Functional and taxonomic diversity of intertidal macroalgae communities from a climate refugia hotspot

Assessing the consequences of climate change in coastal ecosystems is challenging, largely due to their natural variability and the context-dependent responses of organisms. Tropicalization events are reshaping communities, with declines in species sensitive to local climate variability and increases in climate-tolerant and invasive species. Understanding taxonomic and functional biodiversity patterns over space and time is critical to evaluate whether certain regions may act as climate refugia. We investigate the spatial and temporal patterns of intertidal macroalgae community diversity, taxonomic and functional (α-diversity and β-diversity), along the northern Portuguese coast. Data was collected over an 18-year interval from five distinct locations (spanning from 41°42'41.4"N 8°51'43.4"W to 41°03'06.8"N 8°39'28.6"W). The objective of this work was to characterize the spatial and temporal patterns of intertidal macroalgae communities, along with their inherent changes. Our key findings include (1) coastal sea surface temperatures were approximately 2°C cooler than offshore waters, suggesting the area may function as a climate refugia; (2) both taxonomic and functional space contracted over time, indicating losses of species and functions; (3) for both space and time, turnover (β-replacement) was the main driver of taxonomic changes, whereas nestedness (β-richness) primarily drove functional changes. These spatial and temporal shifts in community composition are likely to have significant functional impacts, such as reduced habitat availability and lower productivity rates, with important implications for ecosystem services like blue carbon storage and habitat provision. This knowledge is crucial for mitigating the effects of climate change and best implementing effective conservation management strategies.