Nutrient pollution was killing coral reefs long before climate change, claims study
The analysis shows nitrogen loading from the Florida Keys and greater Everglades ecosystem caused by humans, rather than warming temperature, to be the primary driver of coral reef degradation at Looe Key Sanctuary Preservation Area
Climate change and increasing ocean temperatures have long been known to destroy coral reefs at an alarming rate. However, pollution driven by human activities has now emerged as a major culprit, which is causing a decline among the reefs, according to a new study.
Nutrient run-off from multiple sources — badly-treated sewage, fertilizers, and topsoil — is causing coral bleaching, leaving them even more vulnerable and causing their destruction, says a research team from Florida Atlantic University, University of Georgia, and the University of South Florida.
"Bleaching" is a sign that the corals are in distress. An iconic reef such as the Great Barrier Reef in Australia has been one of the worst victims of bleaching, which killed 50% of its corals.
"Corals and algae depend on each for survival. However, when corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white. The coral is thus left bleached and vulnerable. If the stress caused by bleaching is not severe, coral has been known to recover. But if the algae loss is prolonged and the stress continues, coral eventually dies," according to the National Ocean Service, National Oceanic and Atmospheric Administration (NOAA), US.
The research team examined 30 years of data from Looe Key Reef in the lower Florida Keys. Analysis of data gathered from 1984 to 2014 and seawater samples collected during wet and dry seasons reveal that coral reefs were dying long before rising water temperatures affected them.
The results, say researchers, 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.
"Data makes (it) clear that this is not an either (temperature) or (nutrients) situation, but rather a 'both/and' combination of multiple stressors. Aggressive management is especially important as new threats to coral health continue to emerge," says paper published in Marine Biology.
According to the International Union for Conservation of Nature (IUCN), coral reefs are estimated to directly support over 500 million people globally, who rely on them for daily subsistence. "A 2014 assessment published in the journal Global Environmental Change estimated the social, cultural, and economic value of coral reefs at US$1 trillion. A 2015 study by WWF projects that the climate-related loss of reef ecosystem services will cost US$500 billion per year or more by 2100," says IUCN.
It further states, "Coral reefs are also key indicators of global ecosystem health. They serve as an early warning sign of what may happen to other less sensitive systems, such as river deltas if climate change is not urgently addressed. Once the tipping point for the survival of coral reefs is passed, the deterioration of other systems may cascade more quickly and irreversibly."
Stress and starvation in corals
In the current study, researchers discovered that improperly treated sewage, fertilizers, and topsoil are elevating nitrogen levels, which are causing "phosphorus starvation in the corals, and subsequently reducing their temperature threshold for bleaching".
They found that land-based nutrient run-off has increased the nitrogen-phosphorus ratio in reef algae, suggesting a significantly "higher degree of phosphorus limitation" that can result in "metabolic stress and eventual starvation" in corals. "These coral reefs were dying off long before they were impacted by rising water temperatures. This study represents the longest record of reactive nutrients and algae concentrations for coral reefs anywhere in the world," states the paper.
According to the paper, living corals were monitored and the team collected species of seaweed (macroalgae) for tissue nutrient analysis. They assessed seawater salinity, temperature and nutrient gradients between the Everglades and Looe Key to understand how nitrogen traveled 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.
"Data revealed 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 during the study but increased from 1985 to 1987 and 1996 to 1999 following periods of heavy rainfall and increased water deliveries from the Everglades," the paper reads.
"Between 1991 to 1995, significant increases in Everglades run-off and heavy rainfall resulted in an increase of reactive nitrogen and phytoplankton levels at Looe Key above levels known to stress and cause a die-off of coral reefs. Despite reduced Everglades flows, the water quality has not yet recovered to the levels of the 1980s," the findings state.
According to researchers, nitrogen loading to the coast is predicted to increase by 19% globally owing to rainfall changes as a result of climate change. This, says the team, requires urgent action to stop further degradation. They suggest regular monitoring of seawater and macroalgae to ascertain long-term changes in nitrogen and phosphorus availability, and changes in their ratio — critical to understanding the global crisis of coral reef decline.
The team also recommends better management of water quality at the local and regional levels to maintain balanced nitrogen-phosphorous ratio for reducing the risk of coral bleaching, disease, and death under the current level of temperature stress.
"The future success of the Comprehensive Everglades Restoration Plan will rely on recognizing the hydrological and nitrogen linkages between the Everglades, Florida Bay, and the Florida Keys. 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," say the findings.