Nutrient pollution makes corals more susceptible to bleaching

New research suggests that nutrient pollution will compound the detrimental effects of climate change on coral reefs. According to a study from the ARC Centre of Excellence for Coral Reef Studies, coral bleaching is intensified when reefs are exposed to excessive nutrients.

Nutrient runoff from human activities, as well as naturally occurring nutrients such as nitrogen and phosphorous, is carried into coral environments by circulating ocean currents.

Study lead author Dr. Thomas DeCarlo from the King Abdullah University of Science and Technology (KAUST) said corals are sensitive to high levels of nutrients.

“As the climate warms, mass coral bleaching could occur as often as annually within this century,” said Dr. DeCarlo. “In our study, we found that already heat-stressed corals exposed to excess nutrient levels were even more susceptible to bleaching.”

The findings suggest that ecosystem managers should develop strategies to reduce nutrient stress in waters that are affected by heat stress.

The research confirms the results of previous studies, some of which were conducted on the Great Barrier Reef, that have attributed coral bleaching to a combination of heat and nutrient stresses.

“Our results provide a roadmap for coral reef conservation efforts to be at their most effective,” said study co-author Professor John Pandolfi. “We suggest oceanographic processes should be included when deciding when and where to allocate resources or protection.”

The researchers used the skeletal cores of long-living corals to analyze bleaching events in the Red Sea over the past few decades. They found that coral bleaching was only severe when high sea surface temperatures were coupled with excessive nutrient levels.

Testing the role of nutrients in coral bleaching has been a challenge, primarily because it is difficult to distinguish between which effects are caused by heat stress and which are caused by nutrient pollution. It is also not as easy to measure nutrient loads through satellite imagery as it is to measure ocean temperatures.

The Red Sea provides an ideal study site for examining the impacts of nutrient pollution on corals because it is one of the only marine environments where the effects of summertime nutrients and heat stress are independent of each other. There is only one region of the sea that has a single major source of nutrients in the summer, which are mixed in the ocean as a result of a process called upwelling.

“Incorporating nutrient-supplying ocean currents into coral bleaching forecasts will enhance those predictions that are based on temperatures alone,” said Professor Pandolfi.

“The fact that nutrients are more difficult to measure than temperature may be restricting our recognition of their importance,” said Dr. DeCarlo. “And we need greater longer-term monitoring efforts of nutrient levels on coral reefs.”

“Our research suggests that projections of coral reef futures should move beyond solely temperature-based stress to incorporate the influence of ocean current systems on coral reef nutrient enrichment, and thus susceptibility to bleaching.

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer