Can the Kelp Forest Keep Up?

Kelp forests shelter biodiversity, sequester carbon and provide innumerable benefits to people. But how long can they survive in our rapidly changing world?

By Laura Tangley
Conservation
Jun 27, 2024

Off the coast of California’s Santa Barbara Island, a California sea lion rests in a forest of giant kelp, one of dozens of kelp species that dominate nearshore waters along more than a third of the world’s coastlines. About half of kelp forests feature tall, canopy-forming species such as those found in California. (Photo by Alex Mustard/Nature Picture Library)

TO MOST VISITORS ADMIRING THE VIEW along California’s scenic coastline, the scattered brown splotches of kelp dotting the sea’s surface hardly warrant a second glance. That’s understandable, says Meredith McPherson, an oceanographer and diver who lives in Santa Cruz, California. To appreciate what’s special about kelp forests, she says, you must descend beneath the waves.

Exploring a kelp forest’s rocky floor, McPherson says, “you notice that it’s so quiet, and you’re surrounded by vibrant colors” of fish, sea stars, sponges, snails, anemones and dozens of other creatures grazing on the algae or clinging to the reef the plants attach to. Sprouting from rootlike holdfasts that can be a foot in diameter, the stems of giant kelp—or stipes—sway with the currents, growing to lengths of 100 feet or more. Their leafy blades stretch upward toward the sunlight, spreading across the ocean’s surface to create a dense canopy. Sunbeams filter through the lush, translucent vegetation, a sight some divers compare to cathedral windows. It’s “beautiful, like its own little universe,” says McPherson, a physical scientist at the U.S. Geological Survey’s Pacific Coastal and Marine Science Center.

It’s no wonder that she and other scientists who study California kelp forests have been dismayed to see their rapid decline in recent years. Along the state’s Central Coast, a third or more of the kelp has died off, leaving behind a patchy mosaic of forest fragments. Farther north, the situation is worse. In the first use of satellite imagery to assess the region’s kelp forests, when McPherson was a graduate student at the University of California, Santa Cruz (UCSC), she and her colleagues measured changes in the North Coast’s canopy coverage over time. Published in Communications Biology in 2021, their results—based on 34 years of U.S. Geological Survey Landsat images—revealed that the coast’s once-healthy and nearly ubiquitous kelp forests had declined by more than 95 percent.

Most of that loss took place between 2014 and 2019, a result of an unprecedented marine heat wave preceded by a disease outbreak that killed nearly all sunflower sea stars, a keystone species in California kelp forests. Thanks to the scientists’ long-term data set, “we learned that these forests are naturally dynamic systems that for decades had been resilient to extreme events,” McPherson says. But the one-two punch that hit them 10 years ago, she adds, “led to the complete collapse of the ecosystem—and it still has not recovered.”

An image of a Pacific sea nettle escaping a storm of blacksmith damselfish.

A Pacific sea nettle scuttles away from a school of predatory blacksmith damselfish in a kelp forest off Monterey, California. In a recent report, scientists estimate that 70 percent of kelp forests along the Monterey Peninsula have died off in recent years.

Essential yet unheralded habitats

Kelp forests have value beyond their beauty. Found on every continent along more than a third of the world’s shorelines, they make up the single largest coastal marine habitat on Earth. They also are among the most biologically diverse. “Like corals, kelps are classic foundation species that create habitat and provide food for thousands of other species,” says Mark Carr, a UCSC marine ecologist who has studied kelp forests for decades. A co-author of the Communications Biology study, he ranks these highly productive ecosystems just behind coral reefs when it comes to marine biodiversity.

Unlike coral reefs, however, kelp forests are off the radar for most people, from policymakers to the general public. Without knowing it, “some 750 million people live within 30 miles of these exceptionally beneficial systems, including inhabitants of cities such as New York, London, Seoul, Sydney, Cape Town and Los Angeles,” says Aaron Eger, a marine biologist at Australia’s University of New South Wales and the founder and program director of the Kelp Forest Alliance, an international organization that seeks to raise awareness of and protect these unheralded habitats.

Because many benefits kelp forests provide are difficult, if not impossible, to quantify, Eger and a team of 18 colleagues analyzed three that are not: the market value of commercial fisheries and the potential market values of removing polluting nutrients (mainly nitrogen) from the water and carbon dioxide from the atmosphere. In 2023, they reported in Nature Communications that kelp forests are worth an average of $500 billion a year. That figure would be higher if other quantifiable benefits, such as nature tourism and coastal protection from storms, had been included, he says.

Eger and his colleagues estimate that kelp forests absorb some 5 million tons of carbon annually. The algae are super carbon absorbers due to their extraordinary growth rate of up to 2 feet a day. (By some measures, the underwater forests sequester 20 times more carbon per acre than terrestrial forests.) Converted into biomass as the algae grow, some of the absorbed carbon is exported far offshore when storms and ocean currents sweep kelp fragments out to the deep sea, where they sink and may remain for hundreds or thousands of years. Kelp scientists say this makes the underwater forests a powerful natural ally in our fight against global climate change.

An image of a juvenile sea otter in a giant kelp forest.

When kelp ecosystems are in balance, predators such as sunflower sea stars (below) and southern sea otters (above) keep populations of sea urchins under control. But when sea stars died along most of California’s coast, purple urchins exploded in number and began to destroy entire kelp forests (bottom).

Mowed down by a purple plague

Yet kelp forests are besieged worldwide. Like all marine ecosystems, they are “impacted by everything going on in the ocean,” says Eger, from overfishing and pollution to sedimentation and habitat loss due to coastal development. The two most critical threats today, he says, are rising ocean temperatures caused by climate change and increases in the populations of kelp-consuming herbivores, primarily sea urchins. The latter shifts can be triggered by warming water or by overharvesting urchin predators such as large fish and lobsters.

Globally, the Kelp Forest Alliance estimates that more than 7 million acres of kelp forest have been lost. The hardest hit regions include Tasmania, the Pacific Northwest’s Salish Sea and Western Australia.

As well as Northern California. The kelp catastrophe there can be traced back to 2013 when a deadly, still-mysterious illness known as sea star wasting syndrome started killing large numbers of sunflower sea stars, the region’s primary predators of purple sea urchins. Considered “functionally extinct,” the sea star is now classified as critically endangered by the International Union for Conservation of Nature. Free from their only significant predators, populations of purple urchins quickly exploded.

Then, in 2014, an unprecedented marine heat wave arose in the Northeast Pacific Ocean, spreading down the West Coast through California the following year. Nicknamed “the blob,” this climate change-driven warming occurred just as a strong El Niño began bringing hotter water up from the south. Together, these events—combined with lower nutrient concentrations in warmer seawater—led to a dramatic drop in the growth rate of kelp, which thrives only in cool, nutrient-rich waters.

One result was that fewer kelp stipes and blades broke off and fell to the forest floor. That, in turn, led to behavioral changes in the burgeoning purple urchin population. To hide from predators, urchins normally live within cracks and crevices on the ocean floor, feeding on the bits of kelp that drift by. “It’s kind of like having pizza delivered,” says Carr. But as the algae’s productivity plummeted and less food was delivered, the urchins swarmed out from their former hiding places “and just mowed down all the kelp,” he says, turning once-vibrant forests into “urchin barrens.”

The loss of kelp devastated local economies and communities. A once-robust commercial fishery for red sea urchins, valued at $3 million, collapsed in 2015, reported Laura Rogers-Bennett of the University of California, Davis, and a colleague in Scientific Reports. (Because the purple sea urchin lacks sufficient edible meat, it has zero commercial value.) Likewise, they noted that mass mortality of kelp-dependent abalone forced the closure in 2018 of a recreational fishery worth some $44 million. The impact of the latter on small businesses that worked with abalone divers was particularly “shocking and heartbreaking,” says Michael Esgro, senior biodiversity program manager and Tribal liaison for California’s Ocean Protection Council (OPC). A diver himself, he says, “I saw dive shops that had been around for four decades have to close their doors.”

An image of scuba diver Keith Rootsaert culling purple sea urchins.

Along California’s Central Coast, scuba diver Keith Rootsaert (above) smashes purple urchins with a hammer as part of a project to bring back kelp. With $10 million invested so far, California has become a leader in kelp forest restoration. Off Monterey, a Brandt’s cormorant (below) dives for fish in a thriving kelp forest.

A future for kelp forests?

South of San Francisco, kelp forests fared better, with remnants left off the Central Coast and little recent loss in Southern California. Although the entire coastline was impacted by the marine heat wave and loss of sea stars—followed by urchin population booms—forests toward the south had two advantages. First, they are dominated by giant kelp, a perennial alga species, while the north is made up almost entirely of bull kelp, an annual less resilient to extreme disturbance.

Perhaps more importantly, while sea stars vanished all along the coast, regions south of the Golden Gate Bridge are home to additional urchin predators: California spiny lobsters and large fish called California sheephead in Southern California and southern sea otters along the Central Coast. A three-year study off the Monterey Peninsula—led by Monterey Bay Aquarium ecologist Joshua Smith (then a UCSC graduate student) and published in the Proceedings of the National Academy of Sciences—found that the area’s otters, in response to urchin population growth, began to eat three times more of the spiky prey than they had before the outbreak.

Because otters, unlike lobsters or sheephead, are native to the North Coast—wiped out by fur traders in the 1800s—some conservationists propose reintroducing the mammals to control urchins. But would that plan work? Smith and his team discovered that Monterey’s otters are picky about the urchins they eat, ignoring urchin-barren dwellers—which, with little food themselves, provide virtually no nutrition—and feasting on the animals in kelp forest fragments. While otters are certainly helping protect those fragments, Carr thinks “it’s unlikely they’re going to swoop into an urchin barren, clear out all the urchins and reestablish kelp.”

An image of a Brandt's cormorant diving through the kelp forest.

Smith feels more hopeful, citing “many studies in the far north, such as Alaska and British Columbia, where sea otters moved into urchin barren-dominated areas and consumed the urchins, leading to the recovery of kelp.”

In some urchin barrens lacking natural predators, humans are beginning to step up. Co-published by the Kelp Forest Alliance, the 2022 Kelp Restoration Guidebook documents kelp restoration projects in more than a dozen countries, with many prioritizing sea urchin removal. Some of the most ambitious efforts are in California, where OPC has invested more than $10 million in kelp forest research and restoration since 2019, says Esgro.

He cites one OPC-funded project off the coast of Mendocino. There, the nonprofit Reef Check and other partners paid former red urchin divers, now unemployed, to remove purple urchins from devastated kelp forests. “Collecting urchins is what they do best, and we wanted to invest in the communities hardest hit,” Esgro says. At two sites over the past two years, he reports, the divers “cleared a staggering 50,000 pounds of urchins, and kelp regrowth has been documented—something I never thought I’d see in a place that seemed beyond redemption.”

But even if urchins can be kept at bay and kelp forests regrown—in California and beyond—some worry such successes could be short lived. As climate change continues, both wildlife disease outbreaks and, especially, ocean warming events are expected to be more frequent and severe. “Do you go out and restore a kelp forest that will last only until the next marine heat wave takes it out?” asks Carr. “At what point is this just beating your head against a wall?”

Despite such concerns, most scientists maintain their optimism. “I couldn’t do what I do otherwise,” says Eger, adding that kelps are “such a diverse and adaptable set of species that I can’t imagine a future without kelp forests.” Carr agrees. If there’s one thing he’s learned after nearly 50 years studying the plants, “it’s that kelp is one amazing alga,” he says.

Read more about senior editor Laura Tangley.