Below the gently rolling waves off the Palos Verdes Peninsula, a spiny purple menace is ravaging what should be a thriving kelp forest.
Millions of sea urchins -- scrawny, diseased and desperate for food -- have overrun a band of the shallow seafloor, devouring kelp and crowding out most all other life at a time the giant green foliage is making a comeback elsewhere along the California coast.
In an effort to remedy the situation, scientists and divers will spend the next five years culling the urchins from more than 152 acres of coastal waters degraded years ago by pollution. Once the purple, golf ball-size creatures are under control, young kelp should be able to take hold on the rocky seafloor and grow into the undulating canopies that sustain hundreds of species of marine life.
"Trillions of kelp spores are out there, falling on the seafloor," said Tom Ford, director of marine programs for the Santa Monica Bay Restoration Foundation, the nonprofit leading the project in conjunction with environmental groups, aquariums, fishermen and research institutions. "They just can't get established because they're getting mowed down."
Last month, divers started killing an estimated 4.8 million purple sea urchins -- which rarely are harvested for food -- striking through their pincushion-like exoskeletons with pointed-tip geology hammers. The goal is to reduce their density from as many as 70 per square meter to two. The thinning will leave behind fewer, healthier urchins, clearing the way for kelp to re-establish naturally in a place where it has declined by more than half.
Over the years, environmental groups have repeatedly attempted to restore kelp forests by removing urchins and transplanting healthy seaweed. But the scale of this latest endeavor reflects the growing belief that some ecosystems have been so altered by humans that they require radical intervention to function again.
"We don't want to wait until it's gone so we have to start planting kelp," Ford said. "Let's go in there now while there's still something to build" on.
A fast-growing alga, giant kelp is the backbone of a rich coastal ecosystem. Its translucent green blades, held afloat with gas-filled bladders, tower up from the seafloor to form canopies that provide shelter and nutrients for a diverse marine community.
Yet over the last century, kelp declined steeply off the California coast as storm runoff, erosion and other shore-based pollution clouded the water and made it harder for sunlight to penetrate. As kelp struggled -- and predators like sheephead, lobsters and sea otters declined -- sea urchins moved in.
"Now these [urchin populations] are so large and so well-established that few predators will enter," said Dave Witting, a fish biologist with the National Oceanic and Atmospheric Administration who is working on the project.
On a recent series of dives, half a dozen scientists aboard the Xenarcha, a 28-foot research vessel, pulled on hooded wetsuits and descended 50 feet below the chilly water's surface to the seafloor.
First they examined a healthy kelp forest just down the coast from the urchin zone, taking inventory of the sea life amid the swaying green columns: rockfish, surf perch and kelp bass, among other species. Then they motored over to a corner of the desolate "urchin barren" and got to work, following a 2-meter-wide path and hammering away at the slow-moving creatures that clung to the rocky reef.
It's the beginning of a project that will take years, with twice-a-week expeditions by as many as a dozen dive boats working in a grid pattern to thin the urchins and monitor whether the kelp takes hold.
The project also is drawing on the expertise of commercial divers based in San Pedro, who have joined the urchin culling in hopes of establishing more viable harvesting grounds. The invading purple urchins have gobbled up nutrients and stunted the growth of red urchins, whose bright orange gonads -- known as uni -- are harvested to supply restaurants and sushi bars.
"There's no nutritional value in an empty sea urchin, and that's what you have in an urchin barren," said Bob Bertelli, chairman of the California Urchin Commission.
Regionally, kelp has been making a slow but steady rebound as pollution controls in recent decades have cleaned up coastal waters. A period of cold, nutrient-rich water has accelerated that resurgence in the last several years, with aerial surveys showing the kelp forest more widespread than it's been in decades.
Once a giant kelp anchors itself to a reef, it can grow up to 2 feet a day. But in the barren zone off Palos Verdes, urchins have sat stubbornly in the way of recovery.
Smaller urchin removal efforts -- off Malibu, for example -- were able to bring the canopies back within a year, Ford said, giving his group confidence to move ahead with a larger endeavor. Other scientists, however, warn that removing urchins does not guarantee the return of kelp.