Ag Runoff the New Suspect in Coral Bleaching Epidemic

Dead staghorn coral. (Arc Centre of Excellence Coral Reef Studies)

(CN) – Coral reefs are dying at epidemic levels around the globe.

The Looe Key Reef, south of the Florida Keys, is one such reef that has faced a massive die-off, endangering a critical oceanic ecosystem that supports a complex web of creatures.

In 1984, the reef was comprised of 33% living coral. By 2008, living coral coverage of the reef declined to 6%.

The largest culprit in the coral bleaching phenomenon has been climate change. Scientists say the rising temperatures of the water are stressing the corals, causing them to lose their symbiotic algae and become transparent skeletons incapable of fulfilling their role as teeming cities of the sea.

But researchers from Florida Atlantic University are casting doubt on climate change as the principal factor in coral bleaching and through careful analysis of records dating back decades identified a new and more forcible malefactor: nitrogen.

A study published Monday in the journal Marine Biology by scientists with the Harbor Branch Oceanic Institute makes the case that coral bleaching occurred before waters began to warm but was likely caused by increased nitrogen loading from diverse sources including fertilizer, top soil and treated sewage.

“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 during our long-term study,” said Brian Lapointe, senior author of the study.

Coral reefs need a steady balance between nitrogen and phosphorus to thrive and propagate. But the rapid increase in nitrogen in the Florida Keys due to agricultural and residential runoff via the Everglades has created an imbalance, essentially starving the coral reefs of a nutrient vital to its existence.

Scientists arrived at their conclusions by analyzing data collected between 1984 and 2014 while collecting seawater specimens during wet and dry seasons in the Keys.

Lapointe and his fellow researchers also collected tissue samples from abundant native seaweed while closely monitoring the condition of the coral reefs as they weathered different seasons and run-off periods.

The team was attempting to better understand the flow of nitrogen as it exited the Everglades and travelled south to the Florida Keys National Marine Sanctuary.

What they found was that massive coral bleaching correlated to periods of heavy rainfall that caused large run-off events from the Everglades – specifically during the periods 1985-1987 and 1996-1999.

Between 1991 and 1995, increased nitrogen-rich water deliveries from the Everglades to the sanctuary gave rise to conditions known to stress coral reefs and cause die-offs. While environmental managers have taken steps to reduce run-off from the Everglades, water quality in the marine sanctuary is still below 1984 levels.

Lapointe also points to the Comprehensive Everglades Restoration Plan as an opportunity to address needed nitrogen reduction to protect coral reefs.

“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,” he said.

The scientist pointed to the success story of the Bonaire coral reefs, located in the Dutch Caribbean, where efforts to reduce nitrogen loading have resulted in the resilient reefs beginning to bounce back.

“These coral reefs are beginning to recover following the construction of a new sewage treatment plant in 2011, which has significantly reduced nitrogen loading from septic tanks,” Lapointe said.

The researchers are quick to note warming water temperatures also present a threat to coral reef ecosystems and climate change is harming ocean ecosystems.

But nitrogen is also a culprit and one that can easily be curtailed through better management practices.

“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,” said James Porter, a professor at University of Georgia and co-author. “Our study shows that the fight to preserve coral reefs requires local, not just global, action.”

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