Huge chunks of coral reef are dying due to severe bleaching, and a three-year experiment has found how local stressors and global warming have interacted to cause the damage.
Severe coral bleaching has devastated the central and northern regions of Australia's Great Barrier Reef. It's estimated 90 percent of the reef is bleached, and somewhere between a quarter and a third of the corals have died.
Warming ocean temperatures have been blamed for the damage, but new research published in the Nature Communications journal confirms it's not the only problem.
Instead local stresses of overfishing and nutrient pollution, such as from run-off from coastal development, is also found to be at fault.
Researchers studied corals in controlled experiments near the Florida Keys (Cory Fuchs / Oregon State University)
Added nutrients, especially nitrates and phosphates, cause more algae to grow on the coral than the reef can handle. When the area is overfished, the creatures which usually eat the algae and keep it low are gone.
Combined with higher surface temperatures of the water, it leads to a massive die-off of the coral.
Eighty percent of the coral deaths observed in the study occurred in summer and autumn, but only in areas contaminated with nutrient pollution or lacking in fish.
It's crucial we find out how humans affect a reef's ecosystems, study author Rebecca Vega-Thurber says.
The aftermath of the bleaching event at Lizard Island on the Great Barrier Reef (AAP)
"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.
"That warm climate-change factor interacts with overfishing and nutrient pollution to cause corals to reduce their health... and then ultimately the corals die.
"It's the interaction between the local factors and climate change that seems to be the most important."
And the nutrient pollution has had a secondary, unusual effect on the relationship between fish and coral.
Parrotfish remove algae on the coral by nibbling on it, which researchers say causes no permanent damage to the reef and is crucial to the reef remaining healthy.
Algae-control by parrotfish could be dangerous to unhealthy coral (Cory Fuchs / Oregon State University)
But when nutrient pollution weakens the coral, the interaction turns deadly. When a parrotfish bites weakened coral, the coral dies in nearly two-thirds of the time.
The new information is "grim", but by setting the debate over the cause of coral death it could help with a solution, Dr Vega-Thurber says.
"This makes it clear there's no single force that's causing such widespread coral deaths," she says.
Even if the ocean continues to warm at alarming rates, it means not all hope is lost.
Cleaning up the reefs and protecting the fish which call it home could help prevent corals from dying at such a fast rate as the temperature rises.