A scuba diver swims among a school of fish at NOAA’s Flower Garden Banks National Monument. Photo: NOAA Photo Library/Flickr

A hundred miles south of the Texas-Louisiana border lies one of the smallest yet most remarkable Marine National Sanctuaries in US waters—and scientists are worried that its vibrant reefs could be devastated as floodwaters from Hurricane Harvey start to spill in.

A ‘secret garden’ situated along the edge of the continental shelf, the Flower Garden Banks National Marine Sanctuary features rolling hills of brightly-colored sponges and corals, including some of healthiest populations of stony corals left the Caribbean. The sanctuary is a hotbed of marine biodiversity, home to manta rays, sea turtles, hammerheads, whale sharks, and a panoply of smaller fish and invertebrates. “No one has heard about it, but it’s a really spectacular place,” Rice University biologist Adrienne Correa told Earther.

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The reefs of Flower Garden Banks have held up remarkably well even as nearby Caribbean reefs decline in health due to disease, pollution, and climate change. But the ecosystem has also taken some recent hits, including a 2016 die-off in East Flower Garden Bank, which Correa says might have been partially triggered by an influx of fresh water from Houston’s “Tax Day Floods” last April. That’s part of the reason scientists are now concerned that floodwaters from Harvey, which dumped 13 trillion gallons of rainfall over southeast Texas in August, could create problems for the delicate ecosystem.

Flower Garden Banks National Marine Sanctuary. Photo: NOAA

“With Harvey, basically we saw twice the volume of the Great Salt Lakes dumped on the Texas coast,” Correa said. “This water all runs off and collects on the Texas shelf.”

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On September 28th, an ocean buoy maintained by Texas A&M University measured a 10 percent drop in salinity at Flower Garden Banks, from 36 to 32 parts per thousand. While salinity levels had rebounded by October 4th, it’s possible that this was the first in a string of freshwater pulses that’ll seep into the saltwater-adapted sanctuary from Harvey’s massive plume.

Along with researchers from the University of Houston-Clear Lake, Texas A&M and Boston Universities, Correa is headed out to the reef next week to see whether the freshwater pulse caused any damage. The scientists will be looking at water chemistry, and the composition of microbial communities both in the water and on the corals and sponges, for signs that the reefs are stressed.

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But the first sign of trouble could be something much starker.

“Definitely the biggest concern is a mass mortality event out on the reef,” Correa said. “If it’s anything like what we saw in 2016, all of the invertebrates and little fish can be killed very quickly if there’s an issue with water quality.”

“The minute we dive on the reef, we’ll be able to see visually if there’s a mass mortality,” she added.

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While Correa and her colleagues are concerned with problems arising from changes in salinity, Harvey’s floodwaters also accumulated toxic pollutants and raw sewage as they stagnated over Houston and breached dozens of industrial facilities. At this point, it’s unclear whether pollution unleashed by the storm poses a threat to marine ecosystems.

Corals affected by a 2016 mortality event at East Flower Garden Bank. Photo: FGBNMS/Schmahl via NOAA

“Certainly, there are going to be abundant pollutants in what ran off,” Correa said. But, she added, any reef impacts will depend on how much those pollutants got diluted, something that hasn’t been studied yet.

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Regardless of what impact Harvey’s floodwaters have, Correa says one of the best ways to ensure the long-term survival of these reefs is to expand the sanctuary. Currently, Flower Garden Banks only encompasses three of the dozens of reef banks scattered along the northern Gulf of Mexico’s continental shelf. An expansion proposed last year by NOAA would protect an additional 383 square miles of reefs.

“Expanding the boundaries isn’t going to stop freshwater from reaching Flower Banks,” Correa said. “But it at least can give [these ecosystems] the best chance of staying at maximum health.”