oil into estuarine food webs of the central Louisiana coast, using natural abundance carbon isotope tracers to study transfer of oil carbon to filter feeding oysters, mussels and barnacles. Where oil entered food webs, consumer isotope values were expected to shift from background values towards those of oil, i.e. from -23‰ background towards -27‰ oil for stable carbon isotopes and from about +25‰ background towards -1000‰ oil for radiocarbon. Perhaps surprisingly, the isotope tracers showed little evidence for oil uptake, <20% uptake using stable isotopes in all filter feeders and <1% uptake using the more sensitive radiocarbon measurements for barnacles. Reasons for low uptake of oil potentially include a relatively long and inefficient food chain from oil-degrading microbes to filter feeders, high natural phytoplankton productivity that dilutes any oil signals, and dilution/mixing of oil within the water column. There was some evidence for 1-2‰ lower stable nitrogen isotope values in oil-exposed marsh biota, so that some very localized food web use of oil may be indicated by isotope studies.