An early June heat wave cooked the mussels of Northern California's Bodega Head in their shells, causing the largest die-off of the foundational species in the area in 15 years, The Guardian has reported.

The carnage was first observed by Bodega Marine Reserve research coordinator Jackie Sones, who said she had never seen such a massive death toll in her years of field work along the Northern California coast.

Image: Jackie Sones

"In the past we've seen patches die, but in this case it was everywhere," Sones told Bay Nature. "Every part of the mussel bed I touched, there were mussels that had died."

Marine scientists took the die-off as a clear sign that the climate crisis is already impacting marine life.

"These events are definitely becoming more frequent, and more severe," University of British Columbia biologist Christopher Harley told The Guardian. "Mussels are one of the canaries in the coal mine for climate change, only this canary provides food and habitat for hundreds of other species."

Sones said she first observed the die-off while doing fieldwork in Bodega Bay. First, she noticed algae bleached white and red. Then, she saw the mussels.

"A large percentage of the mussels were open and gaping, some were empty and some still had tissue inside," Sones wrote in a June 18 blog post.

Other scientists reported similar die-offs to what Sones observed along around 140 miles of coastline in Bodega Head, a headland of Northern California's bay, according to The Guardian.

"Mussels are known as a foundation species. The equivalent are the trees in a forest — they provide shelter and habitat for a lot of animals, so when you impact that core habitat it ripples throughout the rest of the system," Sones told The Guardian. "I would expect that this actually impacted the entire region, it's just that you would have to have people out there to document it to know."

Sones believed the die-off happened during a series of mid-day low tides during an early June heat wave. On June 11, temperatures at Bodega Bay reached 75 degrees Fahrenheit. That might not sound like much of a heat wave by human standards, but Northeastern University marine ecologist Brian Helmuth told Bay Nature that a marine creature glued to a rock could see its tissue temperature rise to 105 degrees in those conditions.

The problem wasn't only the heat, but when it happened, as Bay Nature explained:

Typically the hottest days at the California coast occur later in the summer and into the fall, when, fortunately for tidepool animals, the lowest low tides happen early in the morning or late at night. But in the spring and early summer the low tides shift to the late morning or early afternoon. The more unusual early-season heat waves there are, the greater the chance they line up with those mid-day low tides, the harder it gets for mussels. Future die-offs could rewrite the ecology of California's rocky shoreline, where mussels are a foundation species that hundreds of other animals depend on.

The incident is an example of how sensitive ecosystems are to climate change.

"Small changes in temperature can produce big effects," Harley wrote in a paper reported by Bay Nature about a similar die-off in Bodega Bay he studied in 2004.

Helmuth gave the example of lobsters, which have already left New York because ocean temperatures are too warm, but are now thriving in Maine, where temperatures are slightly above average. If those temperatures increase much more, however, they will likely drive the lobsters farther north to Canada.

"The really insidious thing," Helmuth told Bay Nature, "is there's an optimal temperature where organisms do the best, and it's really close to the temperature where they crash."