Protection from harvesting may help kelp forests flourish, but is it enough in the face of climate change?

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Kelp macro shot.
December 07, 2021
Media Contact— Elyse DeFranco / elyse.defranco@gmail.com /

Expecting to study recovery, researchers instead find kelp forests in crisis. 


It’s difficult to overstate the scale of devastation seen in Northern California’s kelp forests over the last decade. Essential to marine life along our coast, these underwater forests create the habitat that so many other species rely on — their loss is the marine equivalent of deforestation.

Mark Carr is a marine ecologist at the University of California, Santa Cruz who is leading the effort to monitor kelp forests along the coast of California. The research aims to understand how kelp forests are responding to protection under the Marine Protected Area (MPA) network, and is funded by the Marine Protected Area Monitoring Program (administered by California Sea Grant in partnership with the Ocean Protection Council and the California Department of Fish and Wildlife.)

“The assumption is that by protecting the fish species from being harvested in the MPAs, there might be ramifications for the invertebrate and algal species in the ecosystem,” says Carr. “We’re trying to capture how entire ecosystems are responding."

Carr’s career trajectory began during his undergraduate years at UC Santa Cruz, when a course on kelp forest ecology captivated him: the same course he now teaches each year. After helping to design and establish the MPA network in California, he set out to understand how kelp forests would respond to protection.

From Santa Cruz north, the primary canopy-forming kelp species is the bull kelp (Nereocystis luetkeana), with giant kelp (Macrocystis pyrifera) dominant south to the Baja California Peninsula. Both species were hit hard by the explosion of purple sea urchins that followed the sudden loss of one of their main predators, sunflower stars, to seastar wasting disease. Purple sea urchins are voracious consumers of kelp, and they have transformed vast swaths of kelp forest into desolate seascapes known as “urchin barrens.”

“We literally have not seen sunflower stars in any of our surveys since 2013 throughout California,” notes Carr.

In a one-two punch, the 2013 seastar die-off was immediately followed by the marine heat wave that hit from 2014 to 2016. Kelp forests still haven’t rebounded from these events: more than 90% of bull kelp forests were lost.

The kelp deforestation has caused enormous challenges for Carr’s team. As they set out to gather data on responses to the establishment of MPAs, the kelp forests were responding to dramatic environmental impacts, confounding their ability to track the kinds of changes they predicted.

“We have data that show that populations were responding [to protection] reasonably well. And then we had the marine heatwave, and those populations declined,” says Carr. “Now we’ll have to wait and see whether, or how, they recover again through time.”

To get an accurate picture of these changes, divers meticulously identify and count species as they swim underwater transect surveys, including fish, invertebrates, kelp and other algae. Each MPA chosen contains four survey sites, supplemented with data from comparable kelp forests located in neighboring areas to the north and south. This design allows them to compare populations and community responses to protection while keeping other conditions consistent.

Their preliminary findings didn’t find many significant differences between how protected vs unprotected kelp forests fared in the face of the marine heatwave and the explosion in purple urchins. Although some of the earliest monitoring sites in central California now have twenty years of data, many other MPAs were established in 2011 and 2012 – only one or two years before the crises hit. However, one notable difference was observed in central California, where the loss of sunflower stars has been buffered by the presence of sea otters, which also prey on purple urchins.

“The sea otters haven’t reduced the number of urchins in the areas that became barren,” says Carr. “But they have protected the remnant forests from being removed.”

A similar effect was seen in the Channel Islands, where MPAs sheltered a greater diversity of urchin predators, including the California spiny lobster and California sheephead. Carr’s team has seen some of the most notable MPA effects in species like these, which are typically favored by fishermen.

The researchers also tested whether large-scale efforts to reduce urchins could give kelp forests a fighting chance. In a study published April 2021, divers painstakingly removed more than 90% of purple urchins from urchin barrens and found that giant kelp forests grew back within six months.

In addition to counting kelp during underwater surveys, the scientists also track them from space. Kyle Cavanaugh at the University of California, Los Angeles is using satellite imagery to track the abundance and distribution of kelp inside and outside of the protected areas. This has the added benefit of providing a visual record prior to, and following, the establishment of the reserves.

Monitoring so many remote ocean locations up and down the expansive California coast requires a large team, and dozens of researchers from four universities contribute to the effort: Humboldt State University, UC Santa Cruz, the University of California, Santa Barbara, and Occidental College. Reef Check, a nonprofit citizen scientist group, is also integral to the partnership.

Despite some promising signs of resilience seen in the protected kelp forests, it has been difficult to grasp the widespread devastation to such a fundamental part of California’s coastal ecosystem. It’s not an overstatement to say that the extent of what was lost wouldn’t be known without extensive long-term research and monitoring efforts like the one conducted by Carr and his team.

“These MPA monitoring programs that are distributed along the state of California are generating the most important data that we have to determine the ecological consequences of climate change,” Carr says.

About California Sea Grant

NOAA’s California Sea Grant College Program funds marine research, education and outreach throughout California. Our headquarters is at Scripps Institution of Oceanography, University of California, San Diego; we are one of 33 Sea Grant programs in the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce.