Video: Low-oxygen dead zone found on seafloor off Alabama coastBy Ben Raines, Press-Register
Sunday, July 04, 2010, 5:00 AM
Recent testing by the Press-Register indicates that a low-oxygen dead zone is hugging the seafloor in places along the Alabama coast, with levels far below the threshold required to support life.
The newspaper sampled oxygen levels in the water column every 5 feet from the surface to the bottom at two natural gas platforms, using an electronic meter made by YSI Inc. The meter is a standard piece of equipment used by state and federal agencies for such field testing.
The paper’s results mirror those obtained by Dauphin Island Sea Lab scientist Monty Graham, who has been surveying oxygen levels at a series of locations, primarily in a north/south line due south of the Sand Island Lighthouse, for several years. He began finding low oxygen levels about a month after the Deepwater Horizon oil spill began in April.
On June 27, the Press-Register tested water at two platforms situated 9 miles apart off the Alabama coast. Both locations were several miles east of the area where Graham has sampled most often.
Healthy oxygen levels of 4 parts per million or higher were found in the upper portion of the water column at each location. But beginning at about 15 feet above the seafloor, levels declined to as low as 0.1 parts per million. Scientists consider 2 parts per million to be the minimum required for most marine life.
The lowest levels were seen at a platform 4 miles southeast of Fort Morgan, which sits in 50 feet of water. Salinity there was 34 parts per thousand, typical of seawater, suggesting the low oxygen was not associated with water flowing out of Mobile Bay, which typically has areas of low oxygen in the summer.
Scuba dives revealed that fish were abundant at both platforms, with large schools of jack crevalle, blue runners, bluefish, spadefish, and red snapper present. At each platform, those fish seemed to avoid the low oxygen areas.
At the platform 13 miles southeast of Fort Morgan, which sits in 70 feet of water, oxygen readings were at 2 parts per million 30 feet above the bottom and grew worse as depth increased. Fish there were concentrated in a band beginning about 35 feet from the bottom and extending to within 10 feet of the surface.
At the platform closer to shore, oxygen levels declined to 2 parts per million at 35 feet below the surface. By 40 feet underwater, oxygen was at 0.1 parts per million.
The myriad creatures that typically grow on the legs of such oil and gas platforms — barnacles, anemones, tunicates, corals, and spiny oysters, among others — were all dead below the 43-foot mark.
Graham has speculated that the low oxygen levels could be related to increased microbial activity as bacteria break down oil, both at the surface and suspended in the water column.
He said there are also indications that the low-oxygen water welled up from deep water off the continental shelf.
Graham conducted a cruise on June 28 and found low oxygen at the same depths that the Press-Register encountered the day before. He said a plankton trawl pulled that day through the area roughly between the platforms visited by the newspaper found there was "definitely nothing (alive) on the bottom."
The low oxygen areas threaten the creatures that live on or buried beneath the seafloor, such as clams and marine worms. The worms, for instance, are exceptionally abundant and provide a primary food source for snapper, grouper and other fish.
"The higher the temperature, the less time they can survive those kinds of oxygen levels," said Vern Minton, head of the state’s Marine Resources Division, adding that the time frame for many of the creatures would be hours, not days. "It’s a significant concern, but not one we can do anything about."
He said the affected areas will recolonize, though how long that takes depends on several factors, including how big the dead area ultimately becomes, and the life cycles of the various species.