It’s not just temperature damaging coral reefs

Nitrogen loading is as much to blame as increasing temperatures for coral bleaching in the Looe Key Reef in the lower Florida Keys, US, a 30-year study suggests. 

And, the researchers say, the reef was dying off long before it was impacted by rising water temperatures. 

Writing in the journal Marine Biology, a team led by Florida Atlantic University’s Harbor Branch Oceanographic Institute says improperly treated sewage, fertilisers and top soil are elevating nitrogen levels, which are causing phosphorus starvation in the corals, reducing their temperature threshold for bleaching. 

The researchers say their study represents the longest record of reactive nutrients and algae concentrations for coral reefs anywhere in the world. They gathered data from 1984 to 2014 and collected seawater samples during wet and dry seasons. {%recommended 6461%}

They also monitored living coral, collected abundant species of seaweed for tissue nutrient analysis, and monitored seawater salinity, temperature and nutrient gradients between the Everglades and Looe Key. 

“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,” says senior author Brian Lapointe.

A key finding is that land-based nutrient runoff has increased the nitrogen-to-phosphorus ratio (N:P) in reef algae, which indicates an increasing degree of phosphorus limitation known to cause metabolic stress and eventually starvation in corals, the researchers say. 

Concentrations of reactive nitrogen are above critical ecosystem threshold levels previously established for the Florida Keys, as are phytoplankton levels for offshore reefs as evidenced by the presence of macroalgae and other harmful algal blooms due to excessive levels of nutrients.

The data reveal that living coral cover at Looe Key Sanctuary Preservation Area declined from nearly 33% in 1984 to less than 6% in 2008. The annual rate of coral loss varied, but increases were noted following periods of heavy rainfall and increased water deliveries from the Everglades. 

“The good news,” says Lapointe, “is that we can do something about the nitrogen problem such as better sewage treatment, reducing fertiliser inputs, and increasing storage and treatment of stormwater on the Florida mainland.”

The bigger picture issue, adds co-author James Porter from the University of Georgia, is that “the fight to preserve coral reefs requires local, not just global, action”.

“Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too,” he says.

“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.”

Underwater video showing the degradation of coral at Looe Key from 1988 to 2019.

Brian Lapointe, Florida Atlantic University’s Harbor Branch Oceanographic Institute.