Oil Spill Dispersants Shifting Ecosystem Impacts in Gulf, Scientists Warn

Oil Spill Dispersants Shifting Ecosystem Impacts in Gulf, Scientists Warn

A seemingly feel-good story showed up this week on the nation’s front pages and newscasts: The oil that befouled the Gulf of Mexico for 86 days is vanishing from the surface, leaving workers with little to clean.

But scientists warn the oil’s ecological impacts are shifting, not ebbing, thanks to massive volumes of dispersants that have kept the crude beneath the waves.

"This is a management decision, to use dispersants," College of William and Mary marine science professor Robert Diaz said yesterday. "It doesn’t make the oil go away, it just puts it from one part of the ecosystem to another."

That dispersed oil now hovers, diluted in the water column, posing a challenge for scientists to track and measure the subsea plumes. Mapping the long-term effects of the nearly 2 million gallons of dispersant used by BP PLC may well be equally difficult, given the array of unanswered questions that surround the products’ rapid breakdown of oil droplets and their chronic toxicity.

In other words, while dispersants may have helped spare the Gulf’s birds, the chemicals are likely shifting dangers to other species lower in the food chain. The National Research Council described dispersant use in 2005 as "a conscious decision" to direct hydrocarbons to one part of the marine ecosystem, "decreasing the risk to water surface and shoreline habitats while increasing the potential risk to organisms in the water column and on the seafloor."

Diaz spoke at a Capitol Hill briefing aimed at guiding future research into dispersants, which remain a politically volatile topic even as their use in the Gulf tapers off thanks to the capped Macondo wellhead. A May meeting at the University of New Hampshire’s Coastal Response Research Center, planned by government scientists and oil industry representatives, yielded a consensus judgment that dispersant use "has generally been less environmentally harmful than allowing the oil to migrate on the surface into the sensitive wetlands."

Another group of scientists, however, issued a public plea (pdf) last week that decried dispersants and warned that, mixed with oil, the products "pose grave health risks to marine life and human health."

Coastal Response Research Center co-director Nancy Kinner sought to put the May statement in context. "Nobody’s saying dispersants are great," she said after yesterday’s briefing, though they are an effective alternative when mechanical methods of collecting spilled oil prove impossible.

Kinner outlined a series of gaps in the current system of testing dispersants. U.S. EPA’s analysis of their toxicity focuses on acute effects of exposure in two representative species, but "we do very little chronic toxicity work," she said. That work would evaluate whether dispersants could heighten the mortality of larvae and other sensitive species that may not die off immediately but studies show are absorbing tiny droplets of dispersed oil into their shells.

Dispersant studies have not examined the products’ long-term effects and their consequences when applied at high pressure, Kinner added, which BP did by spraying the chemicals subsea near the leaking wellhead. Diaz added another mystery to the list, noting that current studies focus on marine organisms that may not be feeling the brunt of this summer’s dispersant assault.

"All the risks we’ve evaluated have used surface, shallow-water species that are easily maintained" in a laboratory setting, Diaz said. "We haven’t been using oceanic species to assess risk, and this is a key issue."

Measuring long-term effects

The Marine Environmental Research Institute’s director, Susan Shaw, the organizer of last week’s statement against dispersant use, agreed that the current extent of testing falls short. Oil mixed with the Corexit dispersant used by BP "is probably having a lethal effect on all these