Runoff from two Houston floods—the 2016 Tax Day flood and the 2017 Hurricane Harvey flood—carried human waste onto coral reefs more than 100 miles offshore, research finds.
“We were pretty shocked,” says Rice University marine biologist Adrienne Correa, coauthor of the study in Frontiers in Marine Science. “One thing we always thought the Flower Garden Banks [National Marine Sanctuary] were safe from was terrestrial runoff and nutrient pollution. It’s a jolt to realize that in these extreme events, it’s not just the salt marsh or the seagrass that we need to worry about. Offshore ecosystems can be affected too.”
Harvey, the most intense rainfall event in US history, dropped an estimated 13 trillion gallons of rain over southeast Texas.
The Flower Garden Banks sit atop several salt domes near the edge of the continental shelf about 100 miles from the Texas and Louisiana coast. Rising several hundred feet from the seafloor, the domes are topped with corals, algae, sponges, and fish. Miles of open ocean separates each bank, or dome-topped ecosystem. The Flower Garden Banks National Marine Sanctuary, which recently expanded, protects 17 banks.
Correa and colleagues sampled sponges at the sanctuary in 2016, 2017, and 2018. They showed samples collected after extreme storm flooding in 2016 and 2017 contained E. coli and other human fecal bacteria. They also used a catalog of E. coli genetic markers contributed by Rice environmental engineer and coauthor Lauren Stadler to show that E. coli on sponges in 2017 came from Harvey floodwaters.
Lead author Amanda Shore, who conducted the research while a postdoctoral fellow in Correa’s lab, says many studies have shown nearshore reefs can be harmed by pollutants that are washed into the ocean by rainfall over land. But marine biologists generally assume ecosystems far from shore are safe from such dangers.
“This shows perhaps they aren’t protected from severe events,” says Shore, an assistant professor of biology at Farmingdale State College in New York. “And these events are increasing in frequency and intensity with climate change.”
Correa says, “That’s the other piece of this. There actually was a massive flooding event in 2015 with the Memorial Day flood. Dips in salinity after that event were detected at surface buoys offshore, but nobody looked or sampled out at the Flower Garden Banks. Nobody imagined you would see something like this 160 kilometers out.”
Houston floods and sponges
In April 2016, widespread flooding occurred in the Houston area when a severe storm dropped more than 17 inches of rain in some places in less than 24 hours. Three months after the flood, recreational divers reported murky waters and dead and dying organisms at East Flower Garden Bank. Marine biologists, including study coauthor Sarah Davies of Boston University, arrived two weeks later to investigate.
Shore and coauthors Carsten Grupstra, a Rice graduate student, and Jordan Sims, a Rice undergraduate, analyzed samples from the expedition, including tissue collected from sponges. Shore says sponges are indicators of water quality because they “are basically filtering seawater to catch organic material to use as food.”
She says previous studies have shown sponges have a microbiome, a population of bacteria that normally live in and on these animals. In this study, Shore characterized the microbiomes on two species: giant barrel sponges, or Xestospongia muta, and orange elephant ear sponges, or Agelas clathrodes. It was the first time the species’ microbiomes had been assayed at Flower Garden Banks, and Correa says that was one reason it took so long to understand what happened in the flood years.
“In 2016, we saw differences between sponge bacteria at a location that showed signs of death and a location that didn’t show signs of death, but we couldn’t get at the cause of the differences because we had no baseline data,” Correa says. “We thought we’d be able to get the baseline data—the normal year—the next year in 2017. But then there was another disaster. We couldn’t get a normal sample in a no-flood year until 2018.”
Shore joined Correa’s lab in 2018, helped collect samples that year and analyzed the microbiomes from each year.
Correa says, “There was a big change in community composition, a shift of the team players, on the sponges that were most affected in 2016. Then, following Harvey in 2017 there was also a shift, but less water made it out there that year, and we think it was less stressful. We didn’t see dead and dying organisms like we had the previous year.”
Harvey, the most intense rainfall event in US history, dropped an estimated 13 trillion gallons of rain over southeast Texas in late August 2017. The researchers says Harvey posed a greater potential threat to the Flower Garden Banks, by far, than the 2016 flood. So why did reefs fare better in 2017?
“Because we got lucky with ocean currents,” Shore says. “Instead of going straight out from Galveston Bay and over the Flower Garden Banks, the water ended up turning a bit and going down the Texas coast instead.”
Harvey’s runoff still sideswiped the banks. Research buoys at the reefs measured a 10% drop in salinity in less than a day on September 28, and Correa’s team found genetic evidence that fecal pollution gathered from the banks in October originated in Harvey floodwaters in Houston.
Houston flooding in the future
The story in 2016 was more complicated, says Correa.
“There was an upwelling event that brought nutrients and cooler waters up from the deep to the top part of the Flower Garden Banks,” she says. “Fresh water is less dense than salt water, and we think the floodwaters came at the surface and sort of sat there like a lens on top of the salt water and kept oxygen from mixing in from the top. The combination of this surface event and the nutrients coming up from the bottom contributed to a bacterial bloom that drew down so much oxygen that things just asphyxiated.”
The big question is whether pollution from extreme storms poses a long-term threat to the Flower Garden Banks. The answer could come from an investment in research that follows the health and microbiomes of individual sponges and corals on the reef over time, says Correa. Her group at Rice and her collaborators are committed to learning as much as they can about the reefs, she says, and are determined to support efforts to conserve and protect them.
Study coauthors are also from Texas A&M University and the University of Houston-Clear Lake. Funding for the work came from the National Science Foundation; the Gulf Research Program of the National Academies of Sciences, Engineering, and Medicine; and Rice.
Source: Rice University