The collapse of Northern California kelp forests will be difficult to stop

Satellite images show that the area covered by kelp forests off the coast of Northern California has dropped by more than 95 percent, with only a few small, isolated patches of bull kelp left. Spicy kelp forests have been replaced by ‘hedgehogs’, where purple sea urchins cover a seabed without kelp and other algae.

A new study led by researchers from UC Santa Cruz documents this dramatic shift in the coastal ecosystem and analyzes the events that caused it. It was not a gradual decline, but a sudden collapse of the kelp forest ecosystem in the wake of unusual ocean warming along the West Coast that began in 2014, part of a series of events that combined to reduce the kelp forests .

Published on March 5 in Communication Biology, the study showed that the kelp forests north of San Francisco have been resilient to extreme warming events in the past, and other strong marine heat waves and El Niño events have survived. But due to the loss of an important hedgehog predator, the sunflower starfish, due to the waste of starfish, the kelp forests in Northern California have left hippopotamus predators, who are greedy kelp spinners.

“There have been many disruptions that have led to this collapse at the same time, and the system now remains in this changed state,” said first author Meredith McPherson, a graduate student in ocean science at UC Santa Cruz. “It’s naturally a dynamic system that was really resilient to extreme events in the past, but the death of sunflower stars has reduced the resilience of the ecosystem. As a result, the kelp forests could not cope with the effects of the marine heat wave and the El. Niño event combined with an uprising of hippos. ‘

The researchers used satellite images from the US Geological Survey’s Landsat missions from 1985 to assess historical changes in the kelp forest roof.

Bulk kelp is the dominant kelp species north of San Francisco Bay, while giant kelp dominates in the south. Both species thrive when strong cold, deep water yields the nutrients along the coast. Marine heat waves and El Niño events suppress coastal swelling, leading to hot water and low nutrients in which kelp grows poorly.

“There used to be big changes when a strong El Niño dramatically reduced the kelp roof, but in the past it always came back,” co-author Raphael Kudela, professor and chair of ocean science at UC Santa Cruz, said. “The loss of resilience is what made this time different – the combination of ocean warming and the loss of starfish took over the hedgehogs.”

Starfish disease first spread in 2013, affecting all types of starfish along the West Coast. The sunflower starfish was one of the hardest hit species and was recently considered critically endangered by the International Union for Conservation of Nature.

Towards the end of 2014, an unusual marine heat wave hit the Northeast Pacific, known as ‘the spot’, when it spread to the West Coast in 2015. A strong El Niño event began to develop around the same time and hot water in the coast of the south. The warm water coincided with an increase in sea urchin population along the North Coast.

“The adaptation of all the events has led to an incredibly dramatic loss of kelp,” Kudela said.

Kelp forests have declined along the coast of California, but not to the same extent as in Northern California. Bulk kelp is an annual species that regrows every year, which can make it more sensitive to these stressors than giant kelp. But another critical difference in Northern California is the absence of other hedgehog predators such as sea otters, which allowed patches to survive in healthy kelp forest in Monterey Bay.

“Sea otters have not been seen on the North Coast since the 1800s,” McPherson said. “According to satellite data from the past 35 years, the kelp did well without sea otters, as long as we still had sunflower stars. Once they were gone, there were no predators of the hedgehogs left in the system.”

What this means for the future, she said, is that the prospects for restoring the kelp forests in Northern California are bleak unless sunflower starfish or another hedgehog predator returns to the system. Even if the temperature and nutritional conditions are good for kelp growth, new kelp plants will find it difficult to establish in the midst of the oak barbs.

Several attempts have been made to manually remove divers’ hedgehogs from selected areas and see if it can help the kelp recover, led by the Reef Check California program (which contributed data for the recording of the tides). An outbreak of sea urchin disease can also lead to mass deaths of hedgehogs and give the kelp a chance to recover. If there is no mechanism to reduce the hedgehog populations, it will be difficult, according to McPherson, to repair and maintain the kelp forests.

“There is a lot of research and discussion right now about the best management strategies for the future,” she said. “It is important to understand and monitor the whole system. If we are going to make recovery efforts, we need to make sure we do so when the kelp represents the temperature and nutritional value.”

Kudela said ocean temperatures along the coast are starting to cool down, having been above normal since 2014. “This year we are finally seeing the ocean temperatures start to cool down, so we hope that it naturally reverses and that the kelp can take it down again,” he said. “There really is not much we can do other than continue to monitor it. The long-term solution, of course, is to reduce our carbon emissions so that we do not have these extreme events.”