Organisms in a food chain are grouped into trophic levels, based on how many links they are removed from the primary producers. Plants or phytoplankton are in the first trophic level; they are at the base of the food chain. Herbivores (primary consumers) are in the second level. Carnivores (secondary consumers) are in the third. Omnivores are found in the second and third levels. At the Dauphin Island Sea Lab in Alabama they decided to track a stable isotope of carbon from the BP oil spill and try and discover how quickly it was being incorporated into the food chain. Dr Monty Graham and his colleagues showed that as the oil approached northern Gulf coastal waters in pulses, there was a dramatic decrease in the carbon isotope weight signature over about a four week period. With all other possible sources of light carbon ruled out, they concluded that oil-carbon entered the plankton food web as micro-organisms fed upon the oil-consuming bacteria.