Unveiling thermal responses on macroalgae assemblages after a 10 years’ time span in a subtropical rocky shore from Southwestern Atlantic Ocean

Biotic thermal stress caused by sea surface temperature (SST) anomalies and marine heatwaves (MHW), driven by ongoing ocean-atmosphere exchanges and the climate change, can severely alter macroalgae community composition. Macroalgae provide vital ecosystem services and serve as key bioindicators. Understanding their dynamic supports impact mitigation in inhabited coastal areas and enhances socio-environmental resilience. We comparatively evaluated changes in richness and composition of seaweed assemblages after a 10-years’ timespan on a rocky shore in southern Brazil. Seasonal biological data from a 2014 baseline checklist were compared to a 2024 resurvey conducted along the same intertidal transects and analyzed alongside local SST and MHW intensity and frequency data retrieved from global databases for the preceding decades. Over this ten-years interval was observed a seasonal inversion on Rhodophyta and Phaeophyceae species richness, being higher during austral winter 2024, contrasting to the higher summer richness observed in 2014. Ulvophyceae was the exception, displaying lower overall richness and biomass-dominance of few opportunistic taxa across both seasons. This intriguing seasonal pattern inversion was strongly related to thermal stress, suggesting that assemblages are shifting toward thermally more suitable seasons, while demonstrating local resilience mechanisms and adaptive capacity. Given the scarcity of ecological studies on macroalgal communities in the SWAO, these biotic shifts enhance our understanding of thermal stressors. Furthermore, this study updates the regional algal diversity database and reports new records, providing a valuable baseline for future long-term monitoring efforts.