The population of kelp forests, which help clean the air, has fallen dramatically. That has environmentalists worried.

ANACAPA ISLAND, Calif. — Frank Hurd gently parted the curtains of giant kelp that reached upward through the cold waters of the North Pacific, looking for signs of life.

Kelp forests cover a quarter of the world’s coastlines, stretching from Antarctica to Australia, Mexico to Alaska, providing food and shelter for thousands of species, while sucking carbon from the atmosphere. But over the past decade, thanks to warming waters and overfishing, they’re disappearing.

On this afternoon, Hurd, a marine biologist at the Nature Conservancy, said he was relieved to find thick kelp canopies surrounding an unpolluted patch off Anacapa Island, part of the Channel Islands National Park in California. But, he says, such refuges are becoming harder to find. “The scale of this problem is dire,” Hurd said.

The fate of the world’s kelp forests may depend on controlling its sworn enemy — sea urchins — and the Nature Conservancy, an Arlington-based environmental group, says it has a plan. It is touting urchins as a culinary cuisine, hoping to appeal to commercial fishermen who could scoop them out of the ocean. It is also attempting to increase the population of their natural predators, sea stars and growing kelp in controlled environments before releasing the algae back into the sea.

“There really is no single silver bullet option to solve this problem. We need to invest in comprehensive solutions to reestablish healthy, resilient ecosystems,” Hurd said.

Kelp are essentially the ocean’s equivalent of trees. They absorb carbon dioxide and nitrogen compounds, helping clean the atmosphere while capturing up to 20 times more carbon per acre than land forests. They also provide a vital habitat for a broad range of marine life; without them, entire ocean ecosystems would crumble.

But the size of kelp forests off the coast of Northern California has shrunk by more than 95 percent since 2014, according to satellite data analyzed by the Nature Conservancy. Other regions have had similar losses: Tasmania’s canopies have decreased by 95 percent, as have Chile’s. Globally, kelp forests along coastlines have declined by a third over the past decade, according to a paper published in the American Institute of Biological Sciences journal BioScience.

“There’s been a catastrophic loss,” said Tom Dempsey, the Nature Conservancy’s California Oceans Program director. “And it’s been all but completely ignored.”

The problem was exacerbated along the California coast by a 2013 heat wave. The average kelp canopy cover off the coasts of Mendocino and Sonoma counties in Northern California fell from 2 million square meters to 60,000 square meters over the course of two years, according to the Nature Conservancy. This marine heat wave, known as “the Blob,” along with a fast-spreading marine virus, all but wiped out the sunflower sea star, the main predator of urchins.

“It’s like this endless parking lot, stretching for miles in every direction, covered in purple urchins,” Dempsey said. With few starfish, the urchin population “exploded to up to 100 times their previous numbers.”

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