Nitrogen from sewage and farms is starving Florida corals to death, study says

Nitrogen from improperly treated sewage and fertilizer runoff from farms and lawns is starving Florida Keys corals to death, according to a new study published in the journal Marine Biology.

The study led by Florida Atlantic University’s Harbor Branch Oceanographic Institute at Looe Key, in the Florida Keys, showed that higher nutrient levels in Florida waters is a key cause of coral bleaching and death. As nutrient runoff from farming and from a growing population increases the amount of nitrogen levels in the water, corals are actually dying before being affected by warmer water temperatures, said Brian Lapointe, one of the authors of the study and a professor at FAU’s Harbor Branch.

“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,’’ Lapointe said in a statement.

Coral reefs are dying at an alarming rate all over the world as warming water temperatures due to climate change increase the frequency of coral bleaching events. Heat stress and other factors lead corals to expel the algae living in their tissue, causing them to turn completely white. Corals have a symbiotic relationship with these microscopic algae that live inside them and give them their color. The algae are also the primary food source for corals.

The research analyzed living corals and seawater samples from 1984 through 2014 during the wet and dry seasons. Lapointe and collaborators from the University of Georgia and the University of South Florida also collected species of seaweed for tissue nutrient analysis. They monitored seawater salinity, temperature and nutrient levels between the Everglades and Looe Key, according to the study.

Data showed that healthy coral cover at Looe Key Sanctuary Preservation Area fell to less than 6% in 2008, from 33% in 1984. Over the 30-year span of the research, the annual rate of coral loss varied, but it rose in years of heavier rainfall, when water flowing from the Everglades increased.

From 1991 to 1995, significant increases in Everglades runoff and heavy rainfall led nitrogen content at Looe Key to rise to levels that have been proven to produce stress and cause coral death, Lapointe said.

Coral reefs create habitats that provide shelter and food for hundreds of marine species. The reef serves as a buffer that protects coastal areas against hurricanes and storm surge. They are also an essential driver of economic growth: coral reefs in southeast Florida generate more than $8.5 billion annually in income and local sales, as well as 70,400 full and part-time jobs, according to the National Oceanic and Atmospheric Administration.

“The future success of the Comprehensive Everglades Restoration Plan will rely on recognizing the hydrological and nitrogen linkages between the Everglades, Florida Bay and the Florida Keys,’’ said Lapointe. “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.’’

The Everglades Restoration Plan seeks to revive the rich ecosystem and restore the natural flow of water, after decades of draining, diversions and canals to direct water to agriculture production and urban development completely altered the circulation.

As part of the plan, the South Florida Water Management District and the U.S. Army Corps of Engineers have been working to send more water south to Florida Bay, which has been devastated by sea grass die-offs due to a lack of freshwater in recent years.