Watching marine life, from otters to wetsuit-clad surfers, frolic in the famously cold water off the coast of Santa Cruz, it’s hard to imagine the devastation that warming water has wreaked just beneath the waves. In central and Northern California, a massive ocean heat wave in 2014 and 2015 set off a cascade of events that killed huge underwater kelp forests, in some areas replacing the dense algal jungles with “urchin barrens.”

The multi-species drama involves not only the kelp but sea urchins and sea stars, and a surprising hero—sea otters—maintaining some of the underwater forests. The work of the otters, and a handful of humans, may be the only hope for this crucial ecosystem to survive in warming waters.

The disintegration of Northern California kelp forests, which grow along rocky coastlines in cool, clear water up to 100 feet deep, is a case study of how global warming triggers cascading effects. Oceans have absorbed about 93 percent of the heat trapped by industrial greenhouse gas pollution. The warming fuels more frequent and stronger marine heat waves that are already devastating kelp and other ocean ecosystems around the world, disrupting coastal communities that rely on salmon, shellfish and other marine resources.

Grant Downie, a fisherman who works diving for urchins with his father in Fort Bragg, first noticed the change in 2014.

“We were on the boat and seeing areas where there should be kelp, and there’s just nothing,” Downie said. “And then doing dives, and the urchins aren’t any good. [My father] said ‘You know, I see something changing.” 

The Northern California urchin fishery normally averages $3 million to $4 million in annual profit, but that dropped to about $1 million in 2015 and declined even further in the following years. Subsequently, the federal government approved $3.3 million in disaster spending for the California red urchin fishery.

“That all stems from the loss of kelp,” Downie said. His own income has dropped by 75 percent, and he and his father have been forced to dive much deeper, to depths over 100 feet, to find quality urchins. Since the kelp collapse started, four urchin divers, including Downie, got the bends—a dangerous condition caused by rising too fast from deep dives—in the area where he works. One man died.

Scientists say they’re not sure if the damage to the kelp is reversible, but prospects for recovery vary greatly along different parts of the coast. In Monterey Bay, small patches of kelp continue to thrive, and new research in the journal Proceedings of the Natural Academy of Sciences credits their survival to sea otters.

A Complete Collapse

Kelp grows all along the western coast of North America, from Alaska to Baja California. Bull kelp, an annual species that dies and comes back once a year, dominates north of Monterey Bay. Giant kelp, a perennial that lives several years, is more common to the south, and the two species overlap along the central coast. 

Both species attach to something solid on the ocean floor, and then grow up towards the sun, elevated by dozens of gas-filled bladders. Kelp can grow 2 feet per day under ideal conditions, reaching up to 175 feet tall. Healthy coastal ecosystems, including kelp, can sequester up to 20 times more carbon per acre than land forests, which also helps slow local ocean acidification.

Kelp forests are some of the most productive habitats on Earth, sheltering hundreds of species and providing important sanctuaries for animals of all sizes. Grey whales have occasionally been known to hide from orcas in kelp, and a huge variety of invertebrates live among its fronds. The kelp forests break up wave action, sheltering coastlines from storms. 

The species is highly dependent on nutrient availability, water temperature and predation, so some variation in kelp abundance is normal, but what happened during and after the heat wave was unprecedented.

“We found that in the past, when there were extreme events such as marine heatwave and El Niños, that the kelp forest was actually resilient and recovered quickly to those events,” said Meredith McPherson, a doctoral student at the University of California, Santa Cruz and the author of a study, on the Northern California kelp die-off, published March 5 in the journal Communications Biology.

Ocean heat waves suppress the upwelling of the cold, oxygenated and nutrient-rich water that kelp needs to thrive, so it’s common for it to die back periodically, when hotter conditions occur.  

But during the 2014 to 2016 warming event, the stress of the heat on the kelp was compounded by the death of millions of starfish, including the sunflower sea star, a key sea urchin predator, from sea star wasting disease, a malady linked with global warming.

In the absence of sea stars, the urchin population grew out of control. In normal conditions, urchins in kelp forests mainly feed on “drift,” the kelp fronds that naturally fall to the bottom. But after the marine heat wave, kelp production appeared to decline dramatically at the same time as the urchin numbers exploded. There wasn’t enough fallen kelp on the seafloor to feed the proliferating urchins, so they grazed on the growing stalks of kelp until the forest was decimated.

The urchins effectively ate themselves out of house and home. When there was no kelp left to eat, they went into a sort of zombie state. Urchins can live without any significant food for years, but their overall health suffers. So fasting urchins are of no value to fishermen. 

“It was just getting harder and harder to find a quality urchin,” Downie said, “and we’d see starfish legs literally melting on the bottom [of the ocean]. You’ve seen a mushroom that gets too old in the wild and starts to melt? That’s what happened to the starfish underwater.”

Ultimately, the proliferation of the urchins combined with the heat wave led to a “complete and total loss of kelp” in the system on the north coast, McPherson said. Kelp coverage for 60 miles along the coast from Point Arena to Bodega Bay declined by 95 percent from 2014 to 2019.

“The 95 percent [decline] was shocking,” McPherson said. But, he added, “I think the more shocking thing is that there are certain areas where it’s just total loss. You zoom in to the Point Arena area, and there’s just 100 percent kelp loss.”

The entire ecosystem has been lost, as the hordes of hungry urchins devour any kelp before it can reproduce. “The kelp can’t, at this time, recover, because concentrations of urchins are so high that any existing patch of kelp can’t release spores and propagate,” McPherson said. “There are just urchins everywhere.”

In Monterey Bay, Otters Tend Patches of Kelp

Monterey Bay was slammed by the same combination of kelp killers as the North Coast. “We saw a rapid decline in kelp that proceeded for about two to three years,” said Joshua Smith, a doctoral student at UCSC. But it never disappeared completely and eventually, the remaining patches of kelp forest reached an “equilibrium point” with the urchin barrens.

New research led by Smith and published March 8 in the Proceedings of the National Academy of Sciences found that this equilibrium can largely be attributed to sea otters. The otters eat the urchins in the healthy kelp areas, keeping the urchin population under control and preventing the collapse of what kelp remains.

“This is fundamentally important for the ecosystem, because it’s contributing to the persistence of kelp forests,” Smith said. “The remnant patches of kelp forests that we have, those otters are maintaining.” 

The vast urchin barrens, however, are untended.The otters, like the commercial fishers, find the urchins in the barrens aren’t worth the effort.

Still, the otters’ dining habits could prove critical to kelp recovery, because the remnant patches of seaweed they maintain could spread if the urchin population declines. The otters, Smith said, are “maintaining these patches of kelp that are ultimately the spores sources to help replenish those barren areas when something else takes out the urchins.” 

Anything from an urchin disease to a bottom scouring swell that sweeps them off their rocks to a recovering starfish population could eventually end the proliferation of urchins. Encouragingly, Smith said there are some signs in Alaska and British Columbia that the sunflower sea star is starting to recover.

A Global Deforestation

The heating of the ocean is continually taking bites from underwater kelp forests at the edge of their ranges around the world, and in recent years, extreme marine heat waves have caused sudden and massive die-offs, similar to coral bleaching events or forest die-offs on land. They are the biggest killer of kelp, and scientists have documented waves of mortality from New Zealand and Australia to the Mediterraean Sea and Mexico. 

2020 research from Rodrigo Beas, a marine researcher with La Universidad Autónoma de Baja California, in the journal Global Change Biology showed the loss of kelp-dependent fish and shellfish is hitting some of Baja’s coastal fishing communities hard and highlight “the urgency of implementing adaptive strategies to sustain livelihoods and ensure food security.”

“We are two or three steps behind, we have not been able to keep up with these pulses of environmental change like marine heatwaves,” he said. “These patterns in nature that we are seeing, we don’t have the capacity to respond to them in time.”

For example, the same marine heat wave from 2014 to 2016 that drove the displacement of kelp by urchins in the waters off Northern California also decimated kelp forests along parts of the Baja coast, driving away commercially important fish,shellfish and crustaceans. Studies as early as 1976 showed how loss of kelp leads to lobster declines.

“We need to enhance our actions to help inform what to do now,” said Beas, who works with coastal communities trying to adapt to rapid change. Research shows that well-connected protected zones, and flexible, timely fisheries management actions are keys to buffering global warming impacts, he added.

Along parts of the U.S. Northeast Coast, kelp forests are being replaced by a less productive ecosystem of low-slung, turf-like algae mats that don’t provide cover for juvenile fish, said Montclair State University marine ecologist Colette Feehan, who tracked the changes in a 2020 Scientific Advances study.

The Northern California kelp study is “particularly significant in that it demonstrates that marine ecosystems can reach ‘tipping points’ at which abrupt or sudden losses of species occur due to compounding climate-change stressors,” Feehan said.

Kelp is not mobile, “so they can’t track climate change the way a fish can,” she said. Some kelp species can only disperse across tens of kilometers, she added, so if they are wiped out across larger areas, recolonization becomes very difficult and slow.

In addition to protecting shorelines by dampening strong waves, Feehan said kelp oxygenates coastal waters and abates ocean acidification by taking carbon dioxide out of the system, storing it and sequestering it in  deep water offshore when it dies and sinks. 

“A loss of kelp forests means losing that carbon sequestration,” she said.

Options Beyond Otters

In California, fishers, including Downie, are helping with kelp restoration by systematically removing urchins in partnership with Reef Check, a citizen science diving organization.

“Me and my father are always trying to compete to see who can pick more, and he’s an animal for how old he is,” Downie said. They culled about 1,100 to 1,200 pounds of urchins per day, all picked by hand. In Monterey Bay, a group of volunteer divers are piloting a similar effort at Tanker’s reef.

But removing all the urchins is “an impossibility,” said Keith Rootsaert, a diver leading that project. “By the time you kill all the urchins, that many have been born.” Instead, the idea is to “create a kelp oasis,” he said. Basically, the divers are acting as backup otters.

Pete Raimondi, an ecologist at UCSC, is working to save kelp spores, a seed banking effort for genetic study and, eventually, restoration. Raimondi thinks this will be necessary as the increasing frequency of marine heat waves continues to buffet kelp ecosystems.

“It’s going to be two steps down, one step up,” he said. “It’s not like one day you’re fine and the next day is completely different. It’s [that] the period of recovery doesn’t compensate.” Eventually, he expects it will be clear that “the net change is down,” he said.

In the meantime, Monterey Bay’s sea otter kelp guardians show that keeping ecosystems intact—before their last remnants are gone—is one way to buffer the worst impacts of global warming.