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.
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. The findings are published in the journal Ecological Applications.
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.
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