A new study into the 7-year stony coral disease outbreak decimating reefs from Florida to the Caribbean has found the disease can be lurking in sand on the ocean floor and rapidly spread lethal infections.
Scientists at the University of Miami and National Oceanic and Atmospheric Administration confirmed for the first time that sand exposed to infected coral can then transmit stony coral disease to healthy coral. Signs of the disease appeared in just a day, far faster than the week to two weeks it takes for the infection to spread among coral.
Researchers have already begun sharing the information with state and federal authorities who oversee coastal construction, including the expansion of South Florida’s busy ports.
“To see visible signs of lesion formation and bleaching after 24 hours was truly surprising,” said Michael Studivan, the study’s lead author and a coral scientist at UM’s Cooperative Institute of Marine and Atmospheric Science and NOAA’s Atlantic Oceanographic and Meteorological Lab. “It emphasizes the potential for sediments to be a rapid way of moving the disease from coral to coral.”
Stony coral disease was first detected off Virginia Key during PortMiami’s dredging of Government Cut to make room for mega ships, appearing on a 300-mile reef tract already struggling as climate change warms waters and stresses reefs. Over the decades, Florida's reefs have lost about 90 percent of their coral.
Confirming the disease can be found in ocean sediment and then transmitted to healthy coral doesn’t necessarily mean the massive $205 million port dredge started the disease, Studivan said. Modeling has shown the dredged sand did not get carried to areas where coral were infected at the time, he said.
But the findings do suggest that such work can trigger future outbreaks by spreading the disease.
The U.S. Army Corps is now considering another expansion at PortMiami, with the number of super ships sailing to the port projected to more than double by 2030.
“It's been one of the big unanswered questions with this outbreak, and I'm not sure if we will ever really understand where it came from,” Studivan said. “But what this study tells us is that perhaps the disease can persist in these sediments, and so it needs to be something that we take into consideration going forward.”
The PortMiami dredge moved 5 million cubic yards of sand to deepen Government Cut about 52 feet to make way for mega ships. Massive plumes of sediment spread from scupper transporting the dredged sand offshore, smothering and killing a half million coral and harming about 250 acres of habitat.
The work, along with planned dredging near Port Everglades, prompted Miami Waterkeeper to sue the U.S. Army Corps of Engineers to improve dredge operations.
"It does not conclusively link these two things, but it provides a possible mechanism by which the dredging could have interacted with a low level disease and provided a spark to turn it into a wildfire," said Waterkeeper executive director Rachel Silverstein. "It provides a plausible hypothesis for how the disease spread and became so deadly."
In the seven years since the disease was first detected amid increased scrutiny near the dredge work, stony coral has spread up and down Florida’s reef tract and to the Caribbean and Yucatan, infecting more than two dozen different species.
Yet scientists still know little about it.
“We don't know if it's viral, bacterial or some other type of agent,” Studivan said.
That’s because of the complex relationship corals have with the ocean microbes, bacteria, viruses, and even fungi that live within them, an ecological unit scientists call a holobiont.
“Corals are tricky animals, despite being considered very simple animals. In reality, they're often more complex than we give them credit,” Studivan said. “It becomes difficult to identify a specific pathogen.”
Other diseases may kill coral in a small subset of species, he said, and then die out or drop to normal, tolerable levels of prevalence. But stony coral not only infects a far high number of valuable reef-building species — the disease has also persisted for much longer.
“It's been estimated that it's killed millions of coral colonies,” he said. “It’s just a population level event.”
Rather than wait to discover what’s causing the disease, Studivan said researchers have looked at what’s spreading it. Water was an easy pathway to first identify. Sediment was another suspect.
They’ve also focused on treatment and prevention, finding ways to better apply antibiotics and breed resistant coral to be transplanted on reefs.
For this experiment, researchers took corals with no signs of disease from near Fisher Island in the summer of 2020 and quarantined them for three months at a lab on Virginia Key. Infected coral were taken from a reef near Lauderdale-by-the-Sea, along with sand from near the infected reefs. Scientists then mixed the sand from near the infected coral with the healthy coral; sick coral with healthy coral and sand from infected and uninfected areas in different tanks to compare the transmission rate. They also profiled the microbes in the coral and the sand.
As results from the study came in, and the speed at which sediment spread the disease became clear, Studivan said researchers reached out to authorities who oversee coastal work like the Florida Department of Environmental Protection and Army Corps.
“The bottom line is that we need to be considering as a whole how sediments might factor into the persistence of the disease outbreak,” he said.
That includes more monitoring for work that can move sediment, like dredging, and calculating the risk that the work can trigger outbreaks, he said.
“The big one that comes to mind is the Port Everglades expansion, which is set to occur in the next several years,” Studivan said.
Researchers are also working to confirm that ballast water can spread the disease after finding outbreaks in the Caribbean that spread despite northbound ocean currents.
They are also focusing on finding biomarkers in the sediment to help them determine if the disease is present and then avoid disturbing the sand.
"What we can do is try and manage how we move sediments, particularly for those larger projects, to ensure that we're not increasing the risk of transmission to a new area," he said. "If we've learned anything after seven years, it's that it's very difficult to pinpoint what's causing it. And so now we need to be focusing on what can we do to stop it."
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