Curbing nutrient overload helps coral resist bleaching

With the right assistance, coral communities can be surprisingly resilient.

According to a new study, published this week in the journal PNAS, reductions in nutrient pollution help corals avoid bleaching during moderate heatwaves.

Several recent research efforts have highlighted the importance of the symbiotic relationship between coral and algae. Coral offer algae protection and nitrogen, while algae share sugars with coral.

When water temperatures rise, the algae accelerates the photosynthesis process, disrupting the chemical balance between the two symbionts. Eventually, coral expel the algae -- the event known as coral bleaching.

If the warming event is brief, algae often return and the coral is able to recover. But if coral are without their algal partners for too long, they will eventually die.

Lab tests have previously shown the deleterious effects of excess nitrogen on the coral-algae relationship, but researchers had not yet -- until now -- studied the systematic effects of nutrient overload on a diversity of coral communities.

For the new study, scientists at the University of California, Santa Barbara, surveyed the effects of nitrogen pollution on coral bleaching across the coral reefs surrounding the island of Moorea in French Polynesia.

In 2016, scientists surveyed the health of more than 10,000 corals around Moorea during a moderate heatwave. To track fluctuations in marine nitrogen concentrations, researchers regularly sampled the large algae species Turbinaria ornata, which absorbs the nutrient.

The sampling efforts allowed scientists to map the amounts of nitrogen available to different coral communities in the months and weeks leading up to the 2016 heatwave.

"These relationships are very complex," lead author Mary Donovan, a postdoctoral researcher at the UCSB's Marine Science Institute, said in a news release. "So, studying them at spatial and temporal scales that match those happening in nature is critical to revealing these really important interactions."

The survey's findings showed coral exposed to elevated nitrogen levels began severing their ties with algae sooner. In other words, the threshold temperature for bleaching was lower when coral were exposed to nitrogen pollution. The greater the nitrogen overload, the lower the bleaching temperature threshold.

Though they expected different corals to have different responses to nitrogen and warm water, scientists found the two most common types of branching coral in Moorea demonstrated identical responses.

Previous studies have shown resilient coral communities can pass along beneficial algae communities to their offspring, increasing their odds of survival. But the latest research suggests nutrient pollution undermines the ability of coral to resist bleaching events. To protect coral, which are under increasing amounts of environmental stress, authors of the new study are urging policy makers to take steps to reduce fertilizer and sewage runoff.

"Marine heatwaves and coral bleaching are the defining challenge facing coral reefs in the 21st century," said study co-author Deron Burkepile, a professor of ecology at UCSB. "Managing CO2 emissions requires global action, so as ecologists and conservationists, we're are also looking for levers we can pull at the local scale to help coral reef ecosystems withstand these global stressors while we also take action to address climate change."

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