As climate change causes ocean temperatures to rise, coral bleaching events are expected to become more frequent. So, scientists are looking for ways to help coral reefs recover more quickly, and a new study from Japan suggests that artificial structures like breakwaters may be helpful.
When oceans become excessively warm, corals can expel the symbiotic algae living within their limestone skeletons, causing them to turn a ghostly white, as if bleached. Severe or frequent bleaching events can kill coral reefs, vital ecosystems that support roughly a quarter of marine species at some point in their lives.
With time, corals can recolonize reefs after bleaching, but warming oceans are expected to cause more frequent bleaching events so corals need to recover as quickly as possible between events for the best chance at long-term survival.
To better understand how to help corals become more resilient, researchers in Japan studied how quickly corals return to artificial structures compared to natural reefs after bleaching.
Previous studies have explored the same question, typically over one to 10 years. The Japanese study used 29 years of field data.
Using government data, the researchers compared coral cover on vertical breakwaters — grooved structures built to protect the shore from waves — with coral recovery on natural reefs in Naha Port, Japan, from 1989 to 2018.
A bleaching event in 1998 affected both types of substrates, but “corals on artificial structures recovered faster from [the] mass bleaching event than the natural coral reef ecosystems,” Toko Tanaya, lead author of the study and senior researcher with the Port and Airport Research Institute in Japan, told Mongabay in a phone call.
Within six years, the breakwaters recovered to near pre-bleaching levels, while “the coral cover on natural reefs barely recovered from bleaching,” the authors write. The breakwaters were dominated by fast-growing species and less diverse than natural reefs, however.
Further research is needed, but a possible explanation is that artificial surfaces are “more robust than natural reefs,” Tanaya said. When coral polyps die, the coral skeletons of natural reefs become fragile and easily broken, making it harder for new coral to grow.
“This paper is outstanding because it looks at coral settlement on breakwaters over almost 3 decades, which is very long for a study of this kind,” Bert Hoeksema an honorary professor of tropical marine biodiversity with the University of Groningen, Netherlands, who wasn’t involved with the study, told Mongabay by email.
As oceans continue to warm and sea levels rise, many countries are looking for ways to both support stressed coral and protect coastlines. Tanaya said her research is a valuable insight into how to do both at the same time.
“Our research suggests that combining natural infrastructure such as coral reefs with artificial infrastructures could be effective to promote coral regeneration, while also demonstrating high disaster prevention,” Tanaya said.
Banner image of coral recovery, courtesy of Port and Airport Research Institute.
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