New Study Reveals Pollution Causes Coral Reef Bleaching and Disease

Disease and coral bleaching can kill off vast swathes of a reef, which act as crucial nurseries and habitat for fish and other species. Now, scientists have confirmed what they've long suspected; it turns out that nutrient loading can lead to both coral disease and bleaching.

For years, researchers have watched as coral reef health has declined near sewage outflows and areas where fertilizers are an issue. Yet until now, almost no large, long-term experiments have been conducted to study the effect of these nutrients on corals.

In order to examine the effect of pollution on coral reefs, the researchers conducted a three-year, controlled exposure of corals to elevated levels of nitrogen and phosphorus at a study site in the Florida Keys. They found that the prevalence of disease doubled while the amount of coral bleaching, an early sign of stress, more than tripled.

"We were shocked to see the rapid increase in disease and bleaching from a level of pollution that's fairly common in areas affected by sewage discharge, or fertilizers from agricultural or urban use," said Rebecca Vega-Thurber, one of the researchers, in a news release.

One disease that was particularly common was "dark spot syndrome." It was found on about 50 percent of diseased individual corals. Yet within one year after nutrient injections were stopped at the study site, the level of dark spot syndrome had receded to the same level as control study plots in which no nutrients had been injected.

"But what was even more surprising is that corals were able to make a strong recovery within 10 months after the nutrient enrichment was stopped," said Vega-Thurber. "The problems disappeared. This provides real evidence that not only can nutrient overload cause coral problems, but programs to reduce or eliminate this pollution should help restore coral health. This is actually very good news."

While the study shows that pollution has a huge influence on coral health, it also shows that corals can bounce back relatively quickly after the pollution is reduced. These findings could be huge when it comes to instituting future policies. The research is evidence that by taking action, we can help reduce coral reef mortality and an actually allow reefs to recover over time.

"Some of the corals left in the world are actually among the species that are most hardy," said Vega-Thurber. "The others are already dead. We're desperately trying to save what's left, and cleaning up the water may be one mechanism that has the most promise."

The findings are published in the journal Global Change Biology.