Low-oxygen pockets found off Alabama coast, raising new fears for sea life
Wednesday, June 09, 2010
Scientists measured two areas of low oxygen off the Alabama coast Tuesday, finding levels below the threshold that marine creatures need to survive.
The first low-oxygen pocket was found on the bottom in about 60 feet of water 12 miles off the coast due south of the mouth of Mobile Bay. The second was in about 100 feet of water 25 miles off the coast.
The low-oxygen layer was about 9 feet thick in each location. Scientists traveled a north-south line during the sampling, so there was no indication as to how wide the areas were.
Scientists said the unusual presence of low-oxygen areas off the coast raises the possibility that the number of oil-eating microbes has swelled because of the millions of gallons of oil in the Gulf.
Water samples collected during the most recent cruise will be analyzed to determine whether oil and oil-consuming microbes were present along the seafloor, said Monty Graham, a University of South Alabama researcher working out of the Dauphin Island Sea Lab.
There is growing concern among scientists that plumes of oil suspended underwater in the Gulf may cause long-term damage to the marine ecosystem.
“I think it adds to the weight of evidence coming from the other people working on those offshore plumes of oil,” said George Crozier, director of the Sea Lab.
Oxygen levels have declined steadily since a May 28 sampling effort, Graham said. Levels were between 4 and 6 milligrams per liter at the research sites on May 28. By June 2, they had declined to 3 milligrams per liter or less. Tuesday, researchers measured 1.7 milligrams per liter.
Scientists consider 2 milligrams per liter the threshold for survival.
“Below 2 milligrams per liter, creatures are stressed. Fish will leave an area looking for more oxygen,” Graham said. Creatures that can’t move — barnacles, oysters, mussels and burrowing animals such as the marine worms — will simply die.
Low oxygen in a wide swath of the Gulf off Louisiana is responsible for the infamous dead zone there. Extra nutrients in the water flowing into the Gulf from the Mississippi River feed microbial blooms. Those microbes, both while they are alive and after they die and decay, consume oxygen.
A similar phenomenon appears to be occurring off Alabama. The question, Graham said, was whether the Deepwater Horizon oil spill is responsible for the increased microbial activity.
He said that it is also possible that heavy rains in April and May caused a flush of nutrients to flow into the Gulf from the rivers that drain into Mobile Bay. However, Graham and other scientists have never measured a similar decline in oxygen off the coast despite years’ worth of data from the same sample locations, he said.
Graham’s research is conducted through the Fisheries Oceanography of Coastal Alabama program, with funding from the Alabama Department of Conservation and Natural Resources and National Science Foundation. Data from the research cruises can be viewed at http://focal.disl.org/oil.html