The coast of California lies an underwater forest of giant kelp, a kind of seaweed that grows to 100 feet tall at the rate of a foot a day. As a terrestrial forest absorbs carbon dioxide, so does the seaweed. The seaweed’s rapid growth plays an important role in absorbing carbon from the water. climate mitigation. “With kelp goes a huge amount of carbon,” says Chris Wilmers, an ecologist at the University of California, Santa Cruz. “As a general rule, kelp forests are much more productive than most terrestrial forests, in that they’re churning through carbon much more quickly.”
But since the 18th century, California’s kelp forest has been steadily mowed down by purple urchins, thanks to the massacre of their natural predator—the sea otter—hunted for its one-of-a-kind fur. (Unlike other marine mammals, sea otters don’t rely on copious amounts of blubber for insulation, but instead on densely packed hairs. They have more than a million per square inch at their thickest. California’s otter population dropped from 20,000 in the 1800s to 50 in the last 100 years.
The native urchin population is at its lowest point without otters watching over the kelp. The spiky invertebrates change their foraging strategy. Instead of hiding in rock crevices waiting for the detritus, they now venture out to eat. “Once the otters are not present, those urchins can overrun the area, and it turns into what’s called an ‘urchin barren,’” says Jess Fujii, the sea otter program manager at the Monterey Bay Aquarium. “And you really won’t see anything else except rocks and hard substrate covered in urchins.” Parts of the West Coast have seen a 10,000 percent increase in urchins in recent years, and California has lost 95 percent of its kelp forests.
Since 2002, the aquarium has been dedicated to bringing back the otters through the most adorable adoption program. Sea otter moms in captivity take in orphaned pups—often left parentless thanks to great white sharks, which bite but don’t actually eat otters, since sharks prefer blubber to fur. The new moms teach the pups how to do sea otter things—like clean themselves, float on their backs, and use rocks to crack open sea urchins on their bellies. “We’re not hand-feeding them and imprinting them on humans—they’re learning how to be an otter from an otter,” says Fujii. “Some of these animals come in when they’re only a day old. They don’t have any notion of what home used to be.”
When the adoptees are ready, Fujii’s team sets them loose in the coastal habitats of California. Each is tagged and monitored closely for the first two weeks to make sure they’re getting along fine. (Alongside observational surveys at Monterey Bay, tagging allows scientists to conduct censuses on the otter population. If not, they’re brought back in and returned to otter school. The team discovered that 37 of the 37 adopted otters released in 2002 and 2016 have survived. just as well as if they’d grown up fully in the wild. The reintroduced animals go on to reproduce more and make more otters. The California coast’s sea otter population has increased to 3,000 thanks to this unique program.
A sea otter, a ravenous ecosystem engineer of high order, is a sea otter. They eat 25% of their body weight per day to stay warm and healthy. They also dive to the seafloor to collect urchins, crabs, and bivalves such as clams. “By having to eat as much as they do in order to survive in their environment, they have really drastic impacts on those habitats, and they’re overwhelmingly positive,” says Fujii. (Another program further up the California coast has tried bringing back a different kind of “urchin slayer”—human divers.)
Keeping the urchin population in check preserves the kelp, which is vital for the ecosystem in two main ways. First, the forest is a food source for fish, which in turn provides habitat for birds and marine mammals such as sea lions. Second, the seaweed is part of what scientists call a “blue carbon” ecosystem, meaning a coastal or marine area that sequesters carbon. (Other areas are wetlands and mangroves.)
But it’s tricky to quantify exactly how much carbon a healthy kelp forest gobbles up. A redwood tree, for instance, grows to be massive over hundreds of years, locking away lots of carbon over long timescales. (Unless it catches on fire, in which case the carbon goes back into the atmosphere.) Things are more in flux underwater. All manner of critters, including sea urchins, are nibbling on kelp—and pooping out the carbon. Plus, the churning sea breaks off bits of the forest, which fall to the seafloor decompose, releasing stored carbon. A kelp forest is constantly in decay, growing back and sequestering carbon.
It’s hard to be sure how long the carbon stays trapped. “The fate of all this kelp is not well understood,” says Wilmers. “Imagine that all that stuff that’s sloughing off is simply sinking to the deep ocean and isn’t going to come back up again for like 1,000 years. That’s a much more significant carbon sequestration benefit than just sloughing off and immediately decomposing and going right back into the atmosphere.”
Wilmers was aware of this uncertainty and has taken some measures to address it. estimates of the potential carbon benefits of healthy otter populations farther north on the Pacific coast, between the Canadian border and the tip of the Aleutian islands. If a kelp forest grows well, and half the carbon it absorbs is sequestered in the deep sea, it’d be the equivalent of canceling the emissions from 5 million automobiles. Even if only 1 percent of carbon is retained in the deep seas, it would still equal the emissions from 100,000 cars.
In Monterey Bay, the otters don’t only protect the kelp. They also venture up the Elkhorn Slough, a large tidal marsh, where they encourage the growth of eelgrass, another coastal plant that sequesters carbon—although the otters affect the plant in a more indirect way. The otters eat sea slugs and invertebrates, which then eat the algae that grows on eelgrass. Because the slugs eat the algae, the plants are kept clean and can absorb more sunlight. The number of crabs that prey upon the slugs is decreasing, which in turn helps the eelgrass. The Elkhorn Slough eelgrass has increased thanks to the return of the Otters. jumped 600 percent in the last three decades.
A mangrove or a river marsh like Elkhorn Slough traps loads of carbon. “They’re removing carbon dioxide at rates that are up to 10 times as fast as we see in terrestrial systems on a per area basis,” says Emily Pidgeon, vice president of ocean science and innovation at the nonprofit Conservation International. “They bury it in the soil below them, and it basically gets locked away for millennia. And so you end up with these very deep, rich stores of carbon in these ecosystems much denser—and hence, with larger amounts of carbon—than you see in forests.”
Aimee Aimee, vice president of ocean conservation strategy at the Monterey Bay Aquarium, stated that restoration marshland plays a crucial role in the ecosystem. “We need these habitats for the services that they provide for us: buffering from storms, food production, filtered water quality,” she says. “That is a great role that otters have played in the Elkhorn Slough estuary, which is one that is notorious for being at the epicenter of a lot of different industrial uses, including agriculture.”
Healthy ecosystems support sustainable fisheries and provide livelihoods for local residents. Monterey’s sea otters are very cute, which in turn attracts tourists and money. These are just a few of the many benefits conservationists see in promoting blue carbon-related policies. nature-based solutions: restoring ecosystems in order to combat climate change. Everyone wins—the locals, the climate, the ecosystem. The sea urchins, crabs and other marine life may not win. But they won’t be missed.