ABOVE: Study coauthor Brain Lapointe analyzes bleached coral on Looe Key reef.

BRIAN LAPOINTE/FLORIDA ATLANTIC UNIVERSITY

A30-year study suggests that pollution may pose a more imminent threat to struggling coral reefs than waters warmed by climate change. The research points to excess nitrogen from topsoil runoff and inadequately treated sewage as the main driver of coral death in Florida’s Looe Key reef, researchers reported July 15 in Marine Biology.

“The good news is that we can do something about the nitrogen problem, such as better sewage treatment, reducing fertilizer inputs, and increasing storage and treatment of stormwater on the Florida mainland,” says coauthor Brain Lapointe, a marine scientist at Florida Atlantic University's Harbor Branch Oceanographic Institute, in an interview with The Miami Herald.

Looe Key reef lies in the southernmost reaches of the Florida Keys and hosts a diverse collection of marine life, including parrotfish, barracuda, and butterfly fish. Coral once covered 33 percent of the Looe Key Sanctuary Preservation Area, a 5.3 square nautical mile region of protected ocean. Between 1984 and 2014, coral cover dwindled to just 5 percent.

The devastation at Looe Key is mirrored across the state; over the last 20 years, Florida has lost half its corals.

The researchers witnessed this extensive coral death as they monitored the state of living corals and changes in seaweed composition, seawater salinity, nutrient levels, and temperature over 30 years. Using satellite images, they also determined how pollutants reached the reef from mainland Florida. The researchers found that fertilizer, topsoil runoff, and human waste drove up nitrogen levels in the ecosystem without raising phosphorus levels. The imbalance likely starved the corals of phosphorous, lowered their temperature threshold for bleaching and led to their demise.

Bolstering these findings, the researchers also found that corals died off at higher rates in times of heavy rainfall and increased runoff from the Everglades.

“We're starting to have enough data to really track the impact of local-scale stressors to corals, over long enough time frames to understand how the communities are changing,” says Michael Fox, a coral reef scientist at the Woods Hole Oceanographic Institution, in an interview with NPR.

Fox, who was not involved in the study, also notes that identifying local and global stressors may help scientists recommend clear courses of action. While warming oceans trigger more and more coral bleaching events, combating local pollution could still help save failing reefs around the world.

Nicoletta Lanese is an intern at The Scientist. Email her at nlanese@the-scientist.com.