(CNN) -- A "shadow" of oil from the Deepwater Horizon spill is in the Gulf of Mexico's food chain, scientists at Alabama's Dauphin Island Sea Lab have found. But that's not necessarily a bad thing.

"Signatures" of oil carbon turned up in zooplankton, animals such as fish larvae and microscopic crustaceans that form the base of the food chain, said Dr. Monty Graham, the lead author of a study published this week.

Plankton is consumed by other organisms, such as crabs, mussels, oysters and shrimp, which are in turn consumed by humans and other species.

On April 20, an explosion aboard the Deepwater Horizon drilling rig killed 11 workers and led to the worst oil spill in U.S. history. The gushing well the rig had been drilling released more than 200 million gallons of oil into the Gulf. After the well was sealed, government and independent scientists concluded as much as 50 percent of that amount had been dispersed, both by natural or chemical means.

"There's a huge amount of angst that remains with people about where the oil has gone," Graham said. "People are having a hard time connecting the dots between all the oil that's been released in the Gulf and the fact that you can't really see any oil any more. We wanted to start the process of accounting where all this oil is going."

Scientists tracked a particular isotope of carbon identified with oil and found it turned up in zooplankton. The study concludes that oil was consumed by microbes, or oil-eating bacteria, which were then consumed by micro-organisms in the plankton food web.

These results were not surprising, Graham said. "We expected early on the microbes would get a hold of the oil and consume it relatively rapidly.

"What we found was that the system works. It doesn't mean everything is OK and it doesn't mean that there isn't anything out there that isn't toxic. It just explains that the ecosystem is working to process this oil as if it were food."

The oil was treated as "fuel" to grow and reproduce, Graham said. "It's all biomass conversion. If I eat a cow that ate grass, I'm not eating grass; I'm eating what got converted into cow biomass."

Carbon is the element that forms the backbone of all life forms, so the evidence of the oil carbon in the zooplankton doesn't necessarily mean the food chain has become contaminated, Graham explained.

"But it does show a way oil components can make their way higher in the food web," he said. "The fact that the carbon did get higher in the food chain shows there is a ... connection between the bacteria and the food chain."

Graham pointed to extensive sampling studies currently being done by the National Oceanic Atmospheric Administration and the Food and Drug Administration.

Graham's study was published Monday in the scientific journal Environmental Research Letters.

Last week, NOAA said that out of 1,735 tests on various types of Gulf seafood, 13 were found to test positive for trace amounts of Corexit, a chemical dispersant that was used to break up the oil during the height of the crisis. The FDA said the amount of dispersant found was so low that it wouldn't be harmful to humans, and that the Gulf seafood currently sold in markets is safe.

Only about 4 percent of the Gulf of Mexico's federal waters remain closed to fishing because of the oil spill.