Coral reef ecosystems around the world are characterized by supporting large numbers of marine species, as well as providing livelihoods for more than half a billion people. They are a source of food and jobs, are used for recreational purposes and provide coastal protection against damage from the sea. This is why the destructive effects of marine heat waves that bleach living corals are a matter of grave concern.
As global temperatures increase, marine heat waves are likely to become more frequent and intense, causing lasting damage to coral reef ecosystems. Warmer water can cause corals to expel the algae living in their tissues, which results in the corals turning white. When corals bleach, however, they do not necessarily die, but may become more vulnerable to other stressors, such as water pollution, which then increase the rate of mortality.
Previous research has shown that many reefs harbor corals that survive ocean heat waves, indicating that the issue of bleaching and its impact on coral mortality is a complex one. Understanding this may be key to conserving coral reefs under conditions of global warming.
For the first time, scientists have now been able to map the locations of living corals before and after a major marine heat wave. Arizona State University researchers were able to take to the sky on the Global Airborne Observatory (GAO), and use the advanced spectrometers and visual equipment to get images of coral reef sites around eight different Hawaiian Islands just before, and again after, a heat wave event in 2019. The GAO aircraft is equipped with advanced remote sensing technologies that enable the mapping of ecosystems both on land and beneath the ocean surface.
The results of the surveys, published today in Proceedings of the National Academy of Sciences, show that coral species and local environment are important factors in determining the likelihood of survival when ocean temperatures rise. They also demonstrate that the GAO approach to visual mapping provides an unprecedented opportunity to monitor coral reefs and assess changes in coastal ecosystems over time.
“Repeat coral mapping with the GAO revealed how Hawaii’s coral reefs responded to the 2019 mass bleaching event,” said Greg Asner, lead author of the study and director of the ASU Center for Global Discovery and Conservation Science. “We discovered coral ‘winners’ and ‘losers.’ And these winning corals are associated with cleaner water and less coastal development despite elevated water temperatures.”
The surveys identified more than 10 potential coral refugia – habitats that may offer a safe haven for corals facing climate change. In these areas, there was up to 40 percent less coral mortality than on neighboring reefs, despite the fact that the warm water infiltrated all the areas, causing similar temperature conditions. The results also identified that reefs near heavily developed coasts are more susceptible to coral mortality from heat waves. Development on land is associated with increased pollution that enters coastal reef systems, creating even more stress for coral reefs already fighting to survive the warming water.
According to Brian Neilson, study co-author and head of Hawaii’s Division of Aquatic Resources, “This study supports Hawaii’s Holomua Marine 30×30 Initiative by not only identifying areas impacted by ocean heat waves, but also areas of refugia. These findings can be incorporated into management plans to aid in building a resilient network of reef regions and sustaining Hawaii’s reefs and the communities that depend on them into the future.”
The Holomua 30×30 initiative aims to establish marine management areas across 30 percent of Hawaii’s nearshore waters. Coral reefs in Hawaii are integral to life on the islands, and are tied to culture and livelihoods. Understanding which corals are surviving, and where, is key to achieving conservation that is targeted and effective.
“Previous approaches have failed to deliver actionable interventions that might improve coral survival during heat waves or to locate places of heat-wave resistance, known as coral refugia, for rapid protection,” said Asner, who is also director of the Global Airborne Observatory. “Our findings highlight the new role that coral mortality and survival monitoring can play for targeted conservation that protects more corals in our changing climate.”