Coral bleaching occurs when corals are stressed by high water temperatures and other factors, causing them to expel the symbiotic algae living within them. Given that fact, scientists are now looking into saving reefs by pumping up cool water from the ocean's depths.
"Upwelling" happens naturally when winds push surface water away from coastal regions, allowing nutrient-rich water from below to move up and take its place. Sometimes, however, deep-ocean pumps are used to manually imitate the process. This is known as "artificial upwelling," and it's done to help increase fish stocks in surface waters, among other reasons.
Led by Dr. Yvonne Sawall of the Bermuda Institute of Ocean Sciences, a team of researchers recently set out to see if artificial upwelling could be used to prevent coral bleaching, by cooling the waters in which reefs are located.
They started by gathering fragments of three species of live coral, at a depth of 15 ft (5 m) from Bermuda's Sea Venture Shoals region. Those corals were then placed in aquaria at the institute, and kept at a range of temperature conditions over a period of three weeks.
Those conditions included an average summer temperature (28°C/82ºF); a warmer temperature known to cause bleaching (31°C/88ºF); that same warmer temperature with daily pulses of cooler 24ºC/75ºF water drawn from a depth of 164 ft/50m; and the same warmer temperature with daily pulses of cooler 20ºC/68ºF water drawn from a depth of 300 ft/100 m.
It was ultimately found that less than two hours a day of exposure to the cool, deep water mitigated thermal stress in the corals – this was evidenced by more of the algae being active in those corals, than in their counterparts that endured the hot temperatures without the cooling pulses. What's more, the deeper and cooler the water, the more pronounced the effect.
"Our study shows the potential benefits of pulsed AU [artificial upwelling] during heat waves," says Sawall. "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."
The research is described in a paper that was recently published in the journal Frontiers in Marine Science.
