Scientists warn that swift action is needed to revive the kelp forests that have faced destruction by extreme weather events and a hungry sea urchin population.
(CN) — Northern California’s kelp forests have weathered many harsh conditions in the past, but the recent loss of a keystone species has led to a sharp 95% decline that scientists warn will be vastly difficult to come back from.
Details of the study were published Friday in the journal Communications Biology, where researchers from the University of California, Santa Cruz detail the disappearance of the kelp forests and propose swift action to return them to their full health.
Kelp forests are unique ecosystems that look downright otherworldly, and provide homes and resources to thousands of species. They are made up of fast-growing giant algae that reach up from the seafloor like trees and can grow over 100 inches tall, thriving in cool, relatively shallow coastal waters.
They are recognized as one of the most important ecosystems for the biodiversity they support and the protection they provide for coastal areas.
Like many other ecosystems, kelp forests have been affected by climate change in one way or another. Violent storms have caused kelp to be ripped or uprooted, though they are typically able to successfully regrow in the spring, and warmer ocean temperatures have created less than ideal conditions for kelp growth.
But the nail in the coffin for the Northern California kelp forests seemed to be a mixture of unfortunate events, starting with an unnatural boom in purple sea urchins and ending with unbearable ocean conditions.
The authors note that despite ocean warming and threatening effects of El Niño storms, the kelp forests outside San Francisco stayed strong. However, that all changed when the famous sunflower sea-star was declared critically endangered by the International Union for Conservation of Nature.
These giant sea stars plummeted in population due to a marine illness called sea star wasting syndrome, resulting in more than a 90% decline with little to no recovery.
Sea stars are keystone predators, meaning they are a staple in the delicately balanced food web of its ecosystem. The loss of the sunflower sea stars meant that their natural prey, the sea urchins, were left to populate with significantly less control, and in turn the kelp forests could not grow fast enough to support the growing urchin population.
“There were a lot of disruptions at one time that led to this collapse, and the system now persists in this altered state,” said first author Meredith McPherson, a graduate student in ocean science at UC Santa Cruz.
“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.”
San Francisco’s kelp forest consists primarily of bull kelp, which has proven to be resilient to temperature fluxuations in the past. They are at their healthiest in cold, nutrient-rich waters, which typically become scarce during heatwaves and El Niño events. While it was normal for them to wither and become weak during this time, they could always bounce back when these events passed.
Unfortunately, it seems that after the spike in urchin feeding, the kelp forest lost its durability. Especially because once a kelp forest was depleted, the area became overrun by urchins, creating decimated regions known as ‘urchin barrens’ unable to sustain vegetation.
“There have been big changes before, when a strong El Niño has reduced the kelp canopy dramatically, but in the past it’s always come back,” said coauthor Raphael Kudela, professor and chair of ocean science at UC Santa Cruz. “The loss of resiliency is what made this time different — the combination of ocean warming and the loss of the sea stars allowed the urchins to take over.”
This kind of extreme die-off event is unprecedented in kelp forests, so to get a better understanding of the timeline the researchers looked at satellite images from 1985 to present day provided by the U.S. Geological Survey’s Landsat missions.
It started with the spreading of sea star wasting disease in 2013, affecting all sea stars but hitting the sunflower species the hardest. Soon to follow was a disastrous marine heatwave from 2014 to 2015 notoriously known as ‘the blob’ which negatively impacted all marine life. Not long after came an El Niño event that brought warm waters, and with it a massive increase in sea urchin population.
“The alignment of all of those events resulted in an incredibly dramatic loss of kelp,” said Kudela.
California saw a decline in kelp forest health all along it’s coast, but Northern California experienced the most extreme loss for two main reasons. The first is that the bull kelp was unable to grow strong again in time for the weather events due to the speed of consumption and it being an off season for their growth.
Second, the urchin’s main predator here was absent and Northern California has few populations of the kelp forest superheroes, the sea otters, who have kept the Monterey Bay kelp forest healthy.
“Sea otters haven’t been seen on the North Coast since the 1800s,” McPherson said. “From what we observed in the satellite data from the last 35 years, the kelp had been doing well without sea otters as long as we still had sunflower stars. Once they were gone, there were no urchin predators left in the system.”
The situation looks bleak for Northern California’s kelp forest, and the authors say that the most viable avenue for recovery would be the reintroduction of urchin predators, such as the sunflower sea stars or sea otters, to clear the dreadful urchin barrens.
McPherson added that it would also likely help if another marine disease took place to kill off many of the urchins, but the most foreseeable solution would be a mechanism to control the booming population.
Efforts to manually clear out the urchins have commenced in some areas conducted by the Reef Check California Program, and are constantly gathering new data and making progress.
“There’s a lot of research and discussion now about the best management strategies for the future,” McPherson said. “It’s important to understand and monitor the whole system. If we’re going to undertake restoration efforts, we need to make sure to do it when the temperature and nutrient conditions are right for the kelp.”
“This year we are finally seeing ocean temperatures starting to cool off, so we’re hoping that it reverses naturally and the kelp is able to take off again,” said Kudela. “There’s really not much we can do except to keep monitoring it. Of course, the long-term solution is to reduce our carbon emissions so we don’t have these extreme events.”