DAUPHIN ISLAND, Alabama -- Scientists at Alabama's Dauphin Island Sea Lab (DISL) confirmed a telling impact of the oil on the coastal marine food web in a recent scientific report titled "Oil carbon entered the coastal planktonic food web during the Deepwater Horizon oil spill," published today in the scientific journal IOP Publishing's Environmental Research Letters.
According to lead author Dr. Monty Graham, "Recently, much has been made of where the oil went. Because of the magnitude of the spill, the fact that the oil seemed to have 'disappeared' so quickly made many people uncomfortable with the unprecedented use of chemical dispersants to move the oil from its floating form on top of the water to micro-droplets within the water." Dr. Graham has been studying this issue as part of a research program funded by the National Science Foundation, the Marine Resources Division of the Alabama Department of Conservation and Natural Resources, and BP's Gulf Research Initiative allocation to the Northern Gulf Institute.
Following the Deepwater Horizon oil spill, about 200 million gallons of crude oil were released into the Gulf of Mexico. According to government and academic researchers, as much as 50 % of this oil may have been naturally or chemically dispersed into the water column. Concern over the ultimate fate of the oil with respect to productive northern Gulf waters has led to much speculation about incorporation of oil-derived carbon into the food-chain.
The evidence is in carbon, the element that forms the backbone of all life on the planet. There are two naturally occurring 'stable isotopes' of carbon: the typical carbon-12 and the slightly heavier carbon-13. The proportion of these isotopes in organisms' bodies has been widely used as a 'tracer' to describe food web connections.
In the Gulf, marine bacteria numbers multiply as they consume the carbon-rich oil. In turn, other micro-organisms feed upon the rich supply of bacteria, and following the paradigm of "you are what you eat," the relative proportion of these carbon isotopes moves up the food chain and can be used to identify the ultimate source of carbon. These can include the dissolved carbon in freshwater, microscopic plants called 'phytoplankton,' terrestrial sources such as grasses and trees…or oil.
Oil carbon lacks much of the heavier carbon isotopes typically found in food for bacteria. So Graham and his colleagues searched for, and found, the light carbon signature in the bodies of zooplankton during the oil spill.
Using this approach, Graham and his colleagues showed that as the oil approached the 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. In fact, oil-carbon repeatedly showed up into August, well after the surface oil slicks were virtually gone.
According to Graham, it was not at all disputed whether the oil would be consumed by marine bacteria, but there was debate on what that would ultimately mean for the rest of the food web. "We showed with little doubt that oil consumed by marine bacteria did reach the larger zooplankton that form the base of the food chain. These zooplankton are an incredibly important food-source for many species of fish, jellyfish and whales," says Graham.
Graham emphasizes that this does not necessarily mean that crude oil toxins were transferred into the zooplankton, but it does show a food web pathway by which other components of the oil could reach higher in the food web. Because the Gulf's waters are so warm, the oil was being degraded extremely rapidly. According to Graham, "the continuing search for where the oil went should not only include direct evidence of existing pools of oil, but also the shadows of where the oil once was as indicated by so-called 'biomarkers' -- such as light carbon isotopes in the bodies of plankton."
Graham and his colleagues are using these findings to prepare experiments at the Dauphin Island Sea Lab to track just how, and how fast, the carbon from oil moves into specific larger plankton groups. They are also trying to determine whether the amount of carbon injected into the northern Gulf ecosystem added to the overall productivity of the system, or merely replaced productivity that otherwise would have occurred naturally.
Download the paper directly from IOP's Environmental Research Letters: http://iopscience.iop.org/1748-9326/5/4/045301/fulltext
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