Mitigation of intertidal temperature extremes by algal canopies

In a rapidly warming world, species must either adapt to changing environmental conditions, shift their distributions through the movement and survival of individuals, or face local or global extinction. Within ecosystems, structural complexity of either environmental or biogenic origin may provide microclimate buffering, promoting persistence. In coastal environments, it has long been stated that macroalgae act as important ecosystem engineers that can moderate thermal extremes, however the magnitude of this buffering has rarely been quantified. Moreover, the extent to which thermal buffering varies among macroalgal species, and how it relates to thermal heterogeneity across scales remains poorly understood. Here, we measured thermal buffering provided by intertidal macroalgae using high-frequency, high-spatial-resolution in situ temperature logging. We compared microclimates beneath algal cover with those on experimentally cleared bare rock across two tidal levels, two solar exposures and two seasons along six shores within a biogeographical transition zone. Thermal buffering was highly heterogeneous across the study area, varying with species identity, algal cover, tidal level, and solar exposure. Together, these factors generated a complex thermal mosaic, with even small algal specimens providing measurable buffering. We found that latitude had little influence on mean daytime low-tide temperatures, whereas local-scale factors had pronounced effects. Lower tidal position, reduced solar exposure, and algal cover each moderated thermal conditions and, in combination, provided an average cooling of approximately 4 °C. These findings highlight the importance of fine-scale habitat heterogeneity in shaping thermal exposure and identify features that could be incorporated into local climate-adaptation strategies, supporting coastal conservation and restoration efforts.