Paradoxical failure of Laminaria ochroleuca (Laminariales, Phaeophyceae) to consolidate a kelp forest inside a Marine National Park

ABSTRACT Kelp forests, one of the world’s most productive ecosystems, have been in decline in many regions in recent years. Climate change, through steady sea warming or marine heatwaves, has led to the disappearance of entire populations, although kelp forest decline may also depend on non-climatic stressors such as grazing. Since the impacts of climate change are projected to continue to intensify for decades, marine protected areas (MPAs) have been suggested as a cost-effective strategy to boost the resilience of marine ecosystems. However, implementing an MPA does not guarantee that all components of the local community will benefit from it. In NW Spain, several stakeholders reported the decline of Laminaria ochroleuca inside the Islas Atlánticas Marine National Park in recent years, but the extent and possible drivers of this decline have not been investigated. Using a combination of quadrat-scale (abundance, biomass) and transect-scale (cover) seasonal surveys over one year, we found striking differences between L. ochroleuca reefs inside and outside the MPA. Populations outside the MPA were the typical canopy forests expected for a perennial kelp, stable year-round and composed mostly of adults. Inside the MPA, however, grazing prevented the canopy phase from being reached. Instead, only mid-sized young plants were detected in autumn, but most had disappeared by winter, and those that remained had been degraded by herbivores to mere blade-less stipes with no growth meristem that eventually perish. Further research seems warranted to monitor whether this phenomenon spreads to nearby kelp beds outside the MPA. Meanwhile, restoring MPA kelp reefs will more likely require efforts to reduce herbivore activity (e.g. translocations, exclusion devices) than to bolster kelp populations. HIGHLIGHTS • Kelp canopies recurrently failed inside a MPA in recent years.• Neither propagule supply nor environmental conditions explain the failure.• Grazers keep a canopy-free state by eradicating recently recruited kelps.