Corals are disappearing across the world's oceans, and most scientists have pointed to warming water temperatures -- the result of climate change -- as the primary driver. But new research suggests nitrogen pollution is the main cause of coral bleaching in Florida.

The study, published this week in the journal Marine Biology, was compiled using three-decades worth of observational data collected at the Looe Key Reef in the lower Florida Keys.

"Our results provide compelling evidence that nitrogen loading from the Florida Keys and greater Everglades ecosystem caused by humans, rather than warming temperatures, is the primary driver of coral reef degradation at Looe Key Sanctuary Preservation Area," lead study author Brian Lapointe, research professor at Florida Atlantic University's Harbor Branch Oceanographic Institute, said in a news release.

Data collected at the test site showed nutrient runoff has boosted the nitrogen-phosphorus ratio in reef algae. As more and more treated sewage and fertilizers from commercial farms rinse into local waterways and flood the oceans with nutrients, including reactive nitrogen, corals are unable to absorb sufficient levels of phosphorous.

According to the new research, phosphorous-starvation lowers the temperature threshold for coral bleaching and increases the chances of coral disease and mortality among reefs.

The symbiotic relationship between coral polyps and the microalgae that provide them food evolved in a low-nutrient environment. Human-caused nutrient loading is disrupting a balance achieved over millions of years.

In addition to measuring the levels of nutrients in macroalgae, or seaweed, among Looe Key reefs, scientists also monitored changes in temperature and salinity. To better understand how nitrogen makes its way to the lower Florida Keys, researchers analyzed nutrient gradients between the Everglades and Looe Key.

In 1984, coral cover in the Looe Key Sanctuary Preservation Area was estimated at 33 percent. By 2008, it was just 6 percent. Today, it's less than 4 percent. The Florida Keys National Marine Sanctuary has the lowest coral cover of anywhere in the Caribbean.

Scientists found periods of accelerated coral decline followed heavy periods of rainfall and water releases from the Everglades. The correlation highlighted the negative impacts of nutrient loading on coral health.

Climate models suggest the region will experience increasing levels of rainfall. Rising water temperatures, however, will only make matters worse, researchers warn.

The new research doesn't discount the negative impacts of global warming on coral health. Coral bleaching has occurred in several remote reefs mostly unaffected by land-based nutrient runoff. But the findings are a reminder that coral face a diversity of threats.

"Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too," said study co-author James W. Porter, emeritus professor of ecology at the University of Georgia. "While there is little that communities living near coral reefs can do to stop global warming, there is a lot they can do to reduce nitrogen runoff. Our study shows that the fight to preserve coral reefs requires local, not just global, action."

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