The kelp forest that only eight years ago formed a lush ocean canopy along California’s northern coast has almost completely disappeared, and scientists studying algae and the species that depend on it are concerned about its inability to recover. .
A new UC Santa Cruz study found that the kelp forest on the Sonoma and Mendocino coast has declined by an average of 95% since 2013. It analyzed satellite images dating back to 1985 to investigate how a number of factors led to the forest. of abrupt algae. decline, including an explosion in the purple sea urchin population, which eats it, and two waves of marine heat. Research shows that the unprecedented destruction was linked to unusual ocean warming and that the kelp forest likely won’t recover anytime soon, in part because removing urchins is so difficult.
“In fact, they can survive in starvation conditions,” said Meredith McPherson, a graduate student in the UC Santa Cruz department of Ocean Sciences and a co-author of the study. “The impact has been that there is basically no kelp forest left, actually.”
Bull seaweedNereocystis luetkeana) generally thrives in the rocky coastal areas of Sonoma and Mendocino counties and creates habitat for many types of fish and invertebrates, including abalone, sea urchin, jellyfish, and sea snails. Its demise has also had impacts on local tourism and other businesses: the abalone fishery closed to recreational divers since 2018, and the Mendocino County commercial red sea urchin fishery is almost completely closed.
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The two warm water events that helped cause the kelp forest to decline include an El Niño phenomenon and what is known as a warm water “patch” that together lasted from 2014 to 2016. Around the same time, a Debilitating disease hit the sunflower starfish population, leaving the purple hedgehog without a predator.
Those hedgehogs quickly took over, eating the remaining algae and starving two other species popular with divers and sushi lovers: the red abalone and the red hedgehog (the purple hedgehog is not as commercially viable). What remains are the hedgehog moors, rocky areas completely covered by spiky purple invertebrates along hundreds of kilometers of the north shore.
“What we’re seeing in Northern California is unprecedented in terms of its scale,” said Tristin McHugh, seaweed project director for the Nature Conservancy’s California Oceans Program, which is developing pilot methods to remove sea urchins from purple sea and restore algae.
Although there were fears that another hot drop of water was forming last year off Alaska, water temperatures on the north coast have returned to normal, McPherson said, and yet the bull algae have not recovered. Unlike other native California algae, it is an annual species that dies off in winter, when it reaches shore in piles that resemble long greenish-brown hoses with bulbous tips. Usually returns every spring.
Scientists had already been monitoring the decline of the kelp forest for years with aerial photography and tidal data, but the new study was the first to use satellite imagery to take a closer look at changes in growth along with the temperature of the sea. ocean and nutrient levels.
“We can see seaweed relatively easily from space using a satellite,” McPherson said.
Seaweed growth relies on cold outcrops in spring that bring nutrients to the surface, and these are reduced when the water temperature rises. Although previous El Niño events, a natural pattern that causes Pacific water temperatures to rise for a year or two, have also caused seaweed to decline, they have generally rebounded.
Farmed urchin from the Norwegian company Urchinomics, which is opening a sea urchin “ranch” in Bodega Bay.
UrchinomicsWhat was different in 2014-16 was the explosion of the purple urchin and the addition of the hot water drop, which at its peak raised the ocean temperature by nearly 7 degrees above average. While there is some evidence that the spot was linked to climate change, more importantly, marine heat waves are becoming more frequent and intense as global temperatures rise due to climate change, McPherson said.
Climate change, pollution and other factors are also to blame for the global decline of kelp forests in the past 20 years, but in California, it’s worst off the coast north of San Francisco. Monterey Bay and other parts of central California have also lost kelp forests to urchin moorlands, but those areas have a mix of bull kelp and giant kelp and also have sea otters that help hunt urchins. In southern California, where giant algae are the main species, the forest has persisted better.
The most concerning part about the decline of the Sonoma-Mendocino kelp forest is that unless the sunflower starfish or other predator returns, the purple sea urchin shows no signs of moving.
“Usually some kind of physical disturbance or illness would wipe out the population,” McPherson said. “It could last for decades and decades.”
Habitat loss to the purple sea urchin is why the California Department of Fish and Wildlife in 2018 put a five-year hiatus in recreational fishing for red abalone, which generally attracts recreational divers from all over the world. California to the north coast. As a result, the last dive shop in the area closed at Fort Bragg about a year ago.
Many jobless professional red urchin divers have been hired into a state sponsored program run by Reef Check, a nonprofit organization, to manually remove purple urchin from the seabed. It is a slow process. In some cases, there are 20 to 30 urchins per square meter of seafloor, and similar efforts in Norway, Japan and New Zealand have shown a reduction to 2 per square meter is necessary for the kelp forest to recover, McHugh. saying.
He added that the private sector has to get involved if there is to be a long-term solution. That is starting to happen. Last year, the Norwegian company Urchinomics signed a partnership with the owners of Bodega Bay to build a sea urchin “ranch”, where purple sea urchins collected from the coast will be fattened to be sold in restaurants and shops.
The Nature Conservancy is also working on pilot programs to see if the traps could be used to capture urchins and is in talks to build an experimental seaweed farm in Humboldt Bay this spring, McHugh said.
McPherson said there are other efforts that give hope to the situation, such as the development of spore banks so that the algae can eventually be replanted when conditions are right.
“It’s a bit bleak for the north coast,” he said. “But there is a lot of work in the area to see how we can maintain seaweed patches for restoration in the future.”
Tara Duggan is a staff writer for the San Francisco Chronicle. Email: [email protected] Twitter: @taraduggan