A huge “dead zone” of water so devoid of oxygen that sea life cannot live in it has spread across 5,800 square miles (15,000 square kilometers) of the Gulf of Mexico this summer, in what has become an annual occurrence caused by pollution.
The extensive area of uninhabitable water may be contributing indirectly to an unusual spate of shark bites along the Texas coast, experts said.
A scientist at the Louisiana Universities Marine Consortium said Tuesday that measurements showed the dead zone extended from the mouth of the Mississippi River in southeastern Louisiana 250 miles (400 kilometers) west to near the Texas border, and was closer to shore than usual because winds and currents.
“Fish and swimming crabs escape (from the dead zone),” said Nancy Rabalais, the consortium’s chief scientist for research into hypoxia, or low oxygen. “Anything else dies.”
Linked to fertilizers
In the last 30 years, the dead zone has become an annual summer phenomenon, fed by rising use of nitrate-based fertilizers by farmers in the Mississippi watershed, Rabalais told Reuters.
The nitrates, carried into the gulf’s warm summer waters by the river, feed algae blooms that use up oxygen and make the water uninhabitable.
The dead zone’s size has varied each year depending on weather conditions, but averages about 5,000 square miles (13,000 square kilometers) and remains in place until late September or early October.
Virtually nothing is being done to stop the flow of nitrates into the river, meaning the dead zone will reappear every year, Rabalais said.
Linked to shark bites
The dead zone forces fish to seek better water, which may be a reason for the recent shark bites on Texas beaches.
Three people have been bitten by sharks along the upper Texas coast this year — a high number for a state that has recorded only 18 shark attacks since 1980.
Terry Stelly, an ecosystem biologist with the Texas Parks and Wildlife Department, said increasing numbers of sharks have been found in recent years in the waters along the Texas-Louisiana border, near the edge of the dead zone.
Along with other factors, “chances are good they (sharks) were looking for higher dissolved oxygen in the water,” he said.
Rabalais agreed, saying “The higher number of sharks in shallow waters may very likely be due to the low oxygen being close to the shore at the time of the attacks.”
“The available habitat for the sharks is definitely less when the low oxygen is so widespread,” she said.
