Kelp forests collapse reduces understorey seaweed β-diversity

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

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.

Changes in maerl-associated macroalgal community dynamics as evidence of anthropogenic pressure

BACKGROUND AND AIMS

Maerl-associated communities have received considerable attention due to their uniqueness, biodiversity and functional importance. Although the impacts of human activities are well documented for maerl-associated macrofauna, the spatio-temporal variations of macroalgae have comparatively been neglected, and the drivers that influence their dynamics are poorly known. We investigate the links between maerl-associated macroalgal communities, anthropogenic pressures and environmental conditions, and hypothesize that sites under human pressure would exhibit different dynamics when compared to reference sites.

METHODS

To better understand community variation through space and time, four subtidal maerl beds under different pressures were consistently monitored over one year in the bay of Brest, Brittany, France. Both macroalgae community monitoring and environmental data were acquired through field sampling and available models.

KEY RESULTS

Higher macroalgal biomass was observed within eutrophic sites, especially in summer (more than ten times higher than in the Unimpacted site), caused by free-living forms of opportunistic red macroalgae. The Dredged site also exhibited distinct macroalgal communities during summer from the Unimpacted site. Nutrient concentrations and seasonality proved to be key factors affecting the macroalgal community composition, although dredging and its effects on granulometry also had a strong influence. Over the long term, fewer than half of the species identified during historical surveys were found, indicating major temporal changes.

CONCLUSIONS

Human pressures have strong impacts on maerl-associated macroalgal communities. Nutrient concentrations and dredging pressure appear as the main anthropogenic factors shaping maerl-associated macroalgal communities. Additionally, our results suggest historical changes in maerl-associated macroalgal communities over 25 years in response to changes in local human pressure management. This study suggests that maerl-associated macroalgal communities could be used as indicators of anthropogenically driven changes in this habitat.