The Gulf of Mexico faces a renewed and enlarged threat to marine life: a low-oxygen “dead zone” about the size of Massachusetts, caused by chemical runoff into the Mississippi River that flows into the sea.
The dead zone, which occurs in Gulf waters in summer and is unrelated to BP Plc’s oil spill, covers an area twice as large as last year, according to a National Oceanic and Atmospheric Administration study released this week. The low-oxygen area this year is the fifth-largest since measurements began in 1985.
Aside from the dead zone, where shrimp and other sea life can’t survive, and the BP spill that dumped an estimated 4.1 million barrels of crude oil into the Gulf, there’s a looming threat of hurricanes. Meteorologists at the U.S. National Hurricane Center say a warming of the Atlantic indicates the storm season could be one of the most active on record.
“You start adding these things up, and there’s a question of what the cumulative effect is and how much additional stress the ecosystem can take,” Kevin Craig, a professor at Florida State University’s Coastal and Marine Laboratory who’s studied oxygen depletion for a decade, said in a telephone interview.
The Atlantic may have 15 or more named storms between now and the end of hurricane season in November, according to researchers at Colorado State University.
Move or Die
This year’s dead zone is bigger because of more nitrogen-and phosphate-rich sewage, fertilizer and other agricultural runoffs flowing from the Mississippi River, Nancy Rabalais, head of the NOAA research team that produced the dead-zone study and director of the Louisiana Universities Marine Consortium, said in a telephone interview. Larger farms that require more fertilizer resulted in higher concentrations of nitrates in the river, she said.
Dead zones, or areas of hypoxic water defined as having 2 milligrams or less of oxygen per liter, form when bacteria use up oxygen to break down fertilizer- and nitrate-fed algae. Many types of marine life can’t survive in hypoxic areas, and must migrate away from the zones or perish. That means fishermen have to trawl farther and spend more on fuel, Rabalais said.
This year’s dead zone, which stretches from the Mississippi River Delta west toward Galveston, Texas, measured 7,722 square miles (20,000 square kilometers) as of July 31, the NOAA study said. The estimate is likely too low, according to Robert Diaz, a professor of marine science at The College of William & Mary in Gloucester Point, Virginia.
Spill’s Possible Impact
Tropical storm Bonnie blew through the region last month, before measurements began, stirring up and re-oxygenating the water, Diaz said in a telephone interview. “The hypoxia can reform in a matter of just a few weeks,” he said.
The oil dumped into the water by the BP spill from April 20 to July 15 could create dead zones by spawning an explosion of bacteria that feed on crude, Rabalais and Diaz said. The spill “may cause a local oxygen drawdown” as bacteria decompose the oil, Diaz said.
Scientists haven’t tied this year’s confirmed dead zone to effects from the oil spill, Rabalais said.
Hypoxia, first documented off the Louisiana coast in 1972, can result in a 10 to 15 percent loss for shrimp fisheries, according to a study conducted by Martin Smith, an associate professor of environmental economics at Duke University. Louisiana’s fishing industry is the second-largest in the U.S., behind Alaska’s.
Fishing Industry Hurt
“Shrimping is very, very slow right now,” Terrie Looney, a coastal and marine agent for the Texas Sea Grant College Program, said in a telephone interview.
The season opened July 15 in Texas, and shrimpers have netted 25 percent of what they normally would, Looney said.
“When you have dead water, you have dead zones,” said Kyle Kimball, a third-generation fisherman in Nederland, Texas. “Fish have to move out of that water because there’s no oxygen.”
Kimball, who usually skippers his 60-foot shrimp boat from Vermilion Bay in Louisiana down the Texas coast, said his catch is 60 to 70 percent below normal this year. After finding much of his usual fishing areas dead and being chased from Louisiana waters closed off because of the oil spill, he said he had to travel more than 100 miles to Freeport, Texas, to net shrimp.
The western Gulf of Mexico, including the dead zone, will have a shrimp harvest of about 45.2 million pounds this year, 20 percent below the historical average, according to the Southeast Fisheries Science Center in Galveston.
“Most of the time the fish and shrimp are smart enough to realize the oxygen is going down and flee,” Diaz said. Even so, the food supply for the creatures is diminished by hypoxia, he said, reducing fish, shrimp and crab populations.
The damage to wildlife from the oil spill and low-oxygen zones could be compounded by hurricanes, said Craig, the Florida State University scientist. Tropical storms could flush animals out of their marsh habitats and damage oyster beds, he said.
While the Gulf can recover from hurricanes and low oxygen, the effects of the oil spill remain a long-term concern, Craig said.
“How it’s going to influence hypoxia or wildlife or fisheries is an unknown,” he said.