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Snorkeling deeper into the clear, warm water, I watch as a bigheaded blue parrot fish with powerful grinders chomps on a fistful of lemon-yellow lobe coral. A couple of Achilles tangs, black with orange spots, nibble at algae. Closer to shore swirls a school of 60 bright-yellow tangs. Awaiting night, a whitetip reef shark sleeps in a shallow cave. Along with sea urchins, those pincushions of the bottom, these fish are the gardeners of coral reefs. They scrape algae like little lawn mowers and gobble the soft animals—called polyps—that make up the jeweled colonies in the shallow seas of our planet, from Hawaii to Florida, the Red Sea to Australia's Great Barrier Reef, the world's largest and most diverse ecosystem. Polyps lead a simple enough life, taking in calcium and carbon dioxide from the water to form their limestone homes, putting up tiny tentacles at night to sting and capture plankton for their food. Their colors come from the symbiotic algae that live within their flesh. These single-celled plants photosynthesize and make sugars from sunlight, giving the corals the energy they need for growth.
Reefs are home to millions of animals—at least 25 percent of all fish species. These underwater worlds are so lush that they've become known as the rain forests of the sea. But just as rain forests are in trouble on land, all is not well beneath the waves. Worldwide, half of all reefs have vanished, and the rest could be extinct by mid-century. Half the coral in U.S. waters is in fair to poor health. Staghorn and elkhorn coral are now on the endangered species list, the first corals ever.
If reefs disappear, we lose a source of extraordinary beauty and biodiversity. We also lose an underwater buffer that holds back waves during hurricanes; a nursery for fish that feed a billion people around the world and provide 200 million jobs in the fishing industry; a home for plants and animals used to treat cancer, HIV, and other diseases; and an estimated $105 billion a year from tourist revenue in the Caribbean alone. The threat to coral is a threat to all of us.
Adjacent to the City of Refuge National Historical Park, the ocean looks as clear as it did five miles away at Kealakekua. But 40 feet down, a blanket of brown goo the size of a football field covers the coral. Resembling a mat of bad Jell-O, it's algae run amok.
I tread water above three research divers from state and federal agencies who are taking time-lapse pictures and snipping samples of algae to study later. Looking at the mess, they guess what happened: A torrential rain passed through local septic tanks and fertilized farm soil on its way to the ocean. This added nitrogen to the reef and made the algae bloom, just as fertilizer boosts your lawn.
"It had been a very happy reef for decades," Bill Walsh, head of the state Division of Aquatic Resources for the Big Island of Hawaii, tells me afterward. "Then, overnight, it's in a state of red alert." Nearby, downslope from a development of million-dollar vacation homes, it's a similar story. Runoff containing tons of dirt and silt made its way down a canyon and onto the reef. It's a coral graveyard now.
I take a quick flight across the Alenuihaha Channel to Maui, where an hour later I'm snorkeling off the famous Maui Ocean Center, in Ma'alaea Harbor. Underwater here, it looks like a rotten tossed salad. I see acres of what resembles butter lettuce, punctuated by thousands of heads of twisted dark hair, an out-of-control double invasion of nasty macroalgae.
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