Aug 14, 2012
Why Deepwater Horizon led to snow
Scientists in the US have put forward several mechanisms by which "marine snow" could have formed after the Deepwater Horizon oil spill. Their study, which offers a rare insight into the behaviour of marine snow in an oil environment, could help researchers understand how the oil was redistributed after the accident and how to tackle oil spills in the future.
Marine snow is the common name for the clumps of organic and inorganic matter – including bacteria, phytoplankton, faeces and bio-minerals – that form near the surface of the sea and fall steadily downwards. Most of the time marine-snow particles are millimetre- or centimetre-sized, although the snow has been found to be up to three metres in diameter in the Mediterranean. How such large particles are formed is a mystery but they are known to suffocate organisms on the sea floor.
Shortly after the Deepwater Horizon oil spill two years ago, in which hundreds of square miles of the Gulf of Mexico were covered with oil, observers found marine snow that had a similar structure (although lesser size) to that of the Mediterranean. But in June 2010, a month after that observation, the Gulf's marine snow had disappeared. The implication was that it had sunk.
Now Uta Passow of the University of California, Santa Barbara, US, and colleagues have examined the formation of the Gulf's marine snow. The researchers believe that their new knowledge could help scientists to figure out how to tackle major oil spills in the future, since it is the presence of marine snow that determines whether oil will migrate from the water surface to the bulk water column, and down to the sea floor.
"Oil that settles on the sea floor will be consumed by organisms that provide food for certain fish," said Passow. "If no marine snow forms and the oil floats at the surface, turtles, birds and mammals are affected ... Thus it is generally of great importance to understand the mechanisms that determine how and in what form oil components are distributed."
Starting in May 2012, Passow, Kai Ziervogel at the University of North Carolina at Chapel Hill and colleagues from the University of Southern Mississippi collected samples of marine snow from the Gulf and tried creating their own marine snow in the lab. To do the latter, they collected samples of oily seawater, put it in large cylindrical jars and then placed the jars on roller tables, which slowly rotated the jars to simulate a marine environment. The scientists measured the properties of the resultant snow, including its sinking velocity, density and porosity.
The team found three mechanisms by which marine snow formed in an oily environment. In one, bacteria degraded the oil at the water's surface, forming mucous webs that could subsequently coalesce. In another, certain components of the oil coagulated and stuck to other suspended matter. In the final mechanism, algae collide to form aggregates, taking oil particles with them.
Although the marine snow left in the Gulf disappeared in a month, some of that kept in the lab was still floating after two years. The researchers believe that this could have been because they kept the snow in the cold and dark, which may have prevented it from ageing.
Passow cannot say what happens to marine snow after it has sunk. Nevertheless, the researchers believe that it could have been key to the cycling of oil after the Deepwater Horizon spill. It may have also led to the oil, or fossil carbon, being eaten by plankton. "Currently it is unknown how the fossil carbon entered the food web after the Deepwater Horizon spill," the researchers said. "However, planktonic grazing on oil-derived marine snow could represent an efficient pathway."
The study is published in Environmental Research Letter.
About the author
Jon Cartwright is a freelance journalist based in Bristol, UK. 12/9/12 This article has been updated to reflect researcher responsibilities.