Lightkeepers: Gulf Coast May Be Permanently Changed by Oil Spill

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Gulf Coast May Be Permanently Changed by Oil Spill

via Wired: Wired Science by Brandon Keim on 5/5/10

If a desperate, last-ditch attempt to cap the Deepwater Horizon wellhead succeeds in coming days, environmental damage to the Gulf of Mexico will still be severe but probably not long-lasting. But if the cap fails, and months pass before a diversionary well can be drilled, the Gulf may be profoundly and permanently altered.

Thousands of miles of marshlands, sea-grass meadows and coral reefs — and the human industries they support — could be damaged beyond recovery. This is a worst-case scenario, and far from certain. But as long as the oil keeps flowing, the odds of it happening get better.

"What worries me is that this could turn into a chronic problem, not just an episode," said ecologist James Cowan of Louisiana State University. "This could be the one additional thing that pushes us past a tipping point."

High concentrations of oil are acutely toxic, but low concentrations have more subtle, widespread effects. As oil percolates through food webs, it retards plant and animal growth, leaving them vulnerable to predation and disease, and less fit to reproduce. With the Deepwater Horizon spill already too large and unpredictable to contain, the question is no longer whether it will cause damage, but what form damage will take.

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If the flow is soon staunched, affected populations should rebound from losses in a few years, and even sooner if the oil stays at sea long enough to be churned by waves and consumed by microbes. Ecosystems will stay intact.

But if oil flows continue, plant and animal populations may be pushed to species-level tipping points, their numbers so low that replenishment is impossible. When this happens, food webs change. Some remaining species become more common, and others less. Disruption favors low-level opportunists that rush into newly open niches. Local ecosystems tip. If that keeps happening, an entire region can tip.

This seems to have happened in the northwest Atlantic, where overfishing for cod led to their permanent replacement by crabs and baitfish. In the northwest Mediterranean, a confluence of overfishing, pollution and climate change fueled the reign of algae and jellyfish. There's no going back from such transformation, at least not at human-relevant timescales. That degree of change is now conceivable across much of the Gulf of Mexico.

"If a perturbation is extensive enough, and lasts long enough, you can shift an ecosystem to an entirely different state," said John Valentine, senior marine scientist at Alabama's Dauphin Island Sea Lab. He called ecosystem tipping "the most extreme of possibilities," but said if the oil flow continues for several months, it could well happen in many areas. "There could be serious consequences for foundation species," he said.

It's not just a single food web that is jeopardized in the Gulf, but an interlocking mosaic of webs. One major component is the underwater sea-grass meadowland that forms a miles-deep fringe along the coastline. These meadows are home to many commercial fish and shellfish species, and are nursery grounds for other migratory fish.

The grass itself is a keystone species, critical for the survival of many other species, and ultimately holding the ecosystem together. If oil seeps deep into the seafloor, the roots of the grass could be choked, preventing it from regrowing. The same holds for above-water grasses in coastal salt marshes, which support other fish and shellfish species. When marsh grasses die, pools of water form, submerging marshes so deeply that new plants can't grow.

Vegetation death doesn't automatically lead to permanent wetland loss, "but it does have that potential," said plant ecologist Irv Mendelssohn of North Carolina State University.

However, "I have worked a number of oil spills where the plants died but were able to recruit back to the site after a few years," Mendelssohn said. His experience underscores the unpredictability of tipping points. A key factor is the presence of other environmental stresses. Tips are rarely caused by a single shock, but require multiple stresses.

Unfortunately, the Gulf is already stressed by fishing and pollution. Mississippi River dams and levees have altered water and sediment flows that historically nourished the delta. In the last 50 years, some 1,500 square miles of wetlands have vanished. Sea-grass losses range from 12 percent to two-thirds. Researchers have reported changes in species compositions, and growing areas of vegetation "patchiness," a pattern considered symptomatic of stressed systems verging on tips.

"The system is already becoming degraded," said University of New Orleans ecologist Denise Reed. It's too soon to know if local systems will tip, but "oil could push a marsh that's already hanging by its fingernails over the edge," she said.

Especially vulnerable are sea grasses and marshes in the western Gulf, home to a fishery worth $2.4 billion annually. The western Gulf has few beaches, which would allow oil to be cleaned with relative ease as it washes ashore. It lacks the barrier islands that line northern and eastern shores, calming waters and slowing the oil's advance. Louisiana's scalloped coastline could soak oil like a sponge.

"There's no first line of defense," said Cowan. And while blocks have been deployed around a few highly productive areas, these cover a tiny fraction of the coast, and could easily be overwhelmed.

Another important type of Gulf ecosystem is found further off the coastline, in coral-rich continental shelves. These are home to complex webs that support many fish species, including most of the red snapper found in U.S. restaurants. These areas will be affected even if oil never reaches land, said Cowan.

Gulf currents loop along coastlines east and west of the spill. Depending on their variability, day-to-day winds, and the spill's ultimate size, the oil could go in either direction, or both. Another current could bring it around the tip of Florida. In a few weeks, the Atlantic Basin hurricane season will begin, bringing storms that could drive oil deep into the wetlands.

"At this point, it all just depends on which way the wind blows," said Reed.

Images: 1) Containment boom at Breton National Wildlife Refuge./United States Coast Guard. 2) Oil slick imaged by Aqua satellite May 4./NASA. 3) Oil slick trajectory map from April 30 to May 5./National Oceanic and Atmospheric Administration. 4) Map of average Gulf of Mexico currents./University of Miami.

See Also:

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.