Quantifying herbivore-driven decline in kelp forests : implications for climate refuge resilienceResistance, Resilience and Phase Shifts

Sara Barrientos¹, Cristina Piñeiro-Corbeira¹, Nerea Alvite¹, Rodolfo Barreiro¹

¹Universidad de A Coruña, Spain

Climate change and local stressors are driving the global decline of kelp forests, key temperate marine ecosystems. NW Spain had been considered a potential climate refuge for kelps, as it is a region influenced by a strong upwelling system. Here, Laminaria ochroleuca and Laminaria hyperborea forests dominated much of the coast some years ago. However, the current conservation status of these kelp forests remained understudied until 2019, when a five-year seasonal monitoring program in the Ría de Vigo revealed that the characteristic perennial canopy of adult kelp had been replaced on some reefs by transient populations of juvenile kelp. These juveniles disappeared annually due to fish overgrazing, impeding perennial kelp forests regeneration.
To assess the extent of this phenomenon beyond the Ría de Vigo, a study was conducted in 2023 along almost the entire NW coast of Spain. Fifty sites with historical kelp forest presence were revisited, revealing that 58% had either completely lost their kelp forests or only retained sparse individuals. For a more detailed investigation, 26 out of these 50 sites were sampled for the present study during two periods (autumn/winter and spring/summer) in 2023-2024. A combination of quadrat-scale (abundance, biomass) and transect-scale (cover) surveys were employed to compare the conservation status. All individuals collected from each quadrat were also used to assess the importance of herbivory pressure on these reefs.
Although the causative agent of kelp decline is difficult to determine in sites where kelp has completely disappeared, this study provides evidence of expanding herbivory pressure along the coast of NW Spain. These findings challenge the notion of NW Spain as a climate refuge for kelps, and highlight the complex interplay between global and local factors in shaping coastal ecosystems. This research underscores the urgent need for continued monitoring and conservation efforts to protect the remaining kelp forests.