Corals are dying everywhere, except in this warm-water refuge

News about coral reefs seems almost unrelentingly bleak. Everywhere they’re bleaching and collapsing, unable to withstand the ravages of fast-heating waters—except, that is, the Northern Red Sea, where it appears that a vast region of exceptionally hardy reefs will survive temperatures far exceeding present-day norms.

In this reef system people might yet learn lessons that can help protect other reefs. And the Northern Red Sea could someday become a refugia: a place where, even as corals elsewhere have vanished, reef species and communities persist until such time as they can spread again.

It’s an extraordinary place. Now it just needs to be protected.

“The global importance of this refugia comes from its scale,” says Eslam Osman, a marine biologist at the University of Essex. “We are not talking about small pockets of resistance as previously recorded. We’re talking about 2000 kilometers of continuous reef system with high diversity and lots of endemic species. This is a huge scale.”

In a study published in Global Change Biology, Osman and colleagues compare records of Red Sea coral bleaching—in which corals expel the algae living symbiotically inside them, turning white and often dying afterwards—with Red Sea sea surface temperatures between 1982 and 2012. Despite periods of prolonged heating, “the northern Red Sea has not experienced mass bleaching,” conclude Osman’s team. Ditto the extreme El Niño-induced heating of 2015-2016, they write, when “bleaching was restricted to the central and southern Red Sea” even though northern waters were even warmer. Laboratory tests further underscored the northern corals’ hardiness.

Just how hardy they are can be understood in terms of a unit called the degree C-week, which combines temperature and duration into one number that scientists use to measure heat stress on reefs. The exact formula isn’t important here, but rather the comparison: whereas 4°C‐weeks typically cause bleaching in corals, and 8°C‐weeks are followed by mass mortality, northern Red Sea corals weathered 11°C‐weeks and 15.1°C‐weeks with minimal damage. Even 18.9°C‐weeks occasioned only moderate bleaching.

This might be a function of a heat-adapted evolutionary history, say the researchers, and unique patterns of wind and water flow that seem to ameliorate the effects of heating. Whatever the explanation, “the entire northern region… may act as a refuge for reef‐building corals from environmental anomalies in times of rapid climate change,” write Osman’s team.

“The northern Red Sea will be the last to bleach” under current climate scenarios, added Osman. “It will sustain life where other reefs collapse.” Yet even as they can handle warmer waters, other human activities, such as habitat destruction and ecological imbalances caused by overfishing, are still a danger.

Though the reefs are healthy now, says Osman, with those along the Red Sea’s Egyptian coastline having benefited from a post-Arab Spring decline in tourism and fishing, the return of tourism will revive those pressures. And while reefs on the Red Sea’s Saudi Arabian coastline have historically been sheltered by restrictions on coastal activities, that country recently designated a 10,000-square-mile region for development, with a proposed $500 billion built-from-scratch megacity called Neom sited along these precious reefs.

There’s a desperate need for protection and conservation, says Osman. “We are lucky because the northern Red Sea has such high tolerance,” he says, “but we should not take it for granted.”

Source: Osman et al. “Thermal refugia against coral bleaching throughout the northern Red Sea.” Global Change Biology, 2018.

This piece was originally published on Anthropocene Magazine, a publication of Future Earth dedicated to creating a Human Age we actually want to live in.

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