A MASS of unidentifiable toxic substances remains in waters and on shores long after oil spills appear to have cleared up. The mystery compounds can cause long-term harm to marine creatures, disrupting their feeding habits and damaging their general health.
To identify the constituents of organic mixtures such as crude oil, scientists use a method called gas chromatography. However, this only picks out certain compounds. Thousands of hydrocarbons cannot be distinguished, leaving an ungainly 鈥渉ump鈥� in the chromatograph trace (see Graphic).
When crude oil is 鈥渨eathered鈥� 鈥� digested by bacteria and battered by storms 鈥� the identifiable compounds tend to break down, leaving the hump of unknown hydrocarbons. Because toxicologists don鈥檛 know what these are, they often ignore them.
Advertisement
Steve Rowland, a geochemist at the University of Plymouth in Britain decided to test whether the unknown hump of substances was harmful. His team collected mussels, which are often used as markers of coastal health, from British waters known to have been contaminated with oil. They extracted the hydrocarbons from inside the mussels and then removed all the constituents they could identify, leaving them with a solution containing just the hump of unknown substances. The researchers then immersed healthy mussels in the solution.
Rowland found that the mussels went off their food, eating up to 70 per cent more slowly after a day鈥檚 exposure. 鈥淭hey鈥檙e not keeling over and dying, but there seem to be long-term effects,鈥� he says.
Meanwhile, John Widdows, a biologist at the Plymouth Marine Laboratory, has seen effects in the real world that might be due to these unknown toxins. He monitored the feeding habits of mussels in the North Sea and Irish Sea to assess coastal health. In polluted areas, the mussels are 鈥渙nly just surviving鈥�, he says. But the toxins he has been able to identify only account for about a seventh of their ill health. Rowland鈥檚 toxic hydrocarbon hump certainly goes some way to explain the anomaly, he says.
Even more strikingly, Jeff Short, a chemist at the US National Oceanic Atmospheric Administration鈥檚 Auke Bay lab in Alaska, who studies the after-effects of the Exxon Valdez spill in 1989, has found that weathered oil can harm fish. He tagged and released thousands of pink salmon after rearing them from eggs left on rocks sprayed with various concentrations of weathered oil. He then kept track of how many returned to spawn. 鈥淭he higher the dose, the fewer came back,鈥� he says. The oil appeared to cause developmental problems that showed up months after their exposure. Short doesn鈥檛 know which compounds are to blame, but says those in Rowland鈥檚 humps are likely candidates.
Rowland鈥檚 team is now working on improving methods to identify the substances in weathered oil. 鈥淭he compounds we鈥檙e finding look like they should be toxic, but we can鈥檛 find any toxicology data on them.鈥�