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

Background/Question/Methods

Heatwave frequency and intensity will increase as climate change progresses, with implications for organism survival and performance and the structure and function of ecological communities. Intertidal sessile invertebrates, many of which 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, further, how heatwaves might impact their ability to provision habitat and thereby affect associated species, is critical. The Pacific Northwest Heat Dome (PNWHD) that occurred in June 2021 drove local temperatures far above historical norms, and coincided with some of the year's lowest tides and longest days, representing a dire thermal challenge for testing questions about spatial patterns of mortality and the limits of facilitation. In this study, we characterize the infaunal community associated with the thatched barnacle, Semibalanus cariosus (Pallass 1788), in coastal British Columbia, Canada and examine patterns in S. cariosus mortality and changes in the composition of its associated community induced by the 2021 Pacific Northwest Heat Dome.

Results/Conclusions

We identified 50 taxa inhabiting S. cariosus beds, with variations in species diversity and community composition between sites. During mortality surveys, we found that neither site-scale shoreline orientation nor algal canopy cover predicted S. cariosus mortality. However, patch-scale variation in substratum orientation was a strong determinant of mortality, with more direct solar irradiance during low tide corresponding with higher mortality. A manipulative shading experiment deployed prior to the PNWHD demonstrated that where substratum temperature was lower, S. cariosus survival, barnacle recruitment, and invertebrate community diversity were higher. Infaunal community composition also differed between shaded and non-shaded plots, suggesting that the facilitative relationship between S. cariosus and associated infauna may change in response to acute thermal stress. These results suggest that while habitat provisioning by foundation species is an important source of biodiversity in the intertidal zone, foundation species alone are not enough to prevent substantial community shifts following extreme heatwaves. As heatwaves become more frequent and more severe, they may further reduce diversity via the loss of biogenic habitat, and spatial variation in these impacts may be substantial.