What do past spills predict for Deepwater Horizon impact?

From ANIMAL PEOPLE, June 2010:

The Deepwater Horizon oil discharge, after 62 days, was
believed by the U.S. Coast Guard to have reached a volume of as much
as 156 million gallons–making it the second worst oil disaster in
history, 15 times larger than the 1989 Exxon-Valdez oil spill in
Prince William Sound, Alaska. The Deep-water Horizon spill is
expected to reach 250 million gallons by the time BP completes
drilling four pressure relief wells in August 2010 and finally caps
the undersea gusher.
The warm waters of the Gulf of Mexico and the Carribean Sea
receive more than four times as much sunlight per year than the
Prince William Sound, however, and that translates into
exponentially greater activity by wind, waves, and microorganisms
to mitigate the effects of oil spills.

The largest previous offshore oil spill anywhere was a 1979
blowout at Ixtoc I, a Pemex drilling site about 500 miles southwest
of the Deepwater Horizon. Ixtoc I spewed 138 million gallons of
crude oil into the Gulf before Pemex stopped the leak with a relief
well completed 290 days later.
“The oil was everywhere, long black sheets of it, 15 inches
thick in some places,” fouling 150 miles of Texas beach, recalled
Glenn Garvin of the Miami Herald.
Marine biologist Wes Tunnell anticipated ecological death for
the region. Marine life declined by as much as 50% in some
samplings; 80% in others. Amphipod marine worms and small
crustaceans, near the bottom of the food chain for fish and birds,
“were practically wiped out,” Tunnell said. The female portion of
the endangered Kemp’s ridley sea turtle population fell to just 300.
Yet Kemp’s ridley sea turtles recovered to record nesting seasons in
2006 and 2007, and the other afflicted species rebounded as well.
“You look around and it’s like the spill never happened,” shrugged
Tunnell. “There’s a lot of perplexity in it for many of us.”
“The environment is amazingly resilient, more so than most
people understand,” said National Autonomous University of Mexico
deep sea biologist Luis A. Soto. “We thought the Ixtoc spill was
going to have catastrophic effects for decades. But within a couple
of years, almost everything was close to 100% normal.”
Soon after Ixtoc I was capped, Hurricane Frederic hit.
“Overnight, half the 3,900 tons of oil piled up on Texas beaches
disappeared,” wrote Garvin. “Human clean-up efforts began putting a
dent in the rest. Even in Mexico, which had neither the resources
nor the hurricanes of the U.S., the oil began disappearing under a
ferocious counterattack by nature. In the water, much of it
evaporated; on beaches, the combined forces of pounding waves,
ultraviolet light, and petroleum-eating microbes broke it down.”
“The environment in the Gulf of Mexico is used to coping with
petroleum,” concluded Tunnell. “The equivalent of one to two
supertankers full of oil leaks into the Gulf every year. The outcome
of that is a huge population of bacteria that feed on oil and live
along the shoreline.'”
The Ixtoc I experience was echoed in the aftermath of other
major oil spills in the Gulf of Mexico and Caribbean Sea. Among the
half dozen largest, in July 1979 the supertankers Atlantic Empress
and Aegean Captain collided off Tobago, spilling 46 million gallons
of crude oil. The Atlantic Empress spilled another 41 million
gallons off Barbados on August 2, 1979 while being towed. On June
8, 1990, 60 miles southeast of Galveston, Texas, the supertanker
Mega Borg lost 5.1 million gallons. On November 28, 2000, on the
Mississippi River south of New Orleans, the oil tanker Westchester
ran aground near Port Sulphur, Louisiana, dumping 567,000 gallons.
More than seven million gallons of oil were spilled during Hurricane
Katrina from various sources, a total volume about two-thirds of the
size of the Exxon Valdez spill. Then, on January 23, 2010, the
tanker Eagle Otome and a barge collided in the Sabine-Neches Waterway
near Port Arthur, Texas, losing about 462,000 gallons.
Two much smaller oil spills that were in the news in early
2010 demonstrated that the location and timing of oil spills are
larger factors in causing harm to wildlife than volume, though the
greater the volume lost, the greater the risk that some oil will
drift into sensitive habitat.
In Edmonton litigation started over the deaths of at least
1,606 ducks who on April 28, 2009 landed in a pond of oil resulting
from Syncrude Canada oilsands extraction work in northern Alberta.
Oilsands extraction in the vicinity, involving three companies, is
also believed to have killed 27 black bears, 31 foxes, 21 coyotes,
and dozens of deer, plus moose, muskrats, beavers, voles,
martens, wolves and bats, according to Mike Hudema of Greenpeace:
164 mammals in all.
On April 6, 2010 the U.S. Fish & Wildlife Service announced a
$16.9 plan to restore migratory seabird habitat damaged by leaks from
the Jacob Luckenbach, a tanker carrying up to 450,000 gallons of oil
that sank in 1953, 17 miles southwest of the Golden Gate Bridge.
The leaks killed more than 50,000 birds between 1990 and 2002, when
the source of the oil was finally identified and the wreck was
drained of oil.
The biggest oil spill ever occurred at the southern end of
the San Joaquin Valley in California, on March 14, 1910.
“Halfway between the towns of Taft and Maricopa in Kern
County,” recalled New York Times science writer Justin Gillis, “a
well blew out and continued spewing oil for 18 months. The ultimate
volume spilled was calculated at 378 million gallons. Nearly half
was recovered and refined by the Union Oil Company. The rest soaked
into the ground or evaporated. Today,” Gillis wrote, “little
evidence of the spill remains.”

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