Scientists unable to identify "smoking gun" in 2016 coral mortality event at Flower Garden Banks National Marine Sanctuary

By Elizabeth Weinberg

August 2018

In July 2016, something mysterious happened at Flower Garden Banks National Marine Sanctuary. On July 25, recreational divers were at East Flower Garden Bank, one of the three banks that make up this sanctuary in the Gulf of Mexico. There, they noticed an area where corals and other organisms were dead and covered in white bacterial mats. The culprit was unclear.

This localized mortality event was unlike anything sanctuary researchers had ever seen before. The coral reefs of Flower Garden Banks National Marine Sanctuary are some of the healthiest in the world, and this patch of dying reef at East Bank looked unlike any known coral disease or damage.

Sanctuary staff quickly alerted scientists from around the world. The scientific community mobilized, conducting surveys on the mortality site a week later and the following year. This February, the sanctuary, in partnership with the Gulf of Mexico Coastal Ocean Observing System (GCOOS) and the U.S. Integrated Ocean Observing System (IOOS), brought together 40 researchers from different disciplines to discuss possible causes of the mortality event. A new report summarizes their findings.

divers over coral
Scientists documenting the bleaching event underway in Flower Garden Banks National Marine Sanctuary. Photo: G.P. Schmahl/NOAA

The initial event

Luckily, the area of dead coral was not far from a long-term monitoring site at East Flower Garden Bank that sanctuary scientists were visiting that day. The recreational divers who noticed the die-off were able to immediately alert the scientists, and within days, Flower Garden Banks National Marine Sanctuary had conducted initial assessment dives in the mortality area.

Approximately six and a half acres within East Flower Garden Bank were affected – an area equivalent to about five football fields. Some surveys showed that up to 80 percent of the coral cover within that area was affected.

The preliminary assessment suggested that the event was neither related to ship traffic nor an oil spill. No ships had been in the vicinity, so it most likely wasn’t linked to a discharge of pollutants. So what caused the die-off?

animation showing salinity over time in the gulf of mexico
This visualization shows salinity levels in the Gulf of Mexico. White areas indicate low salinity levels, which usually stay near the coastline. Notice the intrusion of freshwater south of the Texas/Louisiana border; this increase in freshwater is a possible contributing factor to the die-off. Image: United States Naval Research Laboratory – Stennis Space Center

No “smoking gun”

Since July 2016, researchers have been hard at work trying to find an answer to this conundrum. Unfortunately, though many potential causes have been suggested, there is no “smoking gun” evidence of something that directly caused the mortality event.

What researchers at the recent symposium did agree upon is that low dissolved oxygen in the area was a key factor.

Not long before the mortality event occured, heavy rainfall and widespread flooding took place along the Gulf coast. Much of that water washed into the Gulf of Mexico. Because freshwater is less dense than seawater, a sudden influx of it can lead to layers of different salinity forming within the water column. According to Andrea Kealoha, a Dr. Nancy Foster Scholar and Texas A&M Ph.D. candidate, that hazy freshwater surface layer may have blocked sunlight and photosynthesis, reducing the overall oxygen content of the water. Plus, when the organic matter swept into the sea by the freshwater plume decayed, that decomposition process would have further depleted the area’s oxygen content, explains Dr. Frank Muller-Karger, biological oceanographer at the University of South Florida.

Though corals look like plants, they’re actually animals – so like us, they need oxygen to survive. According to research conducted by Dr. Sarah Davies of Boston University, coral polyps affected by the event seemed to be experiencing oxidative stress – that is, they didn’t have enough oxygen to respire healthily.

At the symposium, Dr. Robert Hetland of Texas A&M University pointed out that July surface temperatures and bottom temperatures were the highest ever recorded. Corals depend on a specific window of temperature to survive, and high temperatures can stress them. Stressed corals could have been more susceptible to whatever happened next.

researchers standing together for a picture
Researchers from different disciplines gathered at Flower Garden Banks National Marine Sanctuary headquarters to discuss possible causes of the mortality event. Photo: NOAA

A model for future monitoring

Although the exact cause of the mortality event is still unknown, this collaboration among researchers points to the importance of monitoring throughout marine environments, both within and beyond national marine sanctuaries. Because scientists were in the sanctuary at an established monitoring site on the day the mortality event was first reported by divers, Flower Garden Banks National Marine Sanctuary scientists were able to respond rapidly. The monitoring sites also helped establish a baseline so that researchers could compare the anomaly to normal conditions.

So far, monitoring activities have shown that this area of East Flower Garden Banks will show some long-term effects. In October 2017, coral cover within the die-off zone was at 17 percent, compared to 50 percent in the healthy long-term monitoring site at East Flower Garden Banks.

Dr. Michelle Johnston, a Flower Garden Banks National Marine Sanctuary research biologist, says scientists will continue to monitor the situation. “While the mortality event affected a localized portion of East Bank, it is important to remember that 99 percent of the bank is still healthy, along with all of West Flower Garden Bank,” she says. “We have a comprehensive long-term dataset, and this will help track any changes to the reef in the future.”

research diver surveying coral
A researcher surveys the area of East Flower Garden Bank affected by the mortality event. Photo: NOAA

Research partners in the investigation of this mortality event include NOAA’s Office of National Marine Sanctuaries, Flower Garden Banks National Marine Sanctuary, NOAA’s Deep Sea Coral Program, the National Weather Service, the U.S. Integrated Ocean Observing System (IOOS), the Gulf of Mexico Coastal Ocean Observing System, Texas A&M University, Texas A&M University – Corpus Christi, the Marine Biodiversity Observation Network, Smithsonian MarineGEO, the Bureau of Ocean Energy Management, University of Southern Mississippi, the Texas General Land Office, Rice University, Boston University, University of Houston, University of Delaware, University of Miami-RSMAS, the U.S. Geological Survey, and University of South Florida. For more information, see the symposium report.

Elizabeth Weinberg is the social media coordinator and writer/editor for NOAA’s Office of National Marine Sanctuaries.