Heat does to coral reefs what peroxide does to human hair — it bleaches them. For these underwater colonies, though, whitening is more than a mere draining of colour. They could become diseased and die. But it's not a death sentence for all corals. While some tolerate high temperatures better than others, the reefs' recovery suffers. This is the paradox gripping the shallow coral reefs of Lakshadweep Islands.
In 1998, Rohan Arthur, who heads the Nature Conservation Foundation's Oceans and Coast Programme, Mysuru, was a young intern in Kenya. When an unusually warm sea, caused by El Niño, blanched the corals, he rushed back to India. Some reefs were already dying by that time, but he managed to document the species, assess which ones suffered the most and gauge the extent of the damage.
About 16 percent of the corals died globally from that disaster while close to 90 percent of the Lakshadweep reefs were destroyed.
Since then, many volunteers, field assistants and biologists, working with Arthur, chronicled changes on six reefs around three islands — Kavaratti, Agatti, and Kadmat. Synthesising all the collected data, Shreya Yadav, Teresa Alcoverro and Rohan Arthur recently published a progress report.
Over the following years, the very shape of the reefs changed. Acropora, a group of fast-growing branched coral struggled to recover. But Porites, a group of slow-growing coral that grows in mounds, withstood the heat of El Niño. Even though entire colonies bleached, it increased in number. This turnover in species, from fragile to resilient ones, built up the resistance of the reef against future El Niño events. But it was not without consequence.
In essence, an El Niño is like a wildfire sweeping through a forest, killing the trees and leaving only fire-resistant bushes in its wake. Instead of intricate branches that could shelter many species at different rungs, the coral reef became two-dimensional, like low shrubland. Reef fish and invertebrates seeking refuge and food in the nooks and crannies of Acropora thickets were stranded. The growth of Porites made little difference to them because the creatures couldn't hide in its boulder-like structure. The researchers calculated that the annual rate of reef recovery was no more than four percent.
By 2010, 12 years after the catastrophe, about 50 percent of the reefs had revived when the next El Niño hit, killing 44 percent of the live corals.
The rising dominance of Porites ensured the mortality figure wasn't as high as the devastation of 1998. The star real estate developer, the Acropora group, which would have helped the reefs bounce back with speed, lost even more ground. However, there was a little glimmer of hope — the surviving coral species didn't react as dramatically as they had in 1998.
For instance, El Niño almost exterminated Acropora in the reefs, but a few isolated patches survive in the lagoons. Although the shallow water becomes hot every summer, the delicate coral thrives, says Arthur. But in the reef that had last experienced unusual temperatures 12 years ago, the same species bleached and died. The periodic exposure to warmth primed the lagoon corals to anticipate and cope with it.
Not only are some species like Porites adapted to handle the heat, some strains of algae, which live within corals and sustain them (see here), also evolved high-temperature thresholds. Corals that have experienced heat-stress may adopt these hardy algal strains to cope with warm seas.