Texas suffers hypoxic areas of its own making

August 02, 2012

By Jim Hiney

COLLEGE STATION, Texas — This is one time it’s better that not everything is bigger in Texas.

Texas A&M University (TAMU) oceanographer Dr. Steven DiMarco and Oceanography doctoral student Ruth Mullins-Perry have documented periodic hypoxic zones — areas of water with very low levels of dissolved oxygen — off the Texas coast. These Texas zones are often not related to the more famous “Dead Zone” spreading from the mouth of the Mississippi River and, at their zenith, cover just one-fourth the area of their better-known cousin.

These zones are also most certainly not dead.

“We tend not to use the term ‘Dead Zone’ anymore because it is a misnomer,” says DiMarco, whose research was funded by the Texas Sea Grant College Program. “‘Dead Zone’ implies there is no life in it and even though an area has low oxygen, it does not mean it is devoid of life. In fact, it could be teeming with life, even in low oxygen conditions.”

That said, hypoxic zones warrant concern among scientists and the general public because there is still much about them and their impacts on sea life that remains unknown.

“If you like to fish or catch crabs, then you should care about hypoxic zones,” Mullins says. “They become a coastal hazard, a stressor to the benthic community and a stressor to the fisheries.”

Hypoxic zones are not new phenomena and are not exclusive to the Gulf of Mexico. Areas of low dissolved oxygen exist around the world on continental shelves and are typically associated with areas where there are large amounts of nutrients entering the area. “Nutrients cause algal blooms and these blooms go through a lifecycle — the phytoplankton live and they die,” DiMarco explains. “When they die, they sink to the bottom where they decay. The process of decay uses oxygen.”

The low oxygen levels persist in places where freshwater inflows from land meet saltwater. “Just like oil and water, freshwater and saltwater have different densities so they don’t mix, they remain in separate layers and this stratification presents a barrier for getting oxygen from the atmosphere down to the lower layers of the ocean,” DiMarco says.

The hypoxic zone off the Mississippi River is both well known and well documented. The river pours nutrients and freshwater into the Gulf of Mexico, creating textbook conditions for persistent low dissolved oxygen levels. Texas, too, has experienced occasional hypoxic zones for years along the upper coast. Conventional wisdom held that the Texas hypoxic zones were created when Mississippi River water traveled down coast in the summertime, when there were no significant winds to mix the layers of water and when heat and dry conditions exacerbated the problem, DiMarco says.

“Then, in 2007 while we were on a cruise off Louisiana, we received data from National Oceanic and Atmospheric Administration (NOAA) that showed a region of hypoxia extending from 70 miles southwest of Freeport,” he says. “That got our attention and rang the alarm that there was this very large area of low oxygen off Texas and we needed to get out into it.”

Rapid response money from Texas Sea Grant allowed DiMarco to fund a data collection cruise to the area. Their research, published in a subsequent paper, found that Brazos River flooding caused by torrential rains dumped larger than normal amounts of freshwater into the Gulf of Mexico, effectively placing a cap on top of the saltwater and allowing dissolved oxygen levels to fall. Through collaboration with TAMU geologists Drs. Ethan Grossman and Josiah Strauss to look at stable isotopes of oxygen, DiMarco and Mullins were able to determine that the freshwater came from the Brazos River. “This discovery changed the game and proved that in some circumstances you can have local Texas causes of the Texas hypoxia,” Di Marco says.

In 2010, DiMarco received a $95,000 research grant from Texas Sea Grant to study the locations, frequencies, durations and possible causes of hypoxic zones in Texas waters. He combined this funding with money from NOAA earmarked for hypoxia research and partnered with the Texas General Land Office to conduct cruises where his team collected data and deployed moored buoys equipped with instruments that continually monitor the surrounding waters. Data collected during the cruises provided DiMarco with a snapshot of water conditions over a large area while the buoys gave him long-term information on specific areas.

When the data revealed hypoxic areas, DiMarco and Mullins tried to tie the incidents to physical causes like freshwater inflows and diminished winds.

“One of the unique things we’re seeing on the Texas side is hypoxic zones are not limited to the summer,” says Mullins-Perry, noting that the Mississippi River hypoxic zone usually appears in April or May, peaks in July and usually disappears in September. “The Brazos is not as big as the Mississippi, but when there is water coming out of the Brazos, it can, at times, provide enough freshwater to stratify the system. Instead of a persistent summertime hypoxia, we will see episodic shorter events, maybe on the order of days, in the early spring and even through the late fall.”

Unlike the Mississippi River, Texas rivers do not carry significant amounts of nutrients — primarily nitrates and phosphates — from large scale agriculture operations to fuel algal blooms. DiMarco believes the nutrients contributing to Texas’ hypoxic zones come from much deeper in the Gulf of Mexico.
“Our idea is the Louisiana side is a large river dominated system. There is a huge signal from the Mississippi and the paradigm is that there is a large amount of nutrients available in the river and the wetlands, and there is a lot of freshwater, so you get this whole system that can very easily go hypoxic” DiMarco says. “Off the Texas coast it is more like a traditional continental shelf. There are benthic processes that contribute to consuming oxygen, but where do the nutrients come from to create that carbon on the sea floor? In most systems around the world that don’t have a huge freshwater source like the Mississippi, the nutrients come from upwelling. Texas appears to be a typical upwelling system.”

Upwelling refers to the process by which warm, less-dense surface water is drawn away from along a shore by offshore currents and replaced by cold, denser water brought up from greater depths.
“In June the winds change a bit here. Cold water comes up along the Texas coast every summer and that water brings nutrients with it,” he continues. “There is typically a phytoplankton bloom associated with this upwelling. Decay of phytoplankton is not an issue until the water stratifies and there is no way to ventilate the bottom layer of water. What we’re seeing in Texas is the need for a freshwater event to cause the stratification and hypoxia.”

DiMarco characterizes his theory of upwelling nutrients as a “working hypothesis” that has yet to be fully tested, and finding proof is not an easy next step. “We must separate the upwelling carbon out from an offshore source or from the Louisiana side, where the bloom comes from the river runoff,” he says. “Finding the driver will take a different kind of approach than what we are doing now, but we want to pursue it.”

Understanding how hypoxic zones affect fisheries may prove just as difficult as finding the cause. Fish, shrimp and other sea life tend to flee hypoxic areas, so they do not die but “they will be displaced, so you may need to go farther offshore or down the coast to find them,” Dimarco says. “There is also some indication now that chronic low oxygen conditions can interfere with the reproduction cycles and physiology of some commercially valuable fish. We are trying to identify quantifiable impacts of hypoxia, but that has been elusive. We’ve been trying to find impacts in Louisiana and haven’t, which may indicate a resilience in the fish. The problem is that in other areas around the world with hypoxia, the fisheries have collapsed, so understanding the system will lead us to have more information about whether the system is close to collapse and how much that particular system can take.

“Fisheries are economically important to Texas and Texans. Our coastal areas are places where we get a lot of seafood and do a lot of recreation,” he continues. “We don’t want to lose these things so understanding the system is important.”

The Texas Sea Grant College Program is a partnership of university, government and industry focusing on marine research, education and outreach. It is administered through the National Oceanic and Atmospheric Administration (NOAA) and is one of 32 university-based Sea Grant Programs around the country. Texas Sea Grant is based in the College of Geosciences Texas A&M University.

Pictured:

Dr. Steve DiMarco (L) and Dr. Antonietta Quigg, associate professor of marine biology at Texas A&M University at Galveston, deploy a CTD profiler into the Gulf of Mexico off the Texas Coast. The profiler collects information on water temperature, salinity and dissolved oxygen as part of DiMarco’s research on hypoxic areas in Texas coastal waters. Photo by Ruth Mullins-Perry

Dr. Steve DiMarco (L) and Dr. Antonietta Quigg, associate professor of marine biology at Texas A&M University at Galveston, deploy a CTD profiler into the Gulf of Mexico off the Texas Coast. The profiler collects information on water temperature, salinity and dissolved oxygen as part of DiMarco’s research on hypoxic areas in Texas coastal waters. Photo by Ruth Mullins-Perry

Water samples taken from different locations in the Gulf of Mexico off the Texas coast reveal varying levels of dissolved oxygen after undergoing chemical treatment. Water samples that turn brown (L) are adequately oxygenated. Hypoxic samples turn white, like those on the right. Photo by Jim Hiney

-30-

Texas Sea Grant is a unique partnership that unites the resources of the federal government, the State of Texas and universities across the state to create knowledge, tools, products and services that benefit the economy, the environment and the citizens of Texas. It is administered through the National Oceanic and Atmospheric Administration and is one of 33 university-based Sea Grant Programs around the country. Texas Sea Grant is a non-academic research center in the College of Geosciences at Texas A&M University. The program’s mission is to improve the understanding, wise use and stewardship of Texas coastal and marine resources.