Kelp forests as natural barriers against invasive turf-forming algae

Kelps are key foundation species on temperate and Artic rocky shores, forming structurally complex habitats that are increasingly being lost and replaced by turf-forming algae. This study aimed to document the transition from Laminaria ochroleuca forests to the proliferation of the invasive turf-forming red algae Falkenbergia rufolanosa, the tetrasporophyte stadium of Asparagopsis armata, in NW Spain. Seasonal surveys (winter, spring and autumn) were conducted over a five-year period across eight rocky reefs to analyze the abundance of kelps and the cover of the invasive species. Our results indicate that the cover of the Falkenbergia stadium was significantly lower in healthy reefs than in degraded ones. Additionally, we observed a general decline in the number of adult Laminaria ochroleuca individuals across both healthy and degraded reef. The results of our study support the idea that healthy kelp forests could act as barriers to invasive turf species, while degraded forests may facilitate their spread. This resistance to invasion underscores the ecological importance of kelp forests and highlights the need for conservation and restoration strategies.