Florida Keys' coral demise linked to nitrogen-enriched runoff in study

Diseased Elkhorn coral in Looe Key in the lower Florida Keys pictured in 2016. (Photo credit: Brian Lapointe, Ph.D., Florida Atlantic University’s Harbor Branch Oceanographic Institute)
Diseased Elkhorn coral in Looe Key in the lower Florida Keys pictured in 2016. (Photo credit: Brian Lapointe, Ph.D., Florida Atlantic University’s Harbor Branch Oceanographic Institute)
Published July 16, 2019

A landmark 30-year study of ailing coral in the Florida Keys shows nutrient-supercharged water from as far north as Orlando is contributing to the death of an ancient ecosystem that evolved to thrive in a fertilizer-free environment.

The research, published Monday in the international journal Marine Biology, was led by Florida Atlantic University's Harbor Branch Oceanographic Institute using measurements that date back to 1984. The breadth of the data makes it the longest record of its kind anywhere in the world, according to Florida Atlantic University.

A key point of the findings is that warming ocean temperatures are not the lone killer of Keys' coral, but part of a knot of man-induced challenges that includes higher rainfall rates from climate change that wash nitrogen-enriched waters through the greater Everglades and into Florida Bay.

While Florida has focused for 30 years on cleaning Everglades-harming phosphorus from the southerly flow of water, nitrogen, which the study links to coral death, is less controlled for even as the effort to send more water south increases under the Comprehensive Everglades Restoration Plan.

Related: Climate change is going to cost Florida a lot of money

Related: Florida newsrooms band together to cover the effects of climate change

Looe Key, which is about 6 miles south of Big Pine Key, was the focus of the study.

"While warming is part of the story, it's only one part," said Harbor Branch research scientist Brian LaPointe, who was the lead author on the study. "When we went back and critically looked at the pattern of coral mortality we see that there are periods after major water releases and heavy rainfall where the rate of coral die off increased."

LaPointe tied higher coral die offs to three periods of time — the mid 1980s, 1996 to 1999, and 2013 — all years of heavy rainfall or increased releases of water to Everglades National Park, where it naturally flows through sloughs into Florida Bay.

The mean cover of coral in the Florida Keys National Marine Sanctuary in 1977 ranged between 30 to 70 percent, according to sanctuary research coordinator Andy Bruckner. By 1995 when permanent monitoring stations were first established, the overall mean cover was about 12 percent, with some sites still as high as 25 to 70 percent.

By 2010, the mean cover was 6 percent. Today it's about 4 percent, although there are some mid-channel patch reefs with as much as 50 percent.

"Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role too," said study co-author James W. Porter, 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."

The study emphasizes yet another way man's reroute of the state's historic plumbing system to make way for homes and farms has harmed the environment, and how challenges continue even as the Army Corps of Engineers and South Florida Water Management District attempt fixes through the Comprehensive Everglades Restoration Plan.

Keep up with Tampa Bay’s top headlines

Keep up with Tampa Bay’s top headlines

Subscribe to our free DayStarter newsletter

We’ll deliver the latest news and information you need to know every morning.

You’re all signed up!

Want more of our free, weekly newsletters in your inbox? Let’s get started.

Explore all your options

The plan was authorized by Congress in 2000 as a blueprint for projects aimed at returning the system to a more natural state and preserving what is left. The plan has an estimated 50-year timeline with a price tag of about $10 billion.

A decade before the Comprehensive Everglades Restoration Plan, the federal government sued Florida for poisoning the Loxahatchee National Wildlife Refuge and Everglades National Park with outside effluent, such as phosphorus. The high levels of phosphorus led to an overgrowth of cattails that out-competed the natural river of grass.

Related: Climate change is here. Will Tampa Bay finally get ready?

In 1992, a court order set limits on phosphorus levels, which forced the creation of storm water treatment areas to clean water before sending it south. Today, more than 57,000 acres of storm water treatment ponds have been built — 20,000 acres more than what was required in a legal settlement.

Nitrogen levels in runoff have dropped with the cleanup of the phosphorus, but are still too high for healthy coral during high runoff events, LaPointe said.

"One of our biggest challenges is that much of the nutrients originate outside of the Florida Keys as a result of Everglades discharges, Florida Bay water and degraded water masses that come from mainland Florida," said Bruckner, the sanctuary research coordinator. "The Florida Keys has made great strides in reducing nutrient input through implementation of sewage treatment plants, but there is still runoff and much of the rest of Florida is still on septic tanks."

An overabundance of nitrogen disrupts the symbiotic relationship in coral that has allowed it to grow in low-nutrient waters. Microscopic algae that live in coral tissue provide oxygen and food in the form of sugar to the coral through photosynthesis, while the coral polyp provides nitrates and carbon to the algae.

Bruckner praised LaPointe's research, but said the primary cause of coral bleaching events is temperature and increased ultraviolet radiation. He noted that mass coral die offs have occurred in remote areas not influenced by artificial nutrients.

Global sea surface temperatures rose at about 0.13 degrees per decade between 1901 and 2015, but can spike during some years such as 2015 when a global coral bleaching alert was issued by the National Oceanic and Atmospheric Administration. That year, temperatures in the Caribbean, Cuba, the Bahamas, Haiti and the Dominican Republic reached 87 to 89 degrees, while average temperatures run 84 to 85.

Still, LaPointe hopes the study will help guide future environmental projects.

"We are seeing all of this worsen as our population grows and nitrogen loads to our waters increase," he said. "There was a balance that evolved over hundreds of millions of years and it was doing very well up until we came along."