From UC San Diego Today Now / March 5, 2026
The growth form of giant kelp (Macrocystis pyrifera) is composed of shoots known as stipes instead of branches. From one parent holding fast to the hard bottom might come as many as 150 stipes.
Typically the tips of the biggest kelp bob at the ocean surface and calm the waters, appearing as patches of gold visible from land — a sign of the good health of the ecosystem that it anchors.
But the kelp as San Diego knows it is in trouble.
In January, a team led by UC San Diego’s Scripps Institution of Oceanography released an unmatched history of kelp forests off La Jolla and Point Loma. Together spanning nearly 19 square kilometers (7.3 square miles), they are the largest on the United States West Coast. Amassed over more than 40 years, their story reveals a progression of steady decline that transcends typical cycles of crash and recovery.
Now, say the researchers, competing organisms usually cast in shadow by the kelp are emerging as winners. The giant kelp are losing, but so might be myriad other organisms – fishes and humans included – as another natural order is disrupted by climate change and other new circumstances.
The downsides range from a decrease in the catch available to recreational fishers in San Diego to the loss of the nurseries that sea stars and open ocean fishes use to protect their larvae. Even the beach wrack – the large piles of decaying kelp that wash up after storms – is diminishing. Though the absence of the pungent kelp will be a relief to some beachgoers, those piles attract the kelp flies that are an important source of food for seabirds.
“It’s like starving the system,” said study lead author Ed Parnell, a marine biologist at Scripps Oceanography. “Giant kelp is an iconic species. It’s highly productive. It provides a lot of food for animals. It’s better for beaches. There are rafts of kelp paddies that pelagic fish use to protect their eggs.”
The Scripps Oceanography study tracks the story of more than 14,000 giant kelp plants over the course of decades. Some of the data gathered dates to the 1970s when veteran study co-authors were early in their careers. The bulk of the story, though, starts in 1983 when Scripps marine biologists Paul Dayton and the late Mia Tegner created the first of 20 stations to follow how kelp grew at various depths and in conditions ranging from rough open ocean-facing waters to relatively tranquil patches. Each station has four permanent transects 25 meters (82 feet) long situated perpendicular to the coast.
“It’s kind of mind boggling to think of how much data we collected,” said Kristin Riser, a study co-author and a staff research associate who started at Scripps in 1990. “It’s probably the longest time series like this in existence and it’s unique in that we followed individual plants.”
The plight of giant kelp elsewhere along the West Coast has garnered enough attention to elicit Congressional action. Legislators have pushed the “Help our Kelp” Act since 2023. The bill seeks to establish a NOAA grant to support conservation and management of American kelp forests.
And in San Diego, the city has supported ongoing research into the status of local kelp forests for more than three decades. The welfare of kelp drew widespread attention in 1988, when the city drew on the expertise of Dayton, Tegner and others after the Environmental Protection Agency sued San Diego to augment its treatment infrastructure for the wastewater San Diego pipes out to the ocean. The researchers deemed that infrastructure unnecessary, convincing the judge in the case and saving the city billions of dollars. Ever since, San Diego has been interested in monitoring the health of its kelp forests to make sure that remains so.
The city provided a portion of the funding for this study, as did the National Science Foundation and California Sea Grant.
The study documents how violent El Niño-powered waves in 1982-83 and 1997-98 ripped established giant kelp from the cretaceous sandstone that anchored it. The researchers saw how the kelp typically recovered quickly nonetheless. What was left clinging to the rocks after storms would release microscopic sporophytes, both male and female, that would find each other and produce offspring. The kelp would repopulate itself thusly and reacquire its former mass in the forests.
That doesn’t happen as much now. The downslide became especially pronounced after 2015, when a pool of unusually warm water settled in the eastern Pacific Ocean and lingered for more than a year. The forests have lived in a range of temperatures between 12 and 15? (54 to 59?) since 1983 with a few spikes reaching 16? (61?) during the El Niños. Since 2015, the temperature in the forests has yet to drop back below 14? (57?).
“Most kelp die after a year,” said Parnell. “Ten years is on the extreme end. What we are seeing now is kelp plants that don’t live as long and can’t make it to the point where they can be highly reproductive.”
These warmer waters encourage parasites like bryozoans to cluster on kelp leaves, dragging them away from the surface sunlight they need. Sea urchins proliferate and overgraze the kelp that are among their favorite food sources. The kelp is also further weakened by the lack of nutrients such as nitrogen in warmer water. Other species of kelp in M. pyrifera’s understory get more access to sunlight as the giant kelp thins out.
“Climate change is the principal driving factor, but understory competition is a major proximal cause,” said Dayton, a marine biologist who started at Scripps Oceanography in 1970.
Dayton and Riser add, however, that the story is nuanced. The demise of one species to the betterment of another happens all the time in nature and there is no saying that giant kelp might not dominate off the San Diego coast again someday. The ecosystem off the coast might be just fine in the meantime, they say. M. pyrifera might start drifting northward in search of cooler waters, Dayton said.
Part of the value of the study is its long duration and the fact that it is still going on, giving scientists the chance to observe nature through full progressions of its cycles, said research team members. Long-term observations are a hallmark of research at Scripps. In this as in other cases, the implications of the current state of the kelp forest might take another 40 years of study to fully appreciate. Maybe longer.
“In ecology, even though we have the longest time series record of giant kelp in existence, it’s still a tiny snapshot. A human lifetime is a blink of an eye,” said Riser. “I’m hoping it’s just a little downtick.”
The study, “Demography and dynamics of giant kelp cohorts across four decades: Lessons for conservation and resilience planning,” was published Jan. 28, 2026 in the journal Ecological Applications. Besides Parnell, Dayton and Riser, study co-authors include Cleridy Lennert-Cody, Lydia Ladah, Brenna Bulach and James Leichter of Scripps Oceanography; Ami Latker of the City of San Diego Public Utilities Department; and Stephen Schroeter of UC Santa Barbara.
