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Using the Ocean to Cool Itself and Save Corals

Coral reef
Stacy Peltier

Due to rising ocean temperatures, marine corals are dying, not only because of the direct heat stress but also because the algae on which they feed are malfunctioning. Now, researchers at the Bermuda Institute of Ocean Sciences (BIOS) have found a novel way to cool them down and save their lives.


When the sea temperature climbs, coral reefs expel their microscopic algae called zooxanthellae. Until that time, the zooxanthellae live in a mutually beneficial relationship with the corals and give them their distinctive bright colors—as well as regular meals. When they are kicked out, the corals turn white in what is known as a bleaching event.

According to the environmental trade publication, OZoCleantech, coral reefs have historically undergone bleaching events every 25-30 years, but since 2010 that gap has shrunk down to every six years. What’s more, a research report issued last year says: “2014–2017 was an unprecedented period of successive record-breaking hot years, which coincided with the most severe, widespread, and longest-lasting global-scale coral bleaching event ever recorded. The 2014–2017 global-scale coral bleaching event (GCBE) resulted in very high coral mortality on many reefs, rapid deterioration of reef structures, and far-reaching environmental impacts.”

Looking for a way to reverse the widespread damage and death of our coral reefs, the BIOS researchers wondered if pumping cooler water up from depths deeper than those at which corals grow—a process known as artificial upwelling (AU)—could cool them cool down and help them thrive. AU was first developed to fertilize surface water with deeper nutrient-dense water to support fish or boost plankton that can sequester carbon dioxide.

“Ocean warming and the occurrence of heatwaves will increase in frequency and intensity over the coming decades, and we need to consider rather unconventional solutions to protect and sustain coral reefs,” said study leader and BIOS assistant scientist, Yvonne Sawall.


To test the theory, the researchers placed corals sourced from a depth of 15 feet off the coast of Bermuda in aquariums set to different conditions. One tank simulated the average summer temperature of 82°F, while another was set at 88°F to simulate heat stress, such as a heatwave during which coral bleaching occurs. Two additional tanks were also set at the heat-stress temperature, but they were cooled with cold-water pulses: one with water from a depth of 164 feet, which was 75°F, and another with water from 300 feet down, which measured 68°F.

Coral in tanks in lab
Yvonne Sawall

Observations showed that even with cold-water pulses that lasted under two hours total per day, the thermal stress was mitigated, with the corals showing higher levels of zooxanthellae than those in the heat-shock tank that wasn’t cooled. The deeper, colder water worked best. The work shows that cold-water pulses during heat-stress events could be a possible way to support corals.

Unfortunately, artificial upwelling has been a controversial practice due to its disruption of a natural ecosystem and because some tests have shown that it could cool the atmosphere or lead to toxic algae build-up. The researchers acknowledge this issue and are now looking for ways to harness its potential benefit while reducing harm.

“Our study shows the potential benefits of pulsed AU during heat waves. The next steps now are to find suitable AU settings to maximize the benefits while minimizing potential harmful side effects of AU for corals and the ecosystem they support,” Sawall said.

The research has been reported in the peer-reviewed journal, Frontiers in Marine Science.