Mussels are in hot water in Southern California, where they lead dangerous lives on the Pacific Ocean’s front porch.
Mussel beds along the state’s southern shorelines are disappearing as climate change alters the region’s ecology, according to a new analysis by aquatic ecologist Corey Garza of the University of Washington. Garza and his team compared modern and historic photos dating to the 1970s to track how mussel beds have changed in three locations within the Southern California Bight, a sweeping curve of coastline and islands centered around Los Angeles. He presented the results in a poster at AGU’s Annual Meeting 2023 in San Francisco.
California mussels live in rocky intertidal areas from northern Mexico to Alaska. They begin their lives as planktonic larvae in the ocean, later anchoring themselves to rocks in tight clusters along the shore. However, rising water temperatures have made it harder for mussel larvae to thrive in the water column. Those that do make it to shore are exhausted.
“It’s almost like running a marathon, to some extent,” Garza said. “They’re petered out.”
Warmer waters also mean that fewer plankton from other species survive, so mussel larvae that do colonize the shoreline have less to eat. The hungry filter feeders must also contend with oceanic predators like sea stars and spiny lobsters, as well as heat stress from solar exposure during low tide.
Decades of Mussel Movements
The historic photographs in Garza’s analysis mostly came from U.S. National Park Service archives and retired scientists. Some were taken for data collection, but not for this particular research.
“There wasn’t really any idea that 50 years from now these are going to have the record of the impact of climate change in Southern California,” said Garza.
To trace that impact, Garza and his team took high-definition drone images of the same mussel beds pictured in the old photos. They then created 3D renderings of the beds, draping the historic images over the new models to compare the past and current mussel beds side by side.
The team then used mapping software to estimate how much of the tidal area was covered by mussels in both the old and new photographs and how the boundaries of the mussel beds moved over time as sea temperatures changed.
Garza’s analyses revealed that climate change has been affecting Southern California since at least the early 1970s.
“The impacts of climate change started a lot earlier than we thought in that system,” he said.
Some mussel beds in Southern California have been depleted by between 70% and 100%.
Fewer food options and dangerous predators have caused the lower boundaries of mussel beds to retreat upward. And although rocks closer to the ocean have become less desirable real estate, mussels farther up shore must still withstand the Sun’s rays and plankton shortages. As a result, Garza found, some mussel beds in Southern California have been depleted by between 70% and 100%.
Rising ocean temperatures are changing mussel habitats throughout their range, but the recession pattern could be unique to Southern California.
“In most of its distribution, exactly the opposite thing is happening, which is the lower boundary is going way lower,” said marine ecologist Peter Raimondi of the University of California, Santa Cruz. In some intertidal habitats, warming oceans have hurt populations of sea stars, so prey species like California mussels and other mollusks are spreading deeper into the ocean.
Garza himself was able to note the decline in mussel colonies in Southern California. He remembers seeing mussel beds on Catalina Island, one of the study’s locations, as a student in the early 1990s.
“Now, I go back with my graduate students, and there’s nothing out there anymore,” Garza said. “It’s clear things have slowly receded up the shoreline.”
—Madeline Reinsel, Science Writer
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