The BP Deepwater Horizon oil spill meets the Gulf hypoxic dead zone

From ANIMAL PEOPLE, June 2010:
NEW ORLEANS–Sixty-three days after the Deepwater Horizon oil
spill started on April 20, the documented toll on wildlife included
997 dead birds, only 265 of them oiled; 749 oiled live birds; 400
dead sea turtles, only eight of them oiled; 128 live sea turtles,
84 of them oiled; and 51 mammals, 47 of them dead, including 38
dolphins, but only four of them oiled.
“These are the consolidated numbers of collected fish and
wildlife reported to the Unified Area Command from the U.S. Fish &
Wildlife Service, National Oceanic & Atmospheric Administration,
incident area commands, rehabilitation centers, and other
authorized sources operating within the Deepwater Horizon/BP incident
impact area,” prefaced the online report, updated daily at
<www.ibrrc.org/gulf-oil-spill-birds-treated-numbers-2010.html>.

“Researchers say there are several reasons for the relatively
small death toll,” summarized Associated Press writers Jay Reeves,
John Flesher, and Tamara Lush. “The vast nature of the spill means
scientists are able to locate only a small fraction of the dead
animals. Many will never be found after sinking to the bottom of the
sea or being scavenged by other marine life. And large numbers of
birds are meeting their deaths deep in the Louisiana marshes where
they seek refuge from the onslaught of oil.”
“Historically, we estimate that 10% of oiled birds are
found,” Tri-State Bird Rescue & Research team member Rebecca Dunne
told Mira Oberman of Agence France-Presse.
Opined Greenpeace marine biologist John Hocevar, “I think
part of the reason why we’re not seeing more yet is that the impacts
of this crisis are really just beginning.”
But none of these factors are unique to the Deepwater Horizon spill.
What is unique is that the Deepwater Horizon spill occurred
within the Gulf hypoxic dead zone, the legacy of more than 30 years
of an entirely different and mostly ignored environmental disaster.
Agricultural runoff from throughout the Mississippi River
drainage basin produces an annual accumulation of nitrogen,
phosphorus, and other nutrients that feed algal blooms where the
freshwater flow from the mouth of the Mississippi mingles with the
warm Gulf of Mexico saltwater. Algal dieback and decomposition then
consumes oxygen faster than the Gulf currents can bring more oxygen
down from the surface.
Fish avoid the dead zone as it expands each spring–and so do
fish-eating birds and marine mammals. The oil spill gave them
additional incentive to depart.
“What’s amazing is there’s so little bird life out here right
now,” National Oceanic & Atmospheric Administration turtle
researcher Kate Sampson told Kim Murphy of the Los Angeles Times on
June 17. “Either they’ve moved on, or the oiling has had a
tremendous impact. We saw a few laughing gulls fly by yesterday,”
Sampson said. “They were oiled, but they could still fly. And we
saw a northern gannet, a diving bird. It was oiled too.” Mostly,
though, Sampson suggested, “I can only imagine that the birds left
because the dining hall is closed.”
Wrote Murphy, “A few dead fish float in the water, though
dolphin-fish, tuna, flying fish, and the occasional shark can still
be seen swimming near the surface, threading their way through the
wavy, sometimes iridescent gobs of crude.”

Big as two states

At peak size each July, the Gulf hypoxic dead zone averaged
about 3,200 square miles in size from 1985 to 1992. It nearly
doubled to 6,200 square miles from 1993 to 2001. Then it expanded to
8,500 square miles in 2002, larger than Connecticut plus Rhode
Island, and has usually been close to that size in recent years,
according to data collected by Gene Turner of Louisiana State
University and Nancy Rabalais, chief scientist for Northern Gulf of
Mexico Hypoxia Studies.
2009 was an exception, with the dead zone occupying just
3,000 square miles, but 2010 is expected to be a record year, as
the Deepwater Horizon oil and methane discharges add to the hypoxic
effect.
The Deepwater Horizon oil contains about 40% methane, eight
times more than most oil deposits, Texas A&M University
oceanographer John Kessler told Associated Press writers Matthew
Brown and Ramit Plushnick-Masti on June 18.
“In early June, a research team led by Samantha Joye of the
Institute of Undersea Research and Technology at the University of
Georgia investigated a 15-mile-long plume drifting southwest from the
leak site,” reported Brown and Plushnick-Masti. “They said they
found methane concentrations up to 10,000 times higher than normal,
and oxygen levels depleted by 40% or more. The scientists found that
some parts of the plume had oxygen concentrations just shy of the
level that tips ocean waters into the category of ‘dead
zone’-uninhabitable by fish, crabs, shrimp and other marine
creatures. Kessler said he has found oxygen depletions of between 2%
and 30% in waters 1,000 feet deep.”
The net effect appears to be that the Deepwater Horizon is
creating a deep water dead zone beneath the algally created dead zone
closer to the surface. “Representatives of the National Oceanic and
Atmospheric Administration acknowledged that so much methane in the
water could draw down oxygen levels and slow the breakdown of oil in
the Gulf, but cautioned that research was still under way to
understand the ramifications,” wrote Brown and Plushnick-Masti.
Texas A&M University oceanographer Steven DiMarco suggested
that giant squid might be affected, since they live in deep water,
and also sperm whales, who feed on squid. One dead sperm whale was
found floating 77 miles south of the Deepwater Horizon site during
the third week of June, reported Kate Spinner of the Sarasota Herald
Tribune. “That it was caused by the spill is hard to say,” Texas
A&M University marine biology professor Randall Davis told Spinner.
“Davis was among a group of researchers in the mid-1990s who
recommended that the Mississippi Canyon area–where the oil spill is
occurring–be protected [as critical habitat] for sperm whales,”
Spinner recalled. “Such protection might have stopped drilling in
the area, or at least put further restrictions on permits. But
Davis said the Minerals Management Service, which funded the
research, took the scientists’ recommendation out of their report.”
As the Deepwater Horizon oil drifts east toward coastal
Alabama and Florida, it may extend the Gulf hypoxic dead zone to
areas not previously affected.
Mark Robson, director of the Florida Fish and Wildlife
Conservation Commission Division of Marine Fisheries Management,
told Reeves, Flesher, and Lush that “his agency has to find any
scientific evidence that fish are being adversely affected off his
state’s waters,” they wrote. “He noted that it is common for fish
to flee major changes in their environment, however,” so that the
evidence of an impact might be the absence of fish, rather than big
fish kills, as occur as result of oxygen depletion in lakes and
rivers.
“In some areas along the coast, researchers believe fish are
swimming closer to shore because the water is cleaner and more
abundant in oxygen,” Reeves, Flesher, and Lush continued. “More
oil could eventually wash ashore and overwhelm the fish. They could
also become trapped between the slick and the beach, leading to
increased competition for oxygen in the water and causing them to die
as they run out of air.”