Centennial persistence of kelp forests on the West Coast of Vancouver Island, Canada

Canopy-forming kelps, bull kelp ( Nereocystis luetkeana ) and giant kelp ( Macrocystis tenuifolia ), form dynamic underwater forests that underpin coastal biodiversity, fisheries, and human well-being. Yet, their persistence under accelerating ocean warming and intensifying marine heatwaves remains poorly understood, particularly at regional scales. We integrate nearly 170 years of evidence, including data from British Admiralty charts (1858–1956) and 10-m Sentinel-2 imagery (2020–2023) to map modern kelp distribution and centennial persistence across 5,600 km of the West Coast of Vancouver Island (WCVI), British Columbia, Canada, partitioned into four ecoregions. Kelp forests in cooler, high-energy ecoregions, containing 97.5% of modern canopy area, remained highly persistent (~88%) over the past century, suggesting the presence of climatic refugia. By contrast, the warmer and more sheltered ecoregion exhibited markedly lower persistence (52%), which was associated with elevated spring and summer sea surface temperatures. These results demonstrate the importance of century-scale baselines for distinguishing natural variability from climate-driven change and identify spatial refugia as priority areas for conservation and restoration within British Columbia’s and First Nations’ marine protection and climate adaptation strategies.