Assimilation of Oil-Derived Elements by Oysters Due to the Deepwater Horizon Oil Spill

During and after the Deepwater Horizon Oil Spill (DWHOS), oysters (Crassostrea virginica) were exposed to oil and susceptible to incidental consumption of surface and subsurface oil materials. We determined the contribution of oil materials from the DWHOS to diet of oysters by comparing carbon (C) and nitrogen (N) stable isotope ratios in oyster shell to ratios in suspended particulate matter (SPM) and in fresh and weathered oil. Average delta C-13 and delta N-15 values in oyster shell (-21 +/- 1 parts per thousand and 9-11 parts per thousand, respectively) were consistent with consumption of naturally available SPM as opposed to values in oil (-27 +/- 0.2 parts per thousand, 1.6 +/- 0.4 parts per thousand). Stable isotope ratios in oyster adductor muscle were similar to shell for delta N-15 but not delta(13C), suggesting either a recent shift in diet composition or differential assimilation of C between tissue types. We found no evidence of assimilation of oil-derived C and N and, therefore, no evidence of an oyster-based conduit to higher trophic levels. Trace elements in shell were inconclusive to corroborate oil exposure. These findings are not an indication that oysters were not exposed to oil; rather they imply oysters either did not consume oil-derived materials or consumed too little to be detectable compared to natural diet.