A Texas A&M research team says processes leading to hypoxia led to the death of parts of the reefs at the Flower Garden Banks National Marine Sanctuary.
(From Texas A&M University/ By Keith Randall ) — 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, Atchafalaya, 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…
Read the full article here: https://today.tamu.edu/2020/01/03/answer-found-for-die-off-of-coral-reefs-in-gulf-of-mexico/