It’s Official: Human Pollution a Big Cause of Reef Loss

coral reef pollution
Patches of bleached green star coral at Flower Garden Banks National Marine Sanctuary. The sanctuary has experienced several significant bleaching events in recent years.

BLAME FOR MASSIVE CORAL DIEOFFS ON TROPICAL REEFS has largely centered on seemingly unstoppable ocean warming. But a new study of coral reef pollution focused on the Florida Keys has confirmed another major cause of coral loss – and, one that can be fixed.

A review of 30 years bringing together data on rainfall events, man-made pollution and coral health has linked massive coral loss in the Keys to direct man-made pollution from improperly treated sewage and agricultural runoff.


In fact, the reefs on the Keys were dying long before rising temperatures began impacting them, say the scientists at Florida Atlantic University and the Universities of Georgia and South Florida, who conducted the study.

The longtime effect of these pollutants is to drive up nitrogen levels and create an imbalance with phosphorous levels, resulting in “phosphorous starvation” that weakens the living coral polyps.

“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, notes Brian Lapointe, senior author and an FAU research professor.


Runoff and pollution have been suggested as key drivers of coral loss in previous studies, especially on the Great Barrier Reef. And, among others, a paper published in 2011 by one of this project’s researchers identified sewage-related bacteria as a principal cause of declines specifically in elkhorn coral populations.

But this study, focused on Looe Key Sanctuary Preservation Area in the Keys, represents the longest record of inactive nutrients and algae concentrations on reefs anywhere in the world. The study was published in July in the journal Marine Biology

coral reef pollution
A brain coral mound exhibiting severe bleaching.


The research team studied data for the period between 1984 and 2014, analyzed living coral and seaweed nutrients and monitored seawater salinity, temperature and nutrient gradients between the Everglades and Looe Key.

They found that:

  • Coral cover at Looe Key dropped from 33 percent in 1984 to less than 6 percent in 2008.
  • The yearly rate of coral loss varied during the study period, but increased following periods of heavy rainfall and increased water deliveries from the Everglades between  1985 and 1987 and 1996 and 1999.
  • Between 1991 and 1995, major increases in heavy rainfall and Everglades runoff led to increases of reactive nitrogen and phytoplankton levels at Looe Key above levels known to stress corals and cause die offs.
  • Without intervention, coastal nitrogen loading is expected to is expected to increase by 19 percent worldwide due to climate change-induced changes in rainfall patterns.

The coral losses at Looe Key are a somewhat more extreme reflection of reef declines for Florida as a whole: Overall, the state has lost half its coral cover of the past 20 years.


The study suggests that while global action is absolutely needed to mitigate global warming and other climate change factors, local action – improvement of sewage treatment facilities, limits on fertilizer runoff – are required, as well.

Construction of an upgraded sewage treatment plant in the Caribbean island of Bonaire that opened in 2011 has been credited with significantly mitigating nitrogen-loading and aiding in reef recovery there, for example.

“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 FAU’s Professor Lapointe in a release from the FAU news office.

PRINCIPAL SOURCES:  Nitrogen enrichment, altered stoichiometry, and coral reef decline at Looe Key, Florida Keys, USA: a 3-decade study, Marine Biology;  THIRTY YEARS OF UNIQUE DATA REVEAL WHAT’S REALLY KILLING CORAL REEFS, Florida Atlantic University News OfficeThere’s Another Thing Killing The Coral Reefs, And We Can Actually Fix This Problem, Science Alert;  Coral Deaths Spurred by Pollutants From Land, The Scientist;  Human Sewage Identified as Coral Killer, Scientific American.