Dead zones regularly haunt coast

Dead zones regularly haunt coast

Low-oxygen waters known as dead zones have appeared as regularly as the tides along the Texas coast, according to a new Texas A&M University study.

Although the first report of a dead zone off Texas came in 1979, researchers now believe the condition has repeated itself annually for at least 23 years and will likely continue — not unlike the lethal cycle that has threatened fish and shrimp in the waters off Louisiana for decades.

The cause is probably the slurry of soil that flows out of several rivers and into the Gulf of Mexico, but A&M oceanographer Steve DiMarco said more water-quality studies are needed.

DiMarco analyzed samples of Texas coastal waters and found that oxygen-starved dead zones have formed between the Louisiana border and Brownsville in all but one year since 1985, the oldest data available.

Evidence previously suggested sporadic dead zones. The new finding surprised DiMarco because the understanding had been that hypoxia, the scientific term for oxygen deficiency, was not persistent along the Texas coast.

Scientists still don’t know the impact of the Texas‘ dead zone, which extends at least 20 miles offshore. DiMarco and industry officials said there are no reports of lower fishing catches or marine animal die-offs.

"We need to be out monitoring the water quality of coastal Texas in a systematic way so that if something really bad happens to the health of the coastal environment, we will at least have some data to have a shot at identifying a cause," DiMarco said.

Mike Segall, owner of Reel Threel Saltwater Charters in Galveston and Freeport, said he has not found any oddities.

"You go offshore, and the fishing should be fine," he said.

The size of the dead zones off Texas has fluctuated over the years, with the largest ever measured coming last year, when the National Oceanic and Atmospheric Administration detected a 1,750-square-mile area of low-oxygen water emanating from the mouth of the Brazos River near Freeport.

The finding prompted DiMarco’s study.

"The question became: If it happened once, how often does it happen?" he said.

The study focused on areas where freshwater enters the Gulf, including the Sabine Pass, Matagorda Ship Channel, Galveston-Bolivar Pass, Aransas Pass and Brazos-Santiago Pass.

The freshwater brings nutrients and organic material into the Gulf, causing algae to bloom. After algae die and sink to the bottom, bacteria that thrive on the plant matter consume additional oxygen, making the water unable to support most forms of sea life.

Sometimes fish can swim away from dead zones, but many bottom-dwellers simply die. The condition also prevents shrimp from reaching depths where they can grow to a more valuable size, said Benny Gallaway, a Bryan-based marine biologist who advises the Texas Shrimp Association.

"They have the potential to be bad news," he said of dead zones.

Gallaway said he believes the Texas condition is linked to Louisiana‘s massive dead zone, which is created each spring by nutrient-rich farm runoff and sewage that flows into the Gulf from the Mississippi River. After a 1993 flood of the river, the dead zone crept west as far as Galveston.

But DiMarco found low levels of nitrates out of the Brazos River last summer, suggesting that the Texas problem may not be man-made. What’s more, the dead zones along the Texas coast do not appear to be continuous.

"It tells us that the hypoxia is linked to local phenomena more than to the Mississippi River," he said.

matthew.tresaugue@chron.com