Corals die when overfishing, pollution and climate change combine

Bleaching linkedto climate change isn't the only human-caused killerthreateningcoral reefs around the world.

Anew study has found thatoverfishing and polluted run-off from farms and lawnsalso interferewith corals'ability to withstand heat waves.

These stressorsdamage corals' microbiome thebacteria that live on corals byincreasingthe proportion of disease-causing bacteria. Thiscausesthe corals to become sickand die, according to thestudy published today in Nature Communications.

That effect isparticularly pronounced during hot summer months, whichare 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'sco-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-Thurbersaid in a video interview released by Oregon State University in conjunction with the study.

For example, parrotfish whichgenerally clean the algae oncorals occasionally bite them, too.Normally, the coralsrecover. But when the researchers exposed corals to pollutants such as nitrogen and phosphorus, 62 per cent of themdied from a parrotfish bite.

On the flip side, Vega-Thurber says humans can help corals survive heat waves by protecting reef fishandreducingpollution.

The researchers had previously found that pollution and a lack of algae-cleaningfishmade corals more vulnerable to dying during heat waves. That work was done in labs;the new studyreplicates those findingsin 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 thebacteria 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 coralsmore 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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