Texas A&M oceanographers have discovered two processes they believe led to hypoxia and likely caused large portions of coral reefs at the Flower Garden Banks National Marine Sanctuary to die three years ago.

Corals, sponges, urchins and sea stars also were harmed in the process. While about 3% of the reef was affected, some parts of the reef — which is located 100 miles from Galveston’s coast — suffered up to 80% loss according to Texas A&M Today. Those areas have yet to recover.  

The researchers believe that hypoxia, or low levels of oxygen, was caused due to a combination of freshwater runoff from the Mississippi, Atchafalaya and Brazos rivers and an upwelling of dense water onto the reef. A&M assistant professor Katie Shamberger was part of the research team that made the discovery, which was published in Coral Reefs, the journal of the International Coral Reef Society.

“We believe the combination of two different processes — river runoff and upwelling — caused localized hypoxia that killed invertebrates on the reef,” Shamberger told Texas A&M Today. “In other words, both processes happened simultaneously to cause hypoxia, and one of them alone may not have caused any trouble.”

The river runoff came first, Shamberger said, making it to the reefs in a thin surface layer. This low-salinity water probably never reached the reefs 60 feet below but was turbid and blocked sunlight, which Shamberger said probably caused there to be more respiration and reduced photosynthesis.

“As a result, oxygen on the reef was being used up faster than it was being produced,” Shamberger said in Texas A&M Today. “This would be no big deal if the water on the reef mixed with surrounding water with normal oxygen levels to replace the oxygen being used up by reef organisms. But we think a second process, called upwelling, helped prevent mixing.”

Within 48 hours, Shamberger said, the upwelling happened, causing deep, dense water to get into pockets on the reef. Mixing led to respiration, which used up the bottom layer of oxygen and killed reef organisms. 

Shamberger said researchers are unsure of how often runoff and upwelling occur simultaneously or how likely they are to cause hypoxia on the reefs again, but she said there is definitely a cause for concern since climate change and ocean warming usually worsen hypoxia.

Andrea Kealoha, who is now at the University of Hawaii Maui College, also was a part of the research team. 

“Hypoxia in coral reefs is an emerging stressor that is gaining more attention,” Kealoha said in Texas A&M Today. “Yet, it is very difficult to measure hypoxia because many reefs, like the Flower Garden Banks, are isolated. By the time anyone can figure out what’s happening, it’s often too late.”

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