Over the past two years, researchers, shellfish managers, and indigenous tribes have collaborated to investigate the impacts of the unprecedented heat dome that engulfed the Pacific Northwest in the summer of 2021. This scorching heatwave broke records, with temperatures reaching a staggering 121 degrees Fahrenheit, causing severe damage to infrastructure and ecosystems. The aftermath of the heatwave was devastating, as it led to the deaths of 650 people in the United States and Canada, along with extensive mortality of shellfish and other marine organisms. Bleached red algae, cockles desperately trying to escape the heat, mussels gasping for relief, and tide pools transforming into hot baths were just a few of the distressing scenes witnessed along the shores.
The intertidal ecosystems, which exist between the high and low tide of the ocean on rocky and sandy shores, were particularly impacted by this extreme weather event. Organisms in these habitats lead a unique life, adapting to both submerged and exposed conditions as the tides fluctuate. Worms, snails, clams, oysters, barnacles, mussels, sea stars, algae, and kelp all thrive in this zone, providing crucial ecological services such as food, water filtration, and oxygen production. Researchers have observed that the locations where these creatures resided played a major role in determining their survival during the heat dome. Living closer to the coast offered some protection compared to inland areas, but even the specific placement of an organism on one side of a rock versus the other could mean the difference between life and death. Although these ecosystems are in the process of recovery, the path to full restoration remains uncertain.
The geographical variations along the coastline were key factors in the diverse impacts of the 2021 heat wave. Tides, similar to long-wavelength waves, result in water sloshing back and forth between the coast and Puget Sound, likened to the ends of a bathtub. In the Pacific Northwest summers, low tides initially occur on the Olympic Peninsula in the cooler morning hours, sparing the biodiversity-rich coastline of Olympic National Park from the worst effects. Low tides then progress inland through the Strait of Juan de Fuca and Puget Sound, reaching the Salish Sea in the scorching mid-afternoon heat. Consequently, higher mortality rates were observed in these areas. An intertidal and nearshore ecologist at the Washington Department of Fish and Wildlife, Wendel Raymond, highlighted the fundamental role of oceanography in shaping the experiences of different organisms during the heatwave.
To exacerbate the situation, the heat dome coincided with exceptionally low tides and extended daylight hours, exposing marine organisms to prolonged periods of hot air. Mussels and clams suffered significant losses, particularly in inland and southern regions. In Puget Sound’s sandy beaches, clams residing deeper in the sand fared better than those closer to the surface. On the rocky Canadian coastline, the most affected organisms, such as seaweeds, mussels, and barnacles, were unable to escape the heat by seeking refuge in deeper waters or hiding under rocks, unlike their mobile counterparts. Christopher Harley, a marine biologist at the University of British Columbia, observed that areas with seaweed cover experienced lower mortality rates. The shade provided by barnacles and the water trapped in their crevices also contributed to cooling the surrounding habitat. Barnacles on north-facing surfaces demonstrated better survival rates and have since managed to repopulate the area. Seaweeds took longer to recover, while mussels, known for their slower recovery rates, have yet to fully rebound.
In contrast, Pacific oysters have thrived in areas where mussels have not yet reestablished themselves. These popular seafood delicacies, originally introduced from Japan, exhibit faster growth and reproduction. Oysters’ thermally tolerant white or light-gray shells give them an advantage over mussels with their shiny black shells. The different shellfish species create diverse habitats for other organisms, but their presence alone does not guarantee their suitability for all species. Harley emphasized that worms and hermit crabs, for instance, might prefer living among mussels rather than oysters. The consequences of an increase in one shellfish species over the other on biodiversity in these regions are still unknown; however, it is certain that this ecosystem has undergone significant transformation.
Raymond anticipates that the shoreline around Vancouver, British Columbia, will largely resemble its pre-heat wave state within another year, disregarding the possibility of another heat wave occurring before recovery is complete. This summer has already witnessed scorching temperatures and the setting of high-temperature records during an August heat wave.
Amelia Hesketh, a researcher, found through her Ph.D. work that organisms struggle to reestablish themselves in high temperatures, even when vacancies in the ecosystem are created due to the death of other organisms. This challenge may result in reduced biodiversity in the long run. Hesketh explained that a heat wave should be perceived as analogous to a forest fire, where some nutrients can still support growth and enable certain organisms to survive in the burned forest. However, it ultimately becomes a less suitable habitat for many creatures. The impact of the 2021 heat wave on oyster and clam reproduction remains unknown, and it will take another year or two of research to unravel the effects. Did some animals die before being able to reproduce, potentially leading to a smaller population? If young offspring had already been produced, were they more susceptible to heat-related mortality compared to the adults? The survival and mortality of these organisms will significantly influence the genetic makeup of future generations.
Researchers have yet to determine whether repeated heat waves will lead to enhanced resilience or increased vulnerability among intertidal organisms. Initial findings from Sandra Emry, a postdoctoral researcher at the University of British Columbia, suggest the latter is more likely. Christopher Harley aptly noted that it is akin to experiencing a sunburn on top of another sunburn. Emry fortuitously conducted research involving the application of space heaters along the shoreline to simulate added heat stress right before the 2021 heat wave. Consequently, she discovered that macroalgae previously exposed to heat stress fared worse during the heat wave. This insight into the compounding impacts of successive heat waves is novel and highlights the significance of understanding this phenomenon.
The Swinomish tribe on Fidalgo Island is taking proactive measures to protect the intertidal zone from future heat waves through the implementation of clam gardens, an ancient practice that helps maintain cooler conditions for clams. One particular beach on the tribe’s reservation, known for subsistence clamming, is highly regulated, prohibiting the harvesting of cockles to facilitate species recovery. Joseph Williams (Squi-qui), a fisherman and former vice-chair of the tribe, emphasized the tribe’s historical role in preserving these beds for thousands of years. The tribe’s ancestors signed a treaty exchanging much of their homeland for the right to continue hunting and fishing in the region.
The consequences of the heat dome extend beyond ecological impacts, as intertidal creatures also contribute to the livelihoods of many individuals in the area. Commercial shellfish growers reported additional mortality of clams and oysters following the 2021 heat wave. These losses are not solely about sustenance but also hold cultural and spiritual significance. Williams emphasized that their traditional foods nourish not only the physical body but also the soul. The far-reaching consequences of the heat dome in a warming world are evident in the changing functionality of intertidal habitats, which, in turn, could influence the response of entire ecosystems to future heat waves in unpredictable ways. While a single extreme event can have devastating effects, the cumulative consequences of multiple events are still largely unknown but undoubtedly significant.
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