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.

KEYWORDS:

• Foundation seaweed
• grazing pressure
• kelp
• Marine Protected Area
• NW Spain
• rocky shore

Acknowledgements

Special thanks to Manuel E. García, Isabella Provera, Pilar Díaz Tapia, Viviana Peña, Iñaki Ferreiro, Iker Irazabal, Alexandro Chamorro, Ignacio Bárbara and Buceo Islas Cíes for their support in the field and laboratory work. We also thank José Antonio Fernández Bouzas, superintendent of Islas Atlánticas de Galicia National Park and park rangers. We deeply thank INTECMAR for providing in situ temperature, salinity and nutrients data. Two anonymous reviewers and the associate editor provided insightful comments that improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary information

The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at https://doi.org/10.1080/09670262.2022.2065365

Supplementary fig. S1. Laminaria ochroleuca. Average seasonal estimates (+SE) outside and inside the MPA for the (a) abundance (as number of individuals per 0.25 m2), (b) biomass (as kilograms WW per 0.25 m–2), and (c) cover (as percentage of occupation in 25 m transects). Ecologically dead are plants that lost their growth meristem due to herbivorism. Juveniles (quadrat samplings only) are plants < 30 cm.

Supplementary fig. S2. Laminaria ochroleuca. Proportion (average + SE) of ecologically dead plants (i.e. plants that had lost their growth meristem to grazing) in the only two seasonal of the year when mid-sized young adult plants were recorded on kelp reefs within the MPA. Estimates for kelp forest outside the MPA also shown for comparison.

Supplementary table S1. Laminaria ochroleuca. Pairwise test for (a) factor “Season” within each level of “Protection” and for (b) factor “Protection” within each level of “Season” for the abundance and biomass of kelps (adults + juveniles). Pseudo-t = statistic analogous to Student’s t-test. P (perm) = p estimated after 9999 permutations. *p < 0.05, **p < 0.01, ***p < 0.001.

Author contributions

S. Barrientos: original concept, conceptualization, methodology, data collection, formal analysis, drafting and editing manuscript. R. Barreiro: original concept, conceptualization, methodology, formal analysis, editing manuscript, funding acquisition, supervision, project administration. C. Piñeiro-Corbeira: original concept, conceptualization, methodology, data collection, formal analysis, editing manuscript, supervision, project administration.

Additional information

Funding

This research was funded by ‘Fundación Biodiversidad y el Ministerio de Transición Ecológica y del Reto Demográfico’ (grant PRCV00797). BioCost Research group acknowledges funding from Xunta de Galicia within the program ‘Axudas para a consolidación e estructuración de unidades de investigación competitivas do SUG’ (grants ED431D 2017/2020, ED431B 2018/49). SB received funding from the Xunta de Galicia and the European Union (European Social Found – ESF) (grant ED481A-2018/101), CPC received funding from the Xunta de Galicia ‘Axudas de apoio á etapa de formación posdoutoral’ (grant ED481B-2021/095).