It will be hard to go back when the kelp forests of Northern California fell

Satellite images show that the area covered with kelp forests off the coast of Northern California has fallen by more than 95 percent, with only a few isolated patches of bull kelp remaining. “Urchin barnacles” have replaced species-rich kelp forests, where purple sea urchins cover the seabed without kelp and other algae.

A new study led by researchers at UC Santa Cruz documents this remarkable shift in the coastal ecosystem and analyzes the events that caused it. This was not a gradual decline, but a sudden collapse in the kelp forest ecosystem following the dramatic warming of an ocean on the West Coast beginning in 2014, part of a series of events that came together to decay kelp forests.

Published March 5 in Communication Biology, the study shows that the kelp forests north of San Francisco withstood extreme warming events in the past, surviving other strong marine heat animations and El Niño events. But the loss of the main urchin predator, a sunflower starfish, as a result of starfish consumption disease, left the kelp forests of Northern California free of any predators of sea urchins, which are active grazing on kelp.

“There was a lot of unrest at one time that caused this collapse, and the system now continues in this altered state,” said first author Meredith McPherson, a graduate student in marine science at UC Santa Cruz. “It’s a very dynamic system that has been incredibly capable of real events in the past, but the death of sunflower stars has caused the strength of the ecosystem to collapse. As as a result, the kelp forests were unable to withstand the effects of the tidal warming event and the El Niño event combined with insurgence of seawater. “

The researchers used satellite images from the Landsat missions of the U.S. Geological Survey dating back to 1985 to assess historical changes in kelp forest canopy cover.

Bull kelp is the main species of kelp that forms the canopy north of San Francisco Bay, while large kelp dominates the south. Both species thrive when deep cold cold water comes up with nutrients to the coastal surface. Ocean heat waves and El Niño events stop coastal uplift, resulting in warm water and low nutrient conditions in which kelp grows poorly.

“There have been major changes in the past, when a strong El Niño has greatly reduced the kelp canopy, but in the past it is always coming back,” said coauthor Raphael Kudela , professor and chair of marine science at UC Santa Cruz. “What made this period different was the loss of stability – the combination of warming of the sea and the loss of starfish allowed the onions to take over.”

Starfish consumption first appeared in 2013, affecting all types of starfish on the West Coast. Sunflower starfish was among the hardest species and was recently listed as endangered by the International Union for Conservation of Nature.

At the end of 2014 an unusual sea wave in the North East Sea known as “the blob” came when it spread down the west coast in 2015. The strong El Niño event began to develop around the same time. , bringing warm water up the coast from the south. The warm water coincided with an increase in sea urchin numbers on the north coast.

“As a result of the alignment of all these events, there has been a huge loss,” Kudela said.

Kelp forests declined along the coast of California, but not to the same extent as in Northern California. Bull kelp is an annual species that grows back each year, which may be more sensitive to these weights than large kelp. But another important difference in Northern California is that there are no other urchin predators such as sea otters, which allowed patches of healthy kelp forest to be maintained in Monterey Bay, for example.

“Sea otters have not been seen on the north coast since the 1800s,” McPherson said. “From what we’ve seen in satellite data from the last 35 years, kelp had been doing well without otters while we still had sunflower stars. they left, there were no urchin predators left in the system. “

That means for the future, she said, the prospects for recovering Northern California’s kelp forests are dire unless sunflower starfish or other urchin predators return to the system. Even if the temperature and nutrition are good for kelp growth, new kelp plants will have a hard time establishing among the urchin barrens.

Efforts have been made to manually remove divers from selected areas and see if this can help kelp recovery, led by the California Reef Check Program (which submitted submarine survey data for the survey ). An outbreak of sea urchin infection can also cause severe death of onions and allow kelp to recover. Without some equipment to reduce urchin counts, however, it will be difficult to restore and maintain the kelp forests, according to McPherson.

“There is a lot of research and debate now about the best management strategies for the future,” she said. “It is important to understand and analyze the whole system. If we are to make recovery efforts, we need to make sure the temperature and feeding conditions are right for the kelp. ”

Kudela said ocean temperatures are starting to cool down the coast, having remained above normal since 2014. “This year we are finally seeing ocean temperatures begin to rise. cooling, so we hope it returns naturally and the kelp can take off again, “he said.” There’s not much we can do but keep an eye on it. In fact, the long – term solution is to reduce our carbon emissions so that we do not have these catastrophic events. “

###

In addition to McPherson and Kudela, the paper’s coauthors include Mark Carr, professor and chair of ecology and evolutionary biology at UC Santa Cruz; Dennis Finger at UC Berkeley; Henry Houskeeper at UC Santa Cruz (now at UCLA); Tom Bell at UC Santa Barbara; and Laura Rogers-Bennett at UC Davis. This work was supported in part by the National Science Foundation.