Summer deluge spurs Texas ‘dead zone’ in Gulf

The 1,750-square-mile area of oxygen-depleted water, stretching from Freeport to Matagorda Bay, is potentially deadly to marine life.

 

A larger dead zone, caused by farm fertilizers, soil erosion and discharge from sewage treatment plants carried into the Gulf by the Mississippi River, forms annually off the Louisiana coast.

 

Until this summer’s heavy rainfall, Texas A&M University oceanographer Steve DiMarco said, Texas rivers typically were incapable of carrying enough fresh water into the Gulf to create dead zones.

 

That changed a month ago, when a National Marine Fisheries Service boat taking fish counts also measured oxygen in the water. Those scientists passed their results to DiMarco.

 

"I’m looking at this data and I’m astounded," he said. "We’ve long expected that Texas had the potential for this to occur, but it typically doesn’t rain in Texas enough for this to happen."

 

DiMarco said he plans to travel to the area, at the mouth of the Brazos River, as soon as this weekend to take further measurements and gauge the impact of Texas’ dead zone, which extends about 25 miles offshore.

So far, he said, he hasn’t had any reports of detrimental effects, such as lower fishing catches or marine animal die-offs.

 

The rains of June and July have pushed the Brazos beyond all known discharge records, according to the U.S. Geological Survey.

 

For the month of July, the Brazos’ average discharge, measured near Rosharon, was nearly 57,000 cubic feet per second, a rate of flow that would fill the Reliant Astrodome in about 11 minutes.

 

That’s more than twice the rate ever measured on the Brazos since 1967, when records were first kept, said Jeffery East, data chief for the USGS Texas Water Science Center in Houston. The average discharge rate for the Brazos in July is a mere 4,700 cubic feet per second.

 

The river’s outpouring has been fairly constant, too, because of several large reservoirs built upstream of the Brazos, such as Lake Whitney. These lakes hold floodwaters and, instead of letting all the water go at once, release it over time.

 

"What this does is elevate the discharge of the Brazos for a lot longer time," East said.

 

Fresh river water is less dense than salt water, so when it reaches the Gulf, it spills on top, DiMarco said. Fresh and salt water mix like oil and water.

 

Scientists believe the fresh water depletes oxygen — a condition known as hypoxia — through a couple of mechanisms.

 

First, because there’s little mixing, oxygen at the surface isn’t transported to the salt water below where marine organisms live.

 

Additionally, the fresh water brings nutrients and organic material into the Gulf, conditions ripe for algae blooms. When this organic material dies, it sinks to the bottom, and bacteria that thrive upon it consume additional oxygen below.

 

Sometimes fish die in large hypoxic areas, but they can swim away. But immobile organisms, such as clams, simply die without access to oxygen.

 

Dead zones are most prominent in summer, when winds, which aid in mixing, are weakest. So the dead zone could remain for a couple of months unless the freshwater supply slows up, or, perhaps ominously, a strong tropical storm or hurricane stirs waters off the coast.

 

"Yep," DiMarco said, "a hurricane would do it."

 

eric.berger@chron.com