Across the tropics corals are dying, much to the alarm of biologists and environmentalists. A major cause of mass die-offs at reefs is climate change as warming surface water temperatures cause many coral species to bleach en masse.
Yet that is hardly the only cause, stresses a team of scientists from Florida Atlantic University’s Harbor Branch Oceanographic Institute. After examining three decades’ worth of data from Looe Key Reef in the lower Florida Keys, the researchers have pinpointed another cause: increased levels of nitrogen in the sea owing to untreated sewage and agricultural fertilizers leaching into the water.
Land-based runoff changes the chemical composition of reef algae, triggering metabolic stress and causing corals, which depend on the algae symbiotically, to starve. Specifically, elevated levels of nitrogen lead to a reduction of phosphorus in corals, which lowers their threshold for enduring warmer water temperatures. The result is a mass die-off of corals even before climate change truly begins to have its impacts on them, the scientists say.
“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,” says Brian Lapointe, a research professor at FAU who is senior author of a new study on the findings.
Decades-long data gathered at Looe Key Sanctuary Preservation Area showed that living coral cover shrank 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 researchers explain.
“Between 1991 [and] 1995, significant increases in Everglades runoff and heavy rainfall resulted in increases of reactive nitrogen and phytoplankton levels at Looe Key above levels known to stress and cause die-off of coral reefs,” they add. “Despite reduced Everglades flows, the water quality has not yet recovered to the levels of the 1980s.”
In other words, water pollution is a primary cause of coral bleaching, which is bad news since runoff of nitrogen is expected to increase in coming years as a result of climate change-driven alterations to rainfall. “The good news is that 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,” Lapointe says.
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