Kylie Mohr writes:

For two years now, scientists, shellfish managers, and tribes have been working to understand how the heat dome that settled over the Pacific Northwest in the summer of 2021 affected the places where the ocean and land meet. That heat wave was like nothing in memory. Temperatures soaring as high as 121 degrees Fahrenheit buckled roads, melted power cables, and scorched forests. By the time the heat subsided, 650 people had died in the U.S. and Canada, and dead and dying shellfish and other marine critters littered beaches, cooking in their shells. Red algae were bleached white. Cockles tried to escape the heat by digging out of the sand, only to be greeted by more heat from the sun. Mussels gaped in an attempt to cool off. Tide pools became tubs of hot water. An estimated 1 billion marine animals perished in Canada alone.

These creatures all inhabited the intertidal ecosystems that exist between the ocean’s high and low tide on both rocky and sandy shores. As the day and the tides turn, organisms there lead life above and underwater. Worms, snails, clams, oysters, barnacles, mussels, sea stars, algae, and kelp all thrive here, providing food, filtering water, and producing oxygen. The people studying these zones have seen how, when the heat dome settled in over these creatures, the places they lived helped determine their fate. Living inland was more dangerous than living closer to the coast, but even living on one side of a rock or another could make the difference between life and death. And although these ecosystems are on the path to recovery, they’re changed—and recovery may be a temporary state.

The wide variety of impacts from the 2021 heat wave had almost everything to do with geography. Tides are like waves with very long wavelengths; experts liken the coast and Puget Sound to two ends of a bathtub, with water sloshing back and forth. During the summer in the Pacific Northwest, low tides hit the Olympic Peninsula first, in the morning when temperatures are cooler. That largely spared the Olympic National Park coastline, a biodiversity hot spot for marine invertebrates and seaweeds. Then, low tides move inland through the Strait of Juan de Fuca and down into Puget Sound; in the summer, they reach the Salish Sea in midafternoon, during the heat of the day. As a result, mortality was greater there. A clam in more-western Neah Bay “had a fundamentally different experience than a clam in Olympia because of the timing of the tide,” Wendel Raymond, an intertidal and nearshore ecologist at the Washington Department of Fish and Wildlife, told me. “That’s just how the oceanography of this place works.”

To make matters worse, the heat dome corresponded with exceptionally low tides and some of the longest days of the year, exposing more organisms to the hot air for longer. Mussels and clams were hit hard inland and farther south. On sandy beaches in Puget Sound, clams deeper in the sand generally fared better than their counterparts closer to the surface. On the rocky Canadian coastline, the creatures that suffered the most—seaweeds, mussels, and barnacles—all had one thing common: “They can’t just pick up and crawl away, swim to deeper water, or hide under a rock,” Christopher Harley, a marine biologist at the University of British Columbia, told me. [Continue reading…]