THE Deepwater Horizon blowout is the largest oil spill in US history, but its ecological impact need not be the worst. It all hinges on the amount and composition of the oil that reaches the Gulf of Mexico’s most sensitive habitat: its coastal marshes. If they can be protected, the region could bounce back in just a few years.
As New Scientist went to press, estimates of the volume of crude so far ejected into the waters of the Gulf ranged from 90 to 195 million litres – dwarfing the Exxon Valdez’s 40-million-litre spill in 1989. But the aftermath of previous spills shows that it is not the volume that matters most.
Consider three vastly different spills (see timeline, right). In 1979, theIxtoc I well off Mexico’s Gulf coast spewed 530 million litres of oil into shallow waters – three times the worst current estimates for Deepwater Horizon. Five years later, “we had to look hard to see any lasting effects”, says Arne Jarnelöv of the Institute for Futures Studies in Stockholm, Sweden, who led a UN team sent to monitor the area.
The Exxon Valdez spilled far less, 40 million litres, yet Alaska’s Prince William Sound is still recovering. And 700,000 litres spilled by the oil barge Florida at West Falmouth on Cape Cod is still affecting species 40 years on. Why such variation? It all comes down to the type of oil and the habitats involved.
The light crude from Ixtoc I made landfall mostly on relatively lifeless sandy beaches, where it quickly degraded into a fairly harmless hard tar. Exxon Valdez’s heavy crude immediately coated rocky inlets that were havens for seabirds and other marine life, and the frigid conditions meant it broke down slowly. West Falmouth suffered disproportionately because the refined fuel oil that spilled is especially toxic, and hit sensitive salt marshes.
The impact of Deepwater Horizon will be difficult to predict because there never has been a sustained spill at such a depth. The good news is that the oil has to rise through 1500 metres of water, and is exposed to the elements for days or weeks before hitting the shore. The most toxic components – benzene, toluene, ethyl benzene and xylenes – are likely to dissolve in the water column and become greatly diluted, or evaporate at the surface.
One big uncertainty relates to the intervention, not the spill – in particular the unprecedented use of chemical dispersants at the leaking well head on the sea floor. The deep ocean plumes this helped create are a mix of dispersant, emulsified oil and water. How marine life will be affected is anyone’s guess.
What is certain is that the plumes are already overlapping with Lopheliacorals that live at depths of 300 to 500 metres. Next month, the US Geological Survey plans to send robotic submersibles to three of itsstudy sites that are close by. “These sites could be impacted quite severely,” warns Cheryl Morrison, a conservation geneticist at the Leetown Science Center in Kearneysville, West Virginia.
The spill could be disastrous for the endangered bluefin tuna – it coincided with the spawning season and there are fears that up to 20 per cent of this year’s larvae may die. “We need all the biodiversity in any year,” says Barbara Block of Stanford University’s Hopkins Marine Station in Pacific Grove, California.
Among coastal habitats, marshes are by far the biggest worry. They are both a crucial wildlife habitat and an important buffer between New Orleans and the hurricane-prone Gulf. The marshes are already eroding at an alarming rate, as a consequence of engineering projects that have constrained the wandering Mississippi and carved out navigation channels. “These marshes are already hanging on by their fingernails,” says Denise Reed, a geomorphologist at the University of New Orleans.
Oil in the coastal estuaries and marshes would also pose the biggest threat to commercial fisheries. They serve as nurseries for both shrimp – which accounts for more than half of Gulf of Mexico fisheries by revenue – and the Gulf menhaden, a member of the herring family. Used for bait and processed into animal feed and fish oils, menhaden comprise more than 70 per cent of Gulf fisheries by weight.
The dispersants that could be bad news for the deep sea may create an emulsion in the marshes that does not readily penetrate sediments, suggests Jacqueline Michel, president of Research Planning, a consultancy in Columbia, South Carolina, that is advising on the spill response. With care, she says, this can be removed without causing further damage. On the other hand, lab experiments by John Nyman of Louisiana State University in Baton Rouge indicate that the combination of Louisiana crude and the main dispersant used on the current spill is more toxic to marsh-dwelling invertebrates than oil alone would be.
Ultimately, the best hope of staving off the worst impacts of the spill is to keep the oil out of the coastal marshes. Plans to construct sand berms to bolster the protection by natural barrier islands may help, say research teams working in the marshes. They will have gaps to allow for tidal flows, so success may depend on the aggressive use of booms and skimmers in those gaps. “The best thing we can do is to stop the oil getting into these wetlands,” says Reed.