The Deepwater Horizon oil spill in the northern Gulf of Mexico (GOM) is effectively a large-scale tracer release. The isotopic signature of the carbon entering the marine environment from the spill will provide a means of tracing the immediate and long term effects of the spill on coastal food webs. Our preliminary data indicates that this pulse of carbon will be assimilated into coastal food webs. Understanding the extent of incorporation and the assimilation pathways will allow us to trace the long-term effects of the spill on coastal ecosystems and their capacity to support living resources. It is particularly urgent to establish pre-impact conditions for biomass at this time, prior to full impact. Living biomass is imprinted with a C14 signature which is set by carbon fixation from atmospheric CO2. This living biomass has slightly greater radiocarbon content than "modern" due to nuclear weapons testing in the 1950's and 1960's which produced an abundance of C14 in the atmosphere. Currently this value is around +107% of modern, but it can vary locally in coastal waters due to variation in the importance of terrestrial inputs or upwelling of older deeper water including contributions from sediments. Because of these factors, we cannot know current C14 values of living biomass a priori - it is necessary to quantify pre-impact C14 levels in organisms in the northern Gulf Coast.