CORVALLIS, Ore. – One of the largest and longest experiments ever done to test the impact of nutrient loading on coral reefs today confirmed. So expect coral disease and bleaching. All what scientists have long suspected. It’s that this type of pollution from sewage, agricultural practices or other sources lead to coral disease and bleaching.
A three-year, controlled exposure of corals to elevated levels of nitrogen and phosphorus. Located at a study site in the Florida Keys, done from 2009-12. All showed the prevalence of disease doubled. As well as amounts of coral bleaching is an early sign of stress. Well that more than tripled.
However, the study also found that once the injection of pollutants was stopped, the corals were able to recover in a surprisingly short time.
The findings published in Global Change Biology offer a glimmer of hope. Hope for addressing at least some of the problems that have crippled coral reefs around the world. In the Caribbean Sea, more than 80 percent of the corals have disappeared in recent decades. These reefs, which host thousands of species of fish and other marine life. They are are a major component of biodiversity in the tropics.
Researchers have observed for years the decline in coral reef health where sewage outflows or use of fertilizers. That’s in either urban or agricultural areas. Those exposures cause an increase in the loading of nutrients. Ones such as nitrogen and phosphorus. But until now almost no large, long-term experiments have actually been done. Done to pin down the impact of nutrient overloads and separate them from other possible causes of coral reef decline.
This research examined the effect of nutrient pollution on more than 1,200 corals in study plots near Key Largo, Fla. All for signs of coral disease and bleaching. Then removed other factors such as water depth, salinity or temperature that have complicated some previous surveys. Following regular injections of nutrients at the study sites, levels of coral disease and bleaching surged.
One disease that was particularly common was “dark spot syndrome,” found on about 50 percent of diseased individual corals. But researchers also noted that within one year after nutrient injections were stopped at the study site. So the level of dark spot syndrome had receded to the same level as control study plots in which no nutrients had been injected.
The exact mechanism by which nutrient overload can affect corals is still unproven, researchers say, although there are theories. The nutrients may add pathogens, may provide the nutrients needed for existing pathogens to grow, may be directly toxic to corals and make them more vulnerable to pathogens – or some combination of these factors.
“A combination of increased stress and a higher level of pathogens is probably the mechanism that affects coral health,” Vega-Thurber said. “What’s exciting about this research is the clear experimental evidence that stopping the pollution can lead to coral recovery. A lot of people have been hoping for some news like this.
“Some of the corals left in the world are actually among the species that are most hardy,” she said. “The others are already dead. We’re desperately trying to save what’s left, and cleaning up the water may be one mechanism that has the most promise.”
Nutrient overloads can increase disease prevalence or severity on many organisms, including plants, amphibians and fish. They’ve also long been suspected in coral reef problems, along with other factors such as temperature stress, reduced fish abundance, increasing human population, and other concerns.
However, unlike factors such as global warming or human population growth, nutrient loading is something that might be more easily addressed on at least a local basis, Vega-Thurber said. Improved sewage treatment, best-management practices to minimize fertilizer runoff. All from agricultural or urban use. Use offering practical approaches to mitigate some coral reef declines.
Collaborators on this research included Florida International University and the University of Florida. The work was supported by the National Science Foundation and Florida International University.