The nitrogen load is just as much to blame as rising temperatures for coral bleaching in the Looe Key reef in Florida Keys low in the US, suggests a 30-year study.
And, say the researchers, the reef was dying well before the impact of rising water temperatures.
Write in the newspaper Marine biology, a team led by the Harbor Branch Oceanographic Institute of Florida Atlantic University, says sewage, fertilizer and soil have high levels of nitrogen, which causes phosphorus deficiency in corals, reducing their threshold bleaching temperature.
The researchers say their study represents the longest record of reactive nutrient concentrations and algae for the world's coral reefs. They collected data from 1984 to 2014 and samples of seawater during dry and wet seasons.
They also monitored live corals, collected many species of algae for analysis of nutrients in the tissues, as well as the salinity of seawater, temperature, and gradients of water. Nutrients between the Everglades and Looe Key.
"Our results provide compelling evidence that nitrogen loading from the Florida Keys and the larger Everglades ecosystem, caused by humans, rather than by global warming, is the primary contributor to coral reef degradation in the area. of Looe Key Sanctuary Preservation during our long-term study "author Brian Lapointe.
A key finding is that nutrient runoff on land has increased the nitrogen / phosphorus (N: P) ratio in reef algae, indicating an increasing degree of phosphorus limitation known to cause metabolic stress and possibly famine among corals, according to the researchers. .
Reactive nitrogen concentrations are above the critical thresholds set by ecosystems for the Florida Keys, as are phytoplankton levels for offshore reefs, as evidenced by the presence of macroalgae and other blooms. Harmful algae due to excessive levels of nutrients.
The data show that live coral cover in the Looe Key Sanctuary Preservation Area has increased from almost 33% in 1984 to less than 6% in 2008. The annual rate of coral shedding has varied, but increases have been observed. observed as a result of heavy rainfall and increased deliveries of water. the Everglades.
"The good news," said Lapointe, "is that we can do something to solve the nitrogen problem, such as improving wastewater treatment, reducing fertilizer inputs, and increasing storage and storage. rainwater treatment on the mainland of Florida ".
The big problem, says co-author James Porter of the University of Georgia, is that "the struggle for the preservation of coral reefs requires local action, not just global action."
"Citing climate change as the exclusive cause of the disappearance of coral reefs worldwide misses the crucial point that water quality also plays a role," he said.
"Although communities living near coral reefs can do little to stop global warming, they can do a lot to reduce nitrogen runoff."