When thermal shifts reshape function in intertidal seaweeds communities.

Climate-driven changes are reshaping marine communities worldwide, yet its functional consequences for intertidal macroalgae remain poorly resolved. We investigated temporal trends in community thermal affinity and functional trait composition of low intertidal macroalgae assemblages along the northern Portuguese coast over seven years spanning more than one decade (2006-2008, 2021-2024). Although mean Community Temperature Index (CTI) remained relatively stable, substantial community turnover occurred. This pattern was primarily driven by changes in the abundance of cold-affinity species, with deborealization and borealization contributing to observed shifts in thermal composition. Furthermore, we demonstrate pronounced temporal shifts in functional traits, including pigmentation, morphology, reproductive mode and structural complexity. Analyses of functional redundancy and originality showed that foundation species disproportionately shape community functional structure. More specifically, Laminaria ochroleuca exhibited high functional originality but low redundancy, indicating potential vulnerability to abundance declines. Community turnover, including the emergence of invasive species like Asparagopsis armata, further illustrates dynamic restructuring of functional space. Together, these results indicate that, despite apparent thermal stability, intertidal macroalgae communities are undergoing functional reorganization, with potential consequences for productivity, habitat complexity, and blue carbon capacity. Conserving functionally original and habitat-forming species is therefore critical for maintaining ecosystem resilience in the face of climate change.