Kane`ohe corals more resilient to bleaching after 50 years of hot

Montipora capitata coral. 

Credit: Keisha Bahr

Corals in Kane`ohe Bay seem more resilient to bleaching in warming waters today than they were 50 years ago—the first evidence that coral may be gaining tolerance to rising global temperatures.

It may not be enough to keep up with the pace of climate change, but it's a hopeful sign.

“Although these results are encouraging in their indication that acclimatization/adaptation of corals and their symbionts can occur at an unexpectedly rapid rate, increased bleaching tolerance may not be enough for widespread coral survival,” said researcher Ku‘ulei Rodgers

A complex study by University of Hawai`i and Bishop Museum researchers looked at how corals responded to hot spells in 1970, and then in 2017 when the research team repeated the earlier studies. They found that corals today take longer to respond to superheated water, that they recover more readily and start growing again more quickly.

There are caveats here, but the indications are hopeful for the future of our reefs.

The new study in the journal PeerJ: The Journal of Life and Environmental Sciences, is entitled "Evidence of acclimatization or adaptation in Hawaiian corals to higher ocean temperatures." The authors are UH Institute for Marine Biology scientists Steve L Coles, Keisha D. Bahr, Ku'ulei S. Rodgers, Stacie L. May, Ashley E. McGowan, Anita Tsang, Josh Bumgarner and Ji Hoon Han. Coles, a veteran coral scientist who also works with Bishop Museum, was part of the original 1970s study.

Science Daily reported on the study. 

Reef corals are two-part organisms. The coral polyp provides a home to single-celled algae called zooxanthellae. It's a mutual relationship, and corals get both their color and some of their food from the algae. When corals are stressed, as when water temperatures rise, they eject their zooxanthellae and begin to starve. They also look white, bleached.

The 1970s experiments studied how corals responded to periods of abnormally warm water. The 2017 experiments recreated those studies.

"Re-running a 50-year old experiment using the same coral species, same experimental setup, and same observer allows us to directly test changes in coral temperature tolerance,” said co-author Keisha Bahr.

After nearly five decades of increasingly warm oceans, those corals seem to behave differently now, the team said. They keep their zooxanthellae longer, and recover quicker after waters return to normal temperatures. The warming trend has been carefully tracked, and offshore sea temperature rise amounts to 1.13 degrees Centigrade from 1958 to 2014, the report says.

Why are the corals more resistant to warming?

It isn’t clear whether that's because corals are adopting more resilient zoozanthellae or whether the corals themselves are more temperature resilient. And Coles warns that it might also have something to do with cleaner waters in Kane`ohe Bay, where in the 1970s, nutrient-rich secondary treatment sewage effluent was being dumped in the bay.

"Elevated levels of dissolved nitrogen have been implicated in stimulating coral bleaching," Coles said.

"Available evidence indicates that the lower concentrations of nutrient pollutants, particularly dissolve organic nitrogen, have played an important role in the increased temperature tolerance of corals after nearly 50 years as was determined by these experiments," the paper said.

In other words, corals can respond better to change when the water is cleaner.

This evidence from a single location is important in a special way, the authors said.

"Our experiments are the first to demonstrate thermal acclimatization/adaptation to elevated ocean  temperature for corals of the same species and from the same location over the past half-century."

The three species of corals they studied are Lobactis scutaria, mushroom or plate coral, Montipora capitata, called rice or pore coral, and Pocillopora damicornis, the cauliflower or lace coral.

© Jan TenBruggencate 2018