Off the coast of Miami, corals are dying. Why? Because of a 16-month-long project to expand the harbor channel. A new study shows how the project killed over half a million corals between 2013 and 2015.
Published in the journal Marine Pollution Bulletin Thursday, the study involved on-the-ground observations of the corals coupled with satellite images to see how much sedimentation the project’s dredging threw into the water—and how all that sediment ultimately affected coral health. The authors found that nearly 2,000 pounds of sediment were suffocating the reefs for every 10 square feet about a mile and a half out from where the dredging was occurring.
As the authors found, up to 74 percent of corals died near the channel in the months after the dredging to expand the shipping channel began. Even two years later, the researchers found that the density of corals some 65 feet from the channel had suffered as a result of the sedimentation.
Corals are pretty fragile organisms. If the water grows too warm, they can begin to die as they expel the algae they need to help produce food. Too much acid in the water (as a result of carbon dioxide), and corals grow weaker and can shrink. Now, if sand and sediment pile on top of corals, they lose access to light, which helps their algae produce food, and they can use a lot of energy trying to remove all that sediment. In short, all these things can kill corals, which are already dying around the world in mass due to climate change.
“If we want to conserve these ecosystems for the generations that come after us, it’s essential that we do all we can to conserve the corals we still have left,” said senior author Andrew Baker, an associate professor of marine biology and ecology at the University of Miami, in an academic release. “These climate survivors may hold the key to understanding how some corals can survive global changes.”
In Miami, more than 560,000 corals were killed in the immediate vicinity of the dredging, according to the study. The authors were able to look at where the sediment plumes hit via satellite images to estimate the distance of the impact. Turns out that they were likely felt more than six miles away from where the dredging occurred. Before the project began, only one coral had been tagged as partially buried by sediment. After the project, the reefs were completely transformed.
An independent consulting firm that the Port of Miami and Army Corps of Engineers hired to conduct environmental monitoring for the project previously attributed the mass coral die off to a disease outbreak. This study, however, controlled for that and looked at how corals closest to the dredging fared during this period. The study’s results contradict what the consulting firm found and showed the value of using satellite images, on-site observations, as well as sediment data, to find the root cause of what’s harming coral.
“It was important to differentiate these multiple impacts occurring on the reefs to understand the direct effects of dredging specifically,” said lead author Ross Cunning, a research biologist at the Shedd Aquarium in Chicago, in the release. “We brought together all the available data from satellites, sediment traps, and hundreds of underwater surveys. Together, the multiple, independent datasets clearly show that dredging caused the major damages observed on these reefs.”
The results are heartbreaking: The threatened staghorn coral, which is protected under the Endangered Species Act, was one of those affected. So is elkhorn coral, another threatened species. These corals need all the protection they can get. And development projects that kill them certainly don’t help the threats they already face from warming oceans.