A new study has suggested that the widespread coral deaths in recent decades are being caused by a combination of multiple local stressors, such as overfishing, nutrient pollution, and pathogenic disease, and global warming.
Based on an experiment that simulated both overfishing and nutrient pollution on a coral reef in the Florida Keys in the southeastern United States over three years, which was one of the longest and largest field experiments of the kind, researchers from six institutions concluded that coral reefs are declining around the world due to these factors.
In a study published this week in Nature Communications, they wrote that these forces greatly weaken corals, allow opportunistic pathogens to build to such levels that corals cannot survive, and a global decline of coral reefs is now reaching catastrophic proportions.
Major findings of the study include:
— Overfishing, nutrient pollution and increased temperature all lead to an increase in pathogens;
— Direct algal contact driven by overfishing and nutrient pollution destabilizes the coral microbiome, in some cases leading to a 6- to 9-time increase in mortality;
— Heat exacerbates these problems, with 80 percent of coral deaths coming in the summer or fall, but only when fish are removed or nutrient pollution is present;
— In a distressed system with many algae, coral disease levels double and coral mortality increases eight times;
— Increased algal cover or elevated temperature can reduce levels of naturally-secreted antibiotics that help protect corals from harmful bacteria.
These findings, researchers said, make it clear that in the face of global warming, some of the best opportunities to protect coral reefs lie in careful management of fishing and protection of water quality. This would give corals their best chance to have a healthy microbiome and resist warmer conditions without dying.
“This is grim news, but at least it will help settle the argument over why corals are dying,” said Rebecca-Vega Thurber, an assistant professor in the College of Science at Oregon State University and corresponding author on the study.
“This makes it clear there’s no single force that’s causing such widespread coral deaths,” she said. “Loss of fish that help remove algae, or the addition of excess nutrients like those in fertilizers, can cause algal growth on reefs. This changes the normal microbiota of corals to become more pathogenic, and all of these problems reach critical levels as ocean temperatures warm.”
The researchers said the problems caused by bacterial infections are in addition to damage from mass coral bleaching events already under way. Only in the early 1980s did researchers observe the first mass bleaching event in recorded history. There have been three such events in the past 20 years.
“About 25-35 percent of the corals on the Great Barrier Reef (in Australia) are dying right now,” Vega-Thurber said.
“In 2014-16 large portions of tropical reef across the planet experienced bleaching, and this past April, 90 percent of the Great Barrier Reef bleached as part of a massive El Nino event. Corals everywhere seem to be dying.”
“We need to know how human activities are affecting coral reef ecosystems,” said David Garrison, program director in the U.S. National Science Foundation’s Division of Ocean Sciences, which funded the research.
“Coral reefs are among the most sensitive indicators of the health of the oceans. This report is a major contribution toward understanding how reefs will fare in the future.”