Algal ‘dirty blizzard’ helped scrub fallout

An undersea “dirty blizzard” helped soot and contaminants from the Deepwater Horizon explosion drop to the Gulf of Mexico’s floor – but toxins were still swirling around five months after the spill was plugged and the fires extinguished.

Researchers in the US and Germany, led by Beizhan Yan from New York’s Columbia University and University of California, Santa Barbara’s Uta Passow tracked oil byproducts and microscopic life that percolated near the spill for more than a year.

They found black carbon, fallout from crude oil burning on the surface, was still present in the Gulf’s waters two months post-cap. But it had a helping hand sinking to the bottom.

The reason? A bloom of oil-loving algae dubbed a “dirty blizzard” because it clumped with black carbon and contaminants as it swirled around the Gulf.

They published their work in the Proceedings of the National Academy of Sciences.

Only a few months after the explosion that destroyed Deepwater Horizon and spilt around 3.2 million barrels of crude oil into the Gulf of Mexico between April and July 2010, biologists noticed deep-water corals 11 kilometres from the well were damaged and dying and found evidence of petroleum at the site.

So how did oil, which floats on water, manage to make it all the way to the bottom of the sea – around 1,370 metres below the surface – after a relatively short period of time?

The link between sea surface and floor is massive, yet all but invisible. Oceans can be filled with “marine snow” – particles less than half a millimetre wide such as algae. As it “falls” from the surface, it snares suspended substances and deposits them on the sea floor.

Following the Deepwater Horizon explosion, NASA’s Moderate Resolution Imaging Spectroradiometer saw evidence of a giant marine snowstorm of algae Skeletonema blooming nearby. Skeletonema thrives on crude oil and is usually found around extraction sites.

To see how the bloom would affect to the spill’s fallout, Yan, Passow and colleagues suspended a two-metre-high funnel-shaped sediment trap around seven kilometres south-west of the Deepwater Horizon well. They chose the spot so it sat underneath the subsurface oil slick, 1,540 metres deep and around 100 metres above the sea floor.

In August and September 2010, they counted more than eight billion algae in the water per cubic metre per day. This marine blizzard was dominated by Skeletonema and carried black carbon, along with toxins such as barium and olefin compounds which are used in drilling mud.

The algal bloom was followed by a drop in numbers, then another, bigger increase March 2011. The short-lived decrease, the researchers write, may be because algae blooms and polluted waters can be patchy.

Worryingly, not all barium sank to the sea floor. The researchers found traces in the carbonate shells of plankton. As plankton are at the bottom of the oceanic food chain, there are repercussions for animals further up the chain.

“The environmental implications of such an unexpectedly long residence time of barium in the water column is significant and worth of further investigation,” they write.