Temporal shifts in kelp forest structure and distribution largely reflect recent ocean warming trends

Ocean warming is driving the redistribution of species at a global scale. Biogeographic transition zones are hotspots of species range shifts, as both warm‐ and cold‐adapted species are found toward contrasting range edges. While anecdotal evidence suggests some distributional shifts have occurred in the northeast Atlantic, the empirical evidence base is relatively limited. As habitat‐forming foundation organisms, kelps strongly influence the structure and functioning of associated communities and underpin ecosystem processes. It is therefore important to document changes in abundance and distribution patterns of kelp species and to understand climate‐driven shifts in kelp habitat structure. During the summer between 2021 and 2023, a total of 34 historical kelp sites were resurveyed, which had been previously surveyed between 1941 and 2016 (i.e. 82 to 5 years prior to resurveys) in intertidal and shallow subtidal reef habitats in the southwest of England (UK), a region within a wider biogeographic transition zone in the northeast Atlantic. The objective of this study was to collect spatially extensive, robust field data on contemporary kelp habitats to test the hypothesis that recent ocean warming trends have led to an increase in warm‐adapted species and, conversely, a decline in cold‐adapted species. Surveys indicated that the cold‐temperate species Laminaria digitata and the warm‐temperate Saccorhiza polyschides are currently the most widespread and abundant habitat‐forming species in the study region. By comparing historical and contemporary survey results, the a priori expectation that cold‐adapted species have declined was strongly supported for Saccharina latissima and partly for Alaria esculenta . Conversely, warm‐adapted Lusitanian species such as S. polyschides and Laminaria ochroleuca have proliferated. The invasive non‐native species Undaria pinnatifida has also increased, but not significantly, and remains spatially restricted across the region. Further, we observed an increase in the community temperature index (CTI) of kelp forests over time, which coincided with shifts in average sea temperatures in the region. Continued ocean warming, combined with other environmental changes, will likely lead to further shifts in the distribution and abundance patterns of habitat‐forming kelp species, with implications for habitat structure and local biodiversity. As ecological functions and traits differ between kelp species, particularly between the ocean warming ‘winners' and ‘losers', wider changes in ecological resilience and ecosystem service provision can be expected.