Twenty-five years on: Widespread kelp forest decline revealed in a potential climatic refugium.

Abstract

Kelp forests are key temperate ecosystems that experience the combined effects of global and local stressors throughout their distribution range. Niche modelling projections identified NW Spain, a region influenced by an intense upwelling system, as one such potential refugium. However, the recent discovery that fish overgrazing has eradicated kelp forests from certain reefs calls into question the validity of these projections. To determine the actual persistence of kelp forests (Laminaria ochroleuca and Laminaria hyperborea) on a regional scale, we resurveyed 50 sites in 2023 where kelp forests had been recorded up to 25 years earlier. Kelp had either disappeared or been reduced to a few scattered individuals at two-thirds (58%) of the sites. Furthermore, where kelp forests persist, L. ochroleuca is now the dominant canopy-forming kelp, while L. hyperborea has only been recorded at two sites. Kelp forest persistence was negatively correlated with summer sea surface temperature and wave exposure. Altogether, our results indicate that kelp forest decline is widespread in NW Spain, challenging the view of this region as a climate refugium and underscoring the difficulty of accurately predicting the trajectory of such complex and fragile ecosystems. Furthermore, in line with the recommendations of the OSPAR Convention, this study lays the foundation for a long-term monitoring network along a region where kelp forests are undergoing rapid change.