Oceanographers fear a massive freshwater plume in the Gulf of Mexico could harm a marine sanctuary 100 miles offshore of Galveston.

A group of scientists will visit the Flower Garden Banks sanctuary this week to assess the effects of the plume, caused by torrential rain during Hurricane Harvey, on the marine ecosystem, said Emma Hickerson, research coordinator at the sanctuary.

Coral reefs in the sanctuary require salty water to survive, according to the National Oceanic and Atmospheric Administration.

“You’ve got an enormous amount of rainfall that has to go somewhere, and that somewhere is into the Gulf of Mexico,” Hickerson said. “What we don’t know at this point is if it reached the reef.”

Hurricane Harvey made landfall Aug. 25 in Rockport, about 200 miles south of the county. The storm dropped 33 million gallons of water in the United States, according to some estimates.

Early numbers show that the resulting plume of freshwater is close to twice the volume of the Great Salt Lake in Utah, said Steve DiMarco, a Texas A&M University professor and principal investigator of the Texas Automated Buoy System, which collects data at various spots around the Gulf.

The lake averages a volume of 5 trillion gallons, according to the Utah Geological Survey, placing the freshwater plume at more than 10 trillion gallons.

Worries about the freshwater plume stem from a “large-scale mortality event” at the sanctuary in 2016, when corals, sponges, sea urchins and other invertebrates died in unprecedented numbers. Scientists now believe that the die-off could be linked to a stream of freshwater that entered the Gulf after the Tax Day floods in April 2016, Hickerson said.

At the time, “huge patches of ugly white mats” coated corals and sponges in the East Flower Garden Bank, killing nearly 50 percent of the corals in the areas they were located.

After that event, researchers had a hard time figuring out how the die-off occurred, said Adrienne Correa, an assistant professor of ecology and evolutionary biology at Rice University.

“The timeline of that event and our recognition of it made it difficult for us to figure out what the specific mechanisms were that underlaid mortality,” Correa said.

Correa will leave for the sanctuary Friday, along with biologists and oceanographers from Texas A&M University at Galveston, University of Houston-Clear Lake and Boston University, officials said.

No conclusions have been made, but the freshwater plume doesn’t seem to have traveled very far off the coast, Correa said.

“Fortunately, the large freshwater mass that we see out there right now, the entirety of it doesn’t seem like it’s going over the banks,” Correa said. “There’s still a chance that winds could bring an eddy or a jet over the banks.”

The mass is moving southwest along the coast, DiMarco said. Researchers were more worried immediately after Hurricane Harvey, when it was still summer, he said.

“There was great, great concern that because it was during the summer season, the winds would be favorable to bring the freshwater farther offshore,” DiMarco said. “Now that we’re into more of the fall season, the winds are unfavorable for that.”

Researchers did detect a slight drop in salinity when they went to the banks in late September, but levels should be restored by now, Correa said.

The sanctuary also is coming off a large coral bleaching event that occurred in October last year. Coral bleaching, which is tied to warmer water temperatures, occurs when coral loses the symbiotic algae that gives it color, Hickerson said.

Bleached corals are not dead but can eventually starve and die if the algae doesn’t return, she said.

Harvey actually cooled off the water and pulled the area out of the danger zone for bleaching, Hickerson said.

Although the coral should be healthier now, being exposed to repeated stressful events, such as bleaching or freshwater exposure, could be a “death by a thousand cuts,” Correa said.

“We can see colonies and different organisms being pushed to the brink over time,” Correa said. “As the frequency and intensity of hurricanes and other extreme storms increase around the world, understanding the impact of those on reefs becomes more important.”

Samantha Ketterer: 409-683-5241; samantha.ketterer@galvnews.com or on Twitter at @sam_kett

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