Kelp forests modulate fish community dynamics and responses to ocean warming

Abstract Kelp forests are foundation habitats that create structurally complex and biodiverse marine ecosystems, yet their extent and ecological integrity are increasingly threatened by rising ocean temperatures and the resulting tropicalisation of temperate coasts. Quantitative assessments of these changes remain very limited, and understanding them is particularly critical in climate transition zones, where ecological shifts can be rapid and pronounced. We conducted a 5‐year (2020–2024) survey for kelp–fish communities at four representative regions along ~600 km of the Iberian coast ( n = 450 transects), spanning a gradient of current and projected tropicalisation. The analysis focused on how kelp species identity and abundance, local sea surface temperature (SST) and reef‐fish assemblage characteristics influence fish population metrics and the community temperature index (CTI). In total, 53 fish species and 4 species of kelps were found. Perennial kelp composition closely followed the SST gradient, with the boreal kelp Laminaria hyperborea dominating in cooler sites (Coruña and Viana), whereas the Lusitanian kelp Laminaria ochroleuca prevailed in warmer sites (Vigo and Peniche). Coruña displayed the highest kelp diversity and abundance, while the southernmost Peniche exhibited the highest fish diversity. Kelp‐abundant regions exhibited lower fish CTIs than expected from local SST, suggesting a potential canopy buffering effect against tropicalisation. In contrast, Peniche, with lower kelp abundance, showed minimal CTI‐SST offset, higher proportions of warm‐affinity taxa and the greatest fish diversity, indicating a more advanced tropicalisation stage. Synthesis . Our spatially resolved benchmark indicates that abundant kelp cover can delay tropicalisation by lowering the realised thermal affinity of reef‐fish communities, while increases in fish biodiversity may signal the early stages of this process. These results underscore the dual role of kelp forests as biodiversity reservoirs and climate buffers, and provide a transferable framework for understanding how foundation species mediate community responses to ocean warming across ecosystems. Safeguarding and restoring kelp forests should therefore be a priority for sustaining temperate‐reef resilience under climate change.