A study using 30 years of unique data has finally revealed the reasons why coral reefs are dying at alarming rates around the world.

A steadily warming planet due to climate change is one well-known reason, but it isn’t the only one directly impacting the health of coral reefs. There’s another sinister reason, and it’s entirely man-made.

A study published in the international journal Marine Biology reveals that the problem of coral bleaching isn’t only due to a warming planet. It’s also due to a planet that’s simultaneously being enriched with reactive nitrogen from multiple sources.

Fertilizers, improperly treated sewage, untreated sewage, and top soil are elevating nitrogen levels in the seas and oceans. These disruptive factors cause phosphorus starvation in corals, reducing their temperature threshold for "bleaching."

These man-made catastrophes mean coral reefs were dying off long before they were impacted by rising water temperatures, which made the situation even worse.

"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, Ph.D., senior author and a research professor at FAU's Harbor Branch.

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

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

Researchers gathered data from 1984 to 2014 and collected seawater samples during wet and dry seasons. Lapointe and collaborators from the University of Georgia and the University of South Florida also monitored the living coral and collected abundant species of seaweed (macroalgae) for tissue nutrient analysis.

They wanted to better understand how nitrogen got from the Everglades downstream to the coral reefs of the Florida Keys National Marine Sanctuary, which now has the lowest amount of coral cover of any reefs in the wider Caribbean region.

The study reveals that living coral cover at Looe Key Sanctuary Preservation Area in Florida plummeted from nearly 33 percent in 1984 to less than 6 percent in 2008.

While the annual rate of coral loss varied during the study, it rose from 1985 to 1987 and 1996 to 1999 after periods of heavy rainfall and increased water deliveries from the Everglades. From 1991 to 1995, significant increases in Everglades runoff and heavy rainfall saw increases of reactive nitrogen and phytoplankton levels at Looe Key above levels known to stress and cause die-offs of coral reefs.

USFWS - Pacific Region, CC BY 2.0

Nitrogen loading to the coast is predicted to increase by 19 percent globally simply as a result of changes in rainfall due to climate change. This finding suggests the need for urgent management actions to prevent further degradation.

"Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too," said James W. Porter, Ph.D., emeritus professor of ecology at the University of Georgia.

"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."