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Landmark Study Pinpoints the Cause of Coral Reef Decline

Posted July 16, 2019

Coral reefs are widely considered to be the most endangered – and the most rapidly declining – ecosystems on the planet, hit especially hard by climbing water temperatures and other stressors caused or exacerbated by climate change.

According to a new study published in the journal Marine Biology, what’s really killing coral reefs is not just warmer oceans, but also the rising water concentrations of reactive nitrogen incoming from improperly treated sewage, fertilisers, and top soil.

Higher concentrations of nitrogen lead to phosphorous starvation in coral, reducing their temperature threshold for “bleaching” – a process that has been going on long before climate change had started to affect marine temperatures.

“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,” said senior author Brian Lapointe.

For the study, the research team gathered data on the Looe Key Reef in the lower Florida Keys from 1984 to 2014, collected abundant seawater and seaweed samples, and ensured the monitoring of living coral.

The loss of coral reefs is related to climate change, but has more to do with human-caused nitrogen runoff. Image: Copernicus Sentinel-2, ESA via, CC BY-SA 3.0

Results showed that increases in Everglades runoff and heavy rainfall increased the nitrogen:phosphorous (N:P) ratio in reef algae, as well as phytoplankton levels, which both lead to metabolic stress and the eventual death of corals.

Given the estimated 19 per cent global increase of nitrogen loading in coastal areas due to changes in rainfall ushered in by climate change, urgent action is needed if we are to prevent a world with no coral reefs at all.

“The future success of the Comprehensive Everglades Restoration Plan will rely on recognising the hydrological and nitrogen linkages between the Everglades, Florida Bay and the Florida Keys,” said Lapointe. “The good news is that we can do something about the nitrogen problem such as better sewage treatment, reducing fertiliser inputs, and increasing storage and treatment of stormwater on the Florida mainland.”

The study emphasises that, when it comes coral reefs, talk of climate change that excludes local issues having to do with nitrogen runoff misses the point that water quality is of huge importance here as well – and while communities living near coral reefs cannot stop climate change, they sure can do a lot to improve the way they handle their water and soil.


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