Coral Reefs Are About to Crash in a Big Way

Coral reefs cover just 0.1 percent of the ocean floor, but provide habitat to 25 percent of sea-dwelling fish species.  That’s why coral scientist C. Mark Eakin, who coordinates the National Oceanic and Atmospheric Administration’s Coral Reef Watch program, is surprised that the warning he has been sounding since last year — that the globe’s corals appear to be on the verge of a mass-scale bleaching event — hasn’t drawn more media attention.

Bleaching happens when corals lose contact with zooxanthellae, an algae that essentially feeds them nutrients in symbiotic exchange for a stable habitat.  The coral/zooxanthellae relationship thrives within a pretty tight range of ocean temperatures, and when water warms above normal levels, corals tend to expel their algal lifeline.  In doing so, corals not only lose the brilliant colors zooxanthellae deliver — hence, “bleaching” — but also their main source of food.  A bleached coral reef rapidly begins to decline.  Coral can reunite with healthy zooxanthellae and recover, Eakin says, but even then they often become diseased and may die.  That’s rotten news, because bleaching outbreaks are increasingly common.

Before the 1980s, large-scale coral bleaching had never been observed before, Eakin says.  After that, regionally isolated bleaching began to crop up, drawing the attention of marine scientists.  Then, in 1998, an unusually strong El Niño warming phase caused ocean temperatures to rise, triggering the first known global bleaching event in the Earth’s history.  It whitened corals off the coasts of 60 countries and island nations, spanning the Pacific Ocean, Indian Ocean, Red Sea, Persian Gulf, Mediterranean, and the Caribbean. We functionally “lost between 15 percent and 20 percent of the world’s coral reefs” in ’98, Eakin said.  Only some have recovered.

Eakin is concerned about a relapse, because the oceans are relentlessly warming, driven by climate change from ever-increasing greenhouse gas emissions.  As heat builds in the ocean, he says, corals become more vulnerable to bleaching.

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