What killed the colorful coral reefs in this Gulf of Mexico sanctuary?

Three years ago, parts of coral reefs in the Flower Garden Banks National Marine Sanctuary were killed in a dramatic event. What caused the death of the large and colorful reefs about 100 miles from the Galveston coast – with up to 80 percent mortality in some areas – was a mystery.

A team of oceanographers from Texas A&M University believes it has an explanation.

Considered some of the healthiest coral reefs remaining in the Caribbean and Gulf of Mexico, the reefs at the Flower Garden Banks grow on top of shallow, submerged banks in otherwise deep water. The Texas A&M team’s research, which was recently published in the current issue of Coral Reefs, found that two separate processes caused hypoxia on the reef, leading to the die-off.

Researchers from the Department of Oceanography included Katie Shamberger, Shawn Doyle, Jason Sylvan, Robert Hetland and Steven DiMarco, along with Andrea Kealoha, now at the University of Hawaii Maui College. The team found that hypoxia – low levels of oxygen – was caused by the transport of freshwater runoff from the Mississippi, Atchafalay and Brazos rivers, and an upwelling of deep, dense water onto the reef.

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

Shamberger said the first process, which was river runoff that flowed offshore, was most likely water from the Mississippi-Atchafalaya Rivers, but about one-fifth of the water was from Texas rivers. These waters made it out to the Flower Garden Banks as a thin, low salinity surface layer. Since it was on the surface, the low salinity water probably didn’t touch the Flower Garden Banks reefs, which are about 60 feet deep, but the runoff was turbid and blocked sunlight from the reef.

“Blocking light from the reef reduces photosynthesis, and we think this resulted in there being more respiration than photosynthesis by reef organisms. As a result, oxygen on the reef was being used up faster than it was being produced,” Shamberger said. “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.”

When upwelling occurred, Shamberger said, deeper and denser water settled into pockets on the reef. This presented mixing and allowed respiration to use up oxygen in the bottom layer, causing the death of reef organisms that couldn’t swim away from the hypoxic water.

“All of this could have happened very quickly – within 48 hours,” Shamberger said.

Only about 3 percent of the reef was affected by the die-off, but some of these areas experienced almost 80 percent mortality, she explained, and those areas have still not recovered. Corals, sponges, urchins and sea stars were some of the marine life most affected.

“While we know that river runoff makes it out to the Flower Garden Banks periodically, and that there is evidence that upwelling at East Bank also occurs, we do not know how often these processes happen simultaneously or how likely it is that they will occur together to cause hypoxia on the coral reefs in the future,” Shamberger said. “We do know that there is cause for great concern because human caused climate change and ocean warming are generally making ocean hypoxia worse.”