Thirty years of unique data collected from Looe Key Reef in the lower Florida Keys shows that large-scale coral bleaching is not just a product of climate change, but an adverse side effect of nitrogen loading caused by improperly treated sewage and fertilizer and topsoil runoff, among other things. This is the conclusion of a study published in Marine Biology.
Coral reefs are a fantastically rich source of biodiversity, but also some of the world's most fragile and threatened ecosystems. Scientific research has shown that they have been declining since the '70s and roughly 40 percent of coral in the Caribbean basin has been lost in the past four decades alone. A similar story can be seen playing out in Australia's Great Barrier Reef.
The main contributor to coral bleaching is generally considered to be rising ocean temperatures caused by climate change – a trend that appears to be accelerating. A study published earlier this year found that ocean warming is taking place at a pace 40 percent faster than the United Nations estimated just five years ago. But, according to researchers led by Florida Atlantic University's Harbor Branch Oceanographic Institute, there could be something else going, exacerbating the situation.
The results of the study suggest that sources of nitrogen runoff, including improperly treated sewage, fertilizer, and topsoil, are increasing nitrogen levels in places like Looe Key Reef, triggering phosphorus starvation in corals and thereby reducing the temperature threshold for bleaching. Essentially, it makes already fragile organisms even more vulnerable to ocean temperature changes.
Using data collected between 1984 and 2014 on coral cover, seawater salinity, temperature, and nutrient gradients between Looe Key and the Everglades, the researchers found that living coral cover at the Looe Key Sanctuary Preservation Area fell from close to 33 percent in 1984 to under 6 percent in 2008. The actual rate of coral loss fluctuated over the years but did increase in certain periods (1985 to 1987, 1991 to 1995, and 1996 to 1999) – apparently in response to heavy rainfall and increased water deliveries from the Everglades.
The team notes that water temperatures exceeded the 30.5°C (86.9°F) bleaching threshold at least 15 times over the course of the study. Yet, the three mass bleaching events took place when inorganic nitrogen and reactive phosphorus ratios rose after a period of heavy rainfall and Everglades runoff.
"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," Brian Lapointe, senior author and research professor at FAU's Harbor Branch, said in a statement.
Just as nitrogen enrichment might exacerbate the effects of climate change (and warming oceans), climate change could increase the amount of nitrogen loading thanks to changing rainfall patterns – by as much as 19 percent globally, say the study's authors. The good news is that this, at least, can be managed, said Lapointe, by improving sewage treatment, reducing fertilizer inputs, and increasing treatment of stormwater on the mainland.
Summarizing the main finding of the study, James W. Porter, emeritus professor of ecology at the University of Georgia, said: "Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too. 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. Our study shows that the fight to preserve coral reefs requires local, not just global, action."