Corals die when overfishing, pollution and climate change combine
As our climate continues to warm, corals face increasing threats, a new study finds
Bleaching linked to climate change isn't the only human-caused killer threatening coral reefs around the world.
A new study has found that overfishing and polluted run-off from farms and lawns also interfere with corals' ability to withstand heat waves.
These stressors damage corals' microbiome — the bacteria that live on corals — by increasing the proportion of disease-causing bacteria. This causes the corals to become sick and die, according to the study published today in Nature Communications.
That effect is particularly pronounced during hot summer months, which are getting even warmer due to climate change, said Rebecca Vega-Thurber, an assistant professor at the College of Science at Oregon State University and the study's co-author.
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"One of the most remarkable and perhaps most worrisome things we found in this study is … when you pollute the environment, you make the corals much more vulnerable to normal things that would happen to them," Vega-Thurber said in a video interview released by Oregon State University in conjunction with the study.
For example, parrotfish — which generally clean the algae on corals — occasionally bite them, too. Normally, the corals recover. But when the researchers exposed corals to pollutants such as nitrogen and phosphorus, 62 per cent of them died from a parrotfish bite.
On the flip side, Vega-Thurber says humans can help corals survive heat waves by protecting reef fish and reducing pollution.
The researchers had previously found that pollution and a lack of algae-cleaning fish made corals more vulnerable to dying during heat waves. That work was done in labs; the new study replicates those findings in natural conditions.
Researchers controlled conditions at four reef plots in the Florida Keys over three years, keeping fish out and adding nitrogen and phosphorus to the water. They monitored the reefs, taking samples to test the bacteria living on the corals.
They found that without fish to clean off the algae, it covered six times more of the coral surface. This made the corals more hospitable to certain bacteria, doubling levels of disease and causing eight times more coral to die.
Eighty per cent of the corals died during summer or fall, and even "modest" increases in heat boosted mortality if fish were removed or the corals were exposed to nutrient pollution.
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