Extreme heatwave drives topography-dependent patterns of mortality in a bed-forming intertidal barnacle, with implications for associated community structure

Heatwave frequency and intensity will increase as climate change progresses. Intertidal sessile invertebrates, which often form thermally benign microhabitats for associated species, are vulnerable to thermal stress because they have minimal ability to behaviourally thermoregulate. Understanding what factors influence the mortality of biogenic species and how heatwaves might impact their ability to provide habitat is critical. Here, we characterize the community associated with the thatched barnacle, Semibalanus cariosus (Pallass 1788), in British Columbia, Canada and attempt to determine what environmental factors — air temperature, time of low tide, shoreline orientation, algal cover, and the angle of solar incidence — explain variations in S. cariosus mortality induced by an unprecedented regional heatwave, the 2021 Pacific Northwest Heat Dome. Further, we used a manipulative shading experiment deployed prior to the heat dome to examine the effect of thermal stress on barnacle survival and recruitment and the barnacle-associated community. We identified 50 taxa inhabiting S. cariosus beds, with variations in species diversity and community composition between sites. Site-scale variables and algal canopy cover did not predict S. cariosus mortality, but patch-scale variation in substratum orientation did, with more direct solar irradiance corresponding with higher barnacle mortality. The shading experiment demonstrated that S. cariosus survival, barnacle recruitment, and invertebrate community diversity were higher under shades, where substratum temperatures were lower. Associated community composition also differed between shaded and non-shaded plots, suggesting S. cariosus was not able to fully buffer acute thermal stress for its associated community . While habitat provisioning by intertidal foundation species is an important source of biodiversity, these species alone may not be enough to prevent substantial community shifts following extreme heatwaves. As heatwaves become more frequent and severe, they may further reduce diversity via the loss of biogenic habitat, and spatial variation in these impacts may be substantial.