Kelp forests off the coast of Northern California have declined by 95 percent, and this collapse will be hard to reverse, according to researchers at UC Santa Cruz. The experts report that the kelp has been replaced by widespread “urchin barrens” that may persist long into the future.
The dramatic shift in the coastal ecosystem of Northern California occurred after a series of extreme events, including unusual ocean warming along the West Coast beginning in 2014. By analyzing satellite images, the researchers found that there are just a few isolated patches of bull kelp remaining.
There is some hope that the kelp forests could start to bounce back. Study co-author Professor Raphael Kudela said that ocean temperatures are beginning to cool down along the coast, after remaining above normal since 2014.
“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 Professor 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.”
The analysis showed that kelp forests north of San Francisco were resilient to extreme warming events in the past, including strong marine heatwaves and El Niño events. However, the widespread loss of the sunflower sea star to a deadly disease has left the kelp forests more vulnerable than they once were.
Sea stars are key predators of sea urchins. When they are not controlled by predators, urchins become larger and more abundant, and feed on kelp until little remains.
“There were a lot of disruptions at one time that led to this collapse, and the system now persists in this altered state,” said study first author Meredith McPherson.
“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.”
Marine heatwaves and El Niño events inhibit coastal upwelling, which leads to poor conditions for kelp including warmer water and low nutrient availability.
“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 Professor Kudela. “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.”
The sunflower sea star was among the hardest hit species by the deadly sea star wasting disease, and was recently listed as critically endangered by the IUCN.
Kelp forests have declined all along the California coastline, but not to the same extent as in Northern California. In Southern regions, such as Monterey Bay, urchin predators like sea otters are present.
“Sea otters haven’t been seen on the North Coast since the 1800s,” said McPherson. “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.”
Unless sunflower sea stars or some other urchin predator returns to Northern California, the chances for kelp forest recovery are poor, said McPherson. Even if other conditions promote kelp growth, it will be difficult for the new plants to become established in the midst of the urchin barrens.
“There’s a lot of research and discussion now about the best management strategies for the future,” said McPherson. “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.”
The study is published in the journal Communications Biology.