La Jolla and Point Loma are home to the largest giant kelp forests on the West Coast, but those golden underwater forests are shrinking as ocean temperatures rise.
A new study from Scripps Institution of Oceanography looks at five decades of data tracking San Diego’s kelp forests. Their findings, published in January in journal Ecological Applications, indicate that San Diego’s giant kelp forests are diminishing — showing that the ocean ecosystem around the city is changing as a major source of food decreases.
Researchers say climate change caused the kelp decline and ecosystem disruption.
“It’s like starving the system,” said the study’s lead author Ed Parnell, a marine biologist at Scripps Oceanography. “Giant kelp is an iconic species. It’s highly productive.”
Giant kelp is not just food. It calms choppy waters, making better beaches for humans. Multiple species use its leafy rafts to protect eggs and larvae. When those fish and sea star populations fall too, recreational fishers will have less to catch. Seabirds will have less to eat as less decaying kelp washes up on shore — the latter attracts the flies the birds feast upon.
The length of the study, with data dating back to 1973, detailed a steady decline in kelp not attributable to crash and recovery cycles. The bulk of data comes after 1983 when researchers set up 20 stations to track individual plants at multiple depths within a seven-mile radius off Point Loma.
“It’s kind of mind boggling to think of how much data we collected,” said Kristin Riser, a study co-author who has worked at Scripps since 1990. “It’s probably the longest time series like this in existence and it’s unique in that we followed individual plants.”
In total, marine biologists tracked 14,000 kelp plants over 40 years, creating family histories of the plants.
“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,” Parnell said.
The downward trend in plant reproductivity and longevity became especially pronounced in 2015 when a pool of warm water settled in the Pacific Ocean and didn’t leave for more than a year.
The city of San Diego funded part of the study, as well as the National Science Foundation and California Sea Grant.
The Sierra Club and EPA sued the city in 1988 to force it to spend over a billion dollars in sewage wastewater infrastructure upgrades, which, in turn, raised rates for residents.
Testimony from some of the study co-authors and other former Scripps researchers convinced the judge that the infrastructure upgrades were unnecessary to protect the giant kelp. San Diego saved money after the judge’s ruling.
Those researchers continued to monitor kelp forests to ensure wastewater upgrades were still unnecessary for the giant kelp. Instead, they blame its decline on rising ocean temperatures.
Parasites like bryozoans cluster on kelp leaves in warmer water, dragging them below the surface and away from the sunlight they need. Warm water also contains fewer nutrients, like nitrogen, that kelp needs. Plus, sea urchin populations thrive in those temperatures, then overgraze on the kelp.
As giant kelp diminishes, with some fish and creatures along with it, other species are competing to take its place — other species of kelp, for instance. Usually relegated to the lower depths with less sun, these species are growing taller as giant kelp thins.
“Climate change is the principal driving factor, but understory competition is a major proximal cause,” said study co-author Paul Dayton. He collected some of the initial data in the study from the ‘70s and went on to create the research stations in 1983 with the late Mia Tegner.
Legislators took notice too. But efforts to create a federal grant to support conservation and management of American kelp forests have yet to pass in the House of Representatives.
Despite concerns over how the giant kelp dying off could affect San Diego’s ecosystem, marine biologists still have hope for the species. Those forests could migrate north in search of colder waters.
In the meantime, Scripps still operates its research stations, continuously collecting data on San Diego’s family of kelp plants.
“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,” Riser said. “I’m hoping it’s just a little downtick.”
