Texas Parks & Wildlife Magazine   


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Texas Dead Zones

Trapped layers of oxygen-starved water threaten the state’s coastal wildlife and coral reefs.

It was early May of 1979, and my dive buddy and I were some four miles off the mouth of the Brazos River near Freeport. Jumping overboard, we allowed our heavy sampling gear to rapidly pull us to the muddy bottom 50 feet below.

We’d made these monthly sampling dives for two years and expected nothing new. Most of the time the water this close to the shore and the river had limited visibility, one or two feet at best. This dive started the same as all the others.

Twenty feet down, everything changed. 

It was like dropping out of a thick cloud. The water became crystal clear. We could see the bottom rushing up at us 20 feet away and we could see a hundred feet in every direction. We plowed into the bottom harder than intended, stunned that we could see it and each other so clearly.

Then we realized that we could see nothing else.

No fish, usually right in our face mask. No crabs crawling around the gear or shrimp hopping away from the big puff of mud where we landed.

Instead, the bottom was covered in dead shrimp. Clams lay on the mud surface gaping open, also dead. On a nearby waterlogged tree branch, the limbs jutting up above the mud were carpeted with polychaete worms that had crawled as high on the branches as possible before dying next to shrimp and crabs.

We didn’t know it at the time, but we were the first scientists to see and document a “dead zone” off the Texas coast.


  Harte Institute;  Rainer Lesniewski | Dreamstime.com

What is a Dead Zone?

Dead zones have become more common and widespread than when we first dove into one in 1979. Some scientists cringe at the term (hypoxia is more appropriate), but many have come to embrace “dead zone” as a term that the public identifies as appropriately alarming.

When I told the story of our 1979 discovery to Paul Montagna, chair for hydroecology at the Harte Research Institute at Texas A&M University-Corpus Christi, he told me he was the first to report on the formation of hypoxia in Corpus Christi Bay, an unknown phenomenon there until he stumbled across it in 1988.

“I thought my instruments were malfunctioning, reading zero oxygen at the bottom of the bay,” he recalled. “I kept recalibrating and testing until I realized it wasn’t my instrument — there was no oxygen in the bottom foot of bay water. At the time it was not thought to be possible in shallow, wind-driven bays like Corpus Christi.”

What we both found were dead zones, where the water had no oxygen. Normal sea water will have a dissolved oxygen content of about 5 to 8 milligrams per liter of water, but when oxygen drops to 2 or 3 milligrams per liter, hypoxia sets in — for most animals, a decreased amount of oxygen can be fatal. Anoxia occurs when no oxygen is present; that is when the term “dead zone” becomes reality.


 Courtesy FWS

Underwater gliders are used to monitor oxygen levels in the Gulf.

Natural and human-caused 

We often think of pollution causing dead zones, but they do occur naturally. The most famous occurrence is called a “jubilee.” In Mobile Bay, Alabama, fish, crab and shrimp literally jump out of the water onto shore where “jubilant” Alabamians fill baskets, boxes and sacks with seafood. Jubilees have happened almost annually in Mobile Bay; the oldest reports date back to the 1860s.

Why does the jubilee happen? In summer, the upswelling of oxygen-poor bay bottom waters force marine life to flee ahead of the rising tide, leaving no place to go but onto the shore and into the waiting arms of those lucky enough to be there at the right time. Fresh seafood for days.

No jubilees have been officially reported in Texas, but there has been a steady increase in the number of fish kills because of hypoxia and dead zones. Hypoxia accounts for over a third of the fish kills since 1951, according to Texas Parks and Wildlife Department records.

Human activity is a common source of hypoxia-related fish kills. Excess nutrients from fertilizers, nonpoint runoff and wastewater discharge feed algae blooms. When the algae die and decompose, the process takes oxygen out of the water. Under the right conditions, hypoxia sets in.

“Just like we have to breathe and take in oxygen, the same thing happens in the ocean — without oxygen, marine organisms must move or die,” says Steve DiMarco, Texas A&M University physical oceanographer. “The oxygen will stay depleted until something comes along and puts it back in.”

Stratification of water can promote development of hypoxia and dead zones, especially when floods flush excess nutrients into coastal bays or, like the Brazos River, directly offshore. Waters of different salinities or temperatures can separate into different layers, like in a cake. If dead algae and decaying vegetation use up oxygen in the bottom layer and there is no mixing because of stratification, dead zones become possible.

DiMarco has monitored and studied Texas dead zones for years, learning about why they form and disappear. His 2007 study indicated that stratification is as important a factor as nutrient loss.

“This was a fairly controversial idea at the time,” DiMarco says. “I’m not saying nutrients are not important; they are very important. But if you don’t know both sides or make assumptions about one’s importance over the other, you’re going to make mistakes. Understanding both is key.”


 Courtesy FWS

Oxygen-depleted water resulted in bleached coral at the Flower Garden Banks.

Coral reef damage

DiMarco also reported on another unusual dead zone, almost 100 miles off the Texas coast at the Flower Garden Banks National Marine Sanctuary. With his marine glider (a robotic autonomous underwater vehicle) in the right place at the right time, DiMarco figured out a mystery that might have gone unsolved.

The massive outflow of fresh water from Hurricane Harvey created a 60-foot-thick layer of fresh water containing no oxygen; that anoxic layer slid over the top of the Flower Garden coral reef. Many of the corals high enough off the bottom to be touched by that layer “bleached” or turned white when their symbiotic alga died. The coral died, too.

I was diving there at the time and recorded the devastating results — blotchy patches of coral everywhere. Fish were few and far between, and, like the bottom off Freeport in 1979, dead marine life had climbed as high on the coral as possible. Before DiMarco’s data became available, there was speculation that nearby oil wells caused the problem, proving the value of monitoring data.

Montagna thinks conditions for dead zones are growing.

“Climate change and water diversions from our estuaries are increasing hypoxia and the chance for dead zone formation in the all the bays we have been sampling, especially since the mid-2000s,” he says, noting the inclusion of San Antonio and Matagorda bays. “Higher temperatures [climate change] and higher salinities [freshwater diversion] decrease the oxygen-carrying capacity of our bay waters.”


 Courtesy FWS

A diver conducts a coral survey.

Impact on People

After Hurricane Harvey hit the Texas coast in 2017, Samantha Aplin Hill’s crabbing business was threatened as flooding made it impossible for her to catch anything for six months straight. The reason for this phenomenon? Dead zones.

As a fourth-generation crabber, Hill has experienced the impact of Texas dead zones through her crabbing business, Aplin Seafood. Her main territory for crabbing runs from the mouth of the Brazos River all the way to Sargent. Since crabs are mobile creatures, they can usually escape the dangers of hypoxia by moving to land, occasionally dipping into the water to keep their gills wet — a sort of Texas jubilee but mostly unseen. However, the crabs already caught in a trap by the time hypoxia occurs are a different story.

“We notice when the water’s bad and I run traps,” Hill says. “One
day, they’re fine, but the next day, when I pull the traps out, the crabs are all dead.”

Because of all the storm surges, Hill says, the mouth of the San Bernard River closed over time, resulting in a lack of water flow.

“The mouth of the river opening again made me realize how crucial it is for us to have more current flow for our area,” she says. “After the river opened, we didn’t have dead zones or problems with water stagnation because the water was circulating.”

Hill learned how to handle hypoxia through her family’s previous experiences, but after Harvey, it was different.

Although not every crabber experienced the same problems, Hill says she kept coming up empty-handed and had to move. 


Samantha Aplin Hill's crabbing business took a hit from Gulf dead zones.


 Courtesy Samantha Aplin Hill

What Can we do?

DiMarco and Montagna think situations such as those faced by Hill will only get worse if we don’t address those contributing factors we can control. Mike Wetz, Harte Research Institute chair for coastal ecosystem processes, agrees but also says he’s encouraged by the health and resilience of Texas coastal waters.

“The Texas Commission on Environmental Quality maintains a robust monitoring program, and we used that data, in part, to produce a report card for the Texas coast,” Wetz says. “The coast scored a B-minus overall, and water quality scored high, which might surprise some people. It’s good news, considering all the stress we put on our coast.”

Wetz and his colleagues recently produced the first synthesis of TCEQ data and found that dissolved oxygen in Texas estuaries has generally been stable over many decades, and hypoxia and dead zones are generally localized in nature, forming where stratification, temperature and salinity come together to cause them. 

The good news is that they seem to quickly dissipate when those conditions disappear. There are only a few places where this may not be true, like in the upper reaches of Galveston Bay near the San Jacinto River and especially Baffin Bay in the Laguna Madre, where excess nutrients are the problem.

“Except for these areas, Texas coastal ecosystems seem resilient enough to bounce back from hypoxia and even hurricanes,” Wetz says. “Our goal must be to maintain that resilience and improve conditions where we can.”

It’s impossible to foretell whether Texas dead zones will worsen or improve. Hurricanes are a natural phenomenon with multiple factors affecting where and how hard they will hit; dead zones and hypoxia are even less predictable.

Scientists agree on the need for sustained monitoring of the ecosystem health along the Texas coast. Texas needs to continue its excellent water quality and fisheries monitoring programs, some of the best in the Gulf of Mexico. Similar programs are needed for benthos (animals that live on and in our bay bottoms and cannot swim away from hypoxia or pollution) as well as birds, seagrass and wetlands.

As our coastal populations grow and industrial development expands, these programs can provide warnings when things begin to get out of balance, so action can be taken before it’s too costly or too late.

Texas is blessed with a healthy and resilient coast, although it isn’t trouble-free or without challenges. The good news: If we take care of our coast, it will take care of us.

Larry McKinney is the chair for Gulf strategies at the Harte Research Institute at Texas A&M-Corpus Christi. Landry Allred is a former intern for Texas Parks & Wildlife magazine.

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