Image Source: https://baynature.org/
An unexpected darkness has recently fallen over the seafloor of the Northern California coast – the shadows cast by bull kelp.
The giant marine alga nearly vanished after a perfect storm of environmental and ecological events, including a marine heatwave and a population boom of seaweed-eating sea urchins, disrupted the marine ecosystem between 2013 and 2015. Kelp forests collapsed by more than 90 percent in Northern California, and with them went both scenic appeal and marine biodiversity. Red abalone, which graze on kelp, starved in droves, and fish departed for deeper waters. What was left, and which persists in much of the region, is a bleak underwater landscape dominated by purple urchins and not much else.
But this year the bull kelp forests of memory have surged back along parts of the Northern California coast. Areas that were completely devoid of kelp as recently as last winter are now marine jungles of tangled underwater stems and dense floating mats of fronds. James Ray, a California Department of Fish and Wildlife biologist and kelp researcher, says the comeback seemed to begin in 2020 “with a little bump in kelp cover.”
“Now we’re seeing a much bigger bump along much of the coast,” he says.
The rapid resurgence, possibly the result of strong springtime upwelling of cold water, has other experts both delighted and a bit mystified.
“The rebound of the forests in Sonoma and Mendocino counties has been surprising and profound considering how devastated they were just a few years ago,” says Franklin Moitoza, a graduate student at Humboldt State who, working with a team of collaborators, has closely tracked kelp forest health and recovery. He says he has seen pronounced kelp regrowth from Bodega Bay to Trinidad within the past year.
Kelp growing back in the midst of dense urchin colonies has challenged what many marine ecologists thought they knew about kelp forest ecosystems. By numerous expert accounts in the past five years, the North Coast’s bull kelp would not recover so long as sea urchins remained in dense numbers.
“Once an environment shifts into an urchin barren state, it tends to stay that way,” marine biologist Cynthia Catton, formerly of the California Department of Fish and Wildlife, told me in 2017. “Everything is pointing toward this being a long-term problem.”
Urchins aggressively eat marine vegetation and can live for decades, even after extinguishing their food supply. Under such circumstances, they have a tendency to overwhelm the environments they dominate – drab ecosystems that scientists call urchin barrens and which can persist for staggering periods of time. In the Aleutian Islands and Tasmania, urchin barrens replaced kelp forests decades ago and have remained ever since, and coastal Hokkaido is home to barrens that developed almost a century ago. It’s this gloomy fate that many marine biologists assigned to the Northern California coast.
The unexpected rebound also complicates a particular theory of ecology that scientists had applied to the California coast’s kelp disappearance – that of alternative stable states. Alternative stable states theory describes a paradigm of alternating ecosystems that share the same location under identical environmental conditions, just at different times. One ecosystem state dominates the space and persists until a powerful environmental disruption causes a rapid transformation, after which a very different ecosystem takes hold. Grasslands, in certain areas, are the alternative stable state to forest. Mucky freshwater lakes clouded with phytoplankton are the alternative stable state to clear-water ecosystems in which predatory fishes prevent the proliferation of phytoplankton. Coral reefs, some scientists believe, are the alternative stable state to underwater seaweed meadows (although this example is debated).
In the case of temperate coastal upwelling zones, kelp forests alternate with urchin barrens. True to the criteria of the theory, which was first introduced to the peer-reviewed literature in 1969, each alternate ecosystem is remarkably stable, and to shift from one to another requires a dramatic environmental disruption that rapidly “flips” the system over. On California’s North Coast, such a perturbation took the form of a multi-year marine heatwave, which stressed the cold-dependent kelp, coupled with a simultaneous viral outbreak that wiped out predatory sunflower sea stars. Their favored prey, purple urchins, proliferated. By grazing on any available vegetation, the urchins prevented kelp recovery even after water temperatures cooled to historical norms in 2017. To scientists studying the collapse of the kelp forests, all signs pointed toward an alternative stable state change, and there was every reason to think the urchin barrens would last indefinitely.
Craig Johnson, a researcher at the University of Tasmania’s Institute for Marine and Antarctic Studies, told me in 2017, “For all intents and purposes, once you flip to the urchin barren state, you have virtually no chance of recovery.”
Such a fatalistic perspective stems both from real-world scenarios as well as the rigid defining criteria of alternative stable states. For instance, each stable ecosystem has an internal positive feedback cycle that helps perpetuate it. Often, it is the species that occupy the ecosystem that lock the states in place. Another key criterion is that it takes a much lesser environmental force to maintain the stable state than it takes to flip the system from one state to the other. For example, an ungodly number of urchins, practically stacked on top of each other, is generally required to demolish a kelp forest and turn it into a barren, but thereafter it takes relatively few of the creatures to maintain the barren. In fact, they must be nearly eradicated to allow a kelp comeback.
At least in theory. But this year divers on the North Coast have seen bull kelp holdfasts fixed to rocks very near clusters of urchins.
“It might be that the stable states aren’t as stable as we thought,” says Sean Craig, a Humboldt State University biologist studying kelp forest ecology. Craig points out that “the general theory does not always fit the reality underwater.”
Moitoza notes that bull kelp never did entirely vanish as many media reports have indicated.
“Patches of bull kelp always remained, and there were large remaining areas in Humboldt County,” he says. “We’re wondering if there are some features of the ecosystem in these places that explain why we saw kelp persist.”
Mark Carr, a kelp forest ecologist at UC Santa Cruz, suggests something else entirely – that the ongoing saga in the waters of the North Coast is not, after all, a stable state alternation but something different: a regime shift caused by a change in environmental conditions. Regime shifts, unlike the more dramatic and rapid alternative stable state shifts, happen when overarching environmental conditions change. The ecosystem changes that follow don’t necessarily occur quickly. Rather, they unfurl in sync with the changing environment.
“It’s an important difference,” Carr says.
In Tasmania, kelp forests vanished through the 1990s due to rapid and long-term ocean warming. The southward push of tropical water also brought a species of warm-water urchin, which became established and, along with the changed ocean conditions, prevented any kelp recovery. The Tasmanian saga, similar in obvious ways to California’s recent experience, is commonly cited as a classic example of an alternative stable state shift – but, by the rules of the theory, it was not.
“It was really a regime shift,” Carr says.
It’s possible that’s what has happened here, he says. The waxing and waning of Northern California’s kelp may be more closely tied to fluctuating water temperatures than urchin densities. If this is the case, kelp forests could be more threatened than we thought. It would also exonerate the urchins that have been cast as the villains. The loss of kelp might be not their fault, but ours.
“If all of a sudden we start having marine heatwaves every 10 years, forget it – you can say goodbye to kelp,” Carr says.
He cautions that the scale of recovery at this point might not amount to an ecosystem change.
“There are still large areas of the North Coast that continue to be urchin barrens,” he says.
Tristin McHugh, The Nature Conservancy’s Kelp Project director, is cautiously optimistic about the years to come. McHugh says she expects “kelp will continue to flourish and potentially increase” so long as ocean temperatures remain cool and favorable – which, of course, they might not. Strong upwelling early in 2021 brought cold, nutrient-rich water into coastal ecosystems – perfect conditions for bull kelp growth. A second La Niña year is in the forecast, which would perhaps mean more of the same in 2022. But with oceans warming worldwide, and with armies of purple urchins still occupying the North Coast’s shallow waters, the bull kelp bounce-back could easily turn around.
Further complicating the study of ecosystem changes is the fact that humans play an increasingly important, if often overlooked, role in ecosystem dynamics. Craig says traditional ecology “has been a bit arrogant because it assumes humans aren’t an important part of how things change.”
In fact, humans have played a fundamental role in the fate of the North Coast’s kelp forests. Hunting sea otters to extinction in most of their range left just a single major predator – the sunflower sea star, Pycnopodia helianthoides – to control purple urchin populations. In effect, humans made the stable kelp forest state much less stable by halving the predatory effect on urchins. Then, when sunflower sea stars vanished, the top-heavy ecosystem caved – but it was human dabbling as far back as the 1700s that was responsible.
Urchin removal efforts are underway in a few locations on the Sonoma and Mendocino coastlines, with the goal of reestablishing bull kelp beds. However, kelp has rebounded at many sites that saw no organized urchin culls. Marissa Baskett, an ecosystem modeler at UC Davis, says understanding how bull kelp has managed to regrow in the presence of a dense urchin population is important not just for the sake of honing ecological theories and predictive models but for resource management purposes.
On the one hand, the kelp could continue to regenerate, urchins or none. On the other, humans may have to help.
“If this really is an alternative stable state, then we know we have to get past a very specific threshold in urchin density to allow kelp recovery,” says Baskett, who recently received funding to assess climate change’s potential effects on kelp restoration efforts, and whether full restoration will even be possible. She will be collaborating with colleagues at UC Santa Cruz, UC Davis and Humboldt State.
McHugh, at The Nature Conservancy, says it’s not yet clear how heavy-handed a role humans must play in future management, and protection, of kelp forests in northern California waters. Culling urchins through removal or underwater smashing may be a long-term maintenance requirement in Northern California waters.
“Even though we’ve seen a comeback [of kelp],” she says, “with no predators more natural predators, we’ll never be out of the woods.”