Researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute analyzed seawater samples from 1984 to 2014 and discovered that warming waters may not be the only culprit responsible for coral bleaching along Florida’s reef tract. Excess nitrogen from sources like improperly treated sewage and fertilizers may also be to blame.  

Why does excess nitrogen lead to coral bleaching? 

Excess nitrogen levels lead to phosphorus limitation, which can lower the temperature threshold for bleaching events. This phosphorus limitation can lead to metabolic stress and eventual starvation in corals.   

Study author Brian Lapointe told the FAU news desk that human-caused nitrogen runoff was the primary driver of coral reef degradation at the Looe Key Sanctuary Preservation Area study site.

Why it matters

Coral reefs in southeast Florida have an asset value of $8.5 billion, generating $4.4 billion in local sales, $2 billion in local income, and 70,400 full and part-time jobs. But, in just 40 years, corals have declined in some areas by more than 90%.  Additionally, increased rainfall thanks to climate change is expected to bring a 19 percent increase in nitrogen loading to coastal areas globally. This means that managing nutrient inputs must be considered as an important step in restoring coral reefs.  

What’s next? 

“Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too,” study co-author James W. Porter, emeritus professor of ecology at the University of Georgia told FAU news. “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.” 

The takeaway message

While warming water temperatures are to blame for coral degradation, they are not the sole contributor. Nitrogen levels in coastal waters must also be considered in coral reef restoration plans.  

Where can I learn more?  

Kudos to:  

The study’s co-authors are Rachel A. Brewton and Laura W. Herren of FAU’s Harbor Branch; James W. Porter, Ph.D., emeritus professor of ecology at the University of Georgia; and Chuanmin Hu, Ph.D., of the College of Marine Science at the University of South Florida.