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Satellite Imagery Shows Northern California Kelp Forests Have Collapsed

Researchers say they’re not sure these iconic coastal ecosystems will be able to make a comeback anytime soon

Satellite images comparing bull kelp canopy cover (gold shading) 2008 and 2019 off the coast of Mendocino and Sonoma Counties in Northern California. (Meredith McPherson)
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The coastal waters of Northern California are changing. A decade ago, hundreds of miles of the rugged seaside were flanked by thick, swaying underwater forests of amber-green bull kelp that were home to fish, abalone and a host of other species. Now, those forests have been nearly wiped out by a series of environmental events that have been falling like ill-fated dominos since 2013.

Bull kelp forest
A healthy patch of bull kelp forest photographed at Pescadero Point (Steve Lonhart / NOAA, MBNMS)

A new study using satellite imagery and underwater surveys is the latest to confirm that these majestic marine ecosystems have all but disappeared, reports Tara Duggan for the San Francisco Chronicle. Satellite images dating back to 1985 show that bull kelp forests off Sonoma and Mendocino counties have declined by a devastating 95 percent since 2013, and, according to the Chronicle, researchers are concerned the kelp may not be able to bounce back anytime soon.

The results, reported last week in the journal Communications Biology, are the first to use satellite images to quantify the ecological losses that have racked up over the last eight years, the Associated Press reports. Across the more than 200 miles of coast encompassed by the study, kelp forests have been almost completely replaced by barren stretches of sea floor covered in spiky purple sea urchins.

Purple sea urchins are marine grazers that love to munch on kelp, and in 2013 one of their biggest predators, the sunflower sea star, abruptly started wasting away due to a still-mysterious disease that has ravaged the many-armed invertebrates from Mexico to Alaska.

Urchin barren
Many of Northern California's kelp forests have been replaced by so-called urchin barrens made up of purple sea urchins like these. (Katie Sowul / CDFW)

As the unchecked purple urchin populations began exploding in number, bull kelp got hit with successive marine heatwaves that made life even harder for the cold water-loving kelp from 2014 to 2016. While these ocean conditions can’t be wholly attributed to climate change, such marine heatwaves are predicted to become more common under climate change.

“There were a lot of disruptions at one time that led to this collapse, and the system now persists in this altered state,” Meredith McPherson, an ocean scientist at the University of California, Santa Cruz and the paper’s first author, in a statement. “It’s a naturally dynamic system that has been really resilient to extreme events in the past, but the die-off of sunflower stars caused the resilience of the ecosystem to plummet. As a result, the kelp forests were not able to withstand the effects of the marine heatwave and El Niño event combined with an insurgence of sea urchins.”

The now impoverished coastal ecosystems forced the closure of the $44 million recreational abalone fishery in 2018 and the commercial red sea urchin fishery has also nearly shuttered, per the Chronicle.

At this point, getting kelp forests to come back to the Northern California coast means the purple urchins carpeting the bottom have got to go. But getting rid of the urchins is no easy feat. Despite having eaten all their favorite foods, the simple, hardy spiked invertebrates somehow manage to persist.

“They can actually survive under starvation conditions,” McPherson tells the Chronicle. “The impact has been that basically there is no kelp forest at all left, really.”

There have been efforts to remove the purple urchins with legions of divers brandishing hammers and even vacuums, but so far there have simply been too many urchins, reported James Steinbauer for Bay Nature in 2020.

Others have suggested bringing in another kelp forest predator, the sea otter, to help fight back the urchins. The problem with this appears to be that sea otters aren’t so interested in the skinny, starved urchins occupying the most barren areas, reports Anuradha Varanasi for Inverse. A separate study published this week in the journal the Proceedings of the National Academy of Sciences, suggests the otters do eat urchins but that they prefer the more well-fed residents of the coast’s remaining kelp forests to the so-called “zombie urchins” clinging to life in the denuded barrens.

Though water temperatures have now returned closer to normal, these legions of zombie urchins make it extremely hard for the bull kelp to make a comeback.

“It is almost impossible for any kelp to regrow because the starving urchins gobble them up immediately,” Joshua Smith, an ecologist at the University of California, Santa Cruz and the PNAS study’s lead author, tells Inverse.

McPherson tells the Chronicle that though things are “a bit gloomy for the North Coast” right now, “there’s a lot of work in the area to see how we can maintain patches of kelp for restoration in the future.”

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