Thermal refugia reinforce macroalgal resilience against climate change in the southeastern Bay of Biscay.

Rising global temperatures present unprecedented challenges to marine ecosystems, demanding a profound understanding of their ecological dynamics for effective conservation strategies. Over a comprehensive macroalgal assessment spanning three decades, we investigated the spatiotemporal evolution of shallow-water benthic communities in the southern Bay of Biscay, uncovering climate-resilient areas amidst the ongoing phase shift in the region. Our investigation identified seven locations serving as potential climate refugia, where cold-affinity, canopy-forming macroalgal species persisted and community structure was similar to that observed in 1991. We unveiled a clear association between the emergence of these refugia, sea surface temperature (SST), and the Community Temperature Index, positioning SST as a significant driver of the observed phase shift in the region. Warming processes, defined as tropicalization (increase of warm-affinity species) and deborealization (decrease of cold-affinity species), were prominent outside refugia. In contrast, cooling processes, defined as borealization (increase of cold-affinity species) and detropicalization (decrease of warm-affinity species), prevailed inside refugia. Refugia exhibited approximately 35% lower warming processes compared to non-refuge areas. This resulted in a dominance of warm-affinity species outside refugia, contrasting with the stability observed within refugia. The persistence of canopy-forming species in refuge areas significantly contributed to maintaining ecosystem diversity and stability. These findings underscored the pivotal role of climate refugia in mitigating climate-driven impacts. Prioritizing the protection and restoration of these refugia can foster resilience and ensure the preservation of biodiversity for future generations. Our study illustrates the importance of refining our understanding of how marine ecosystems respond to climate change, offering actionable insights essential for informed conservation strategies and sustainable environmental management.

The thermal journey of macroalgae: Four decades of temperature-induced changes in the southeastern Bay of Biscay.

Global warming is triggering significant shifts in temperate macroalgal communities worldwide, favoring small, warm-affinity species over large canopy-forming, cold-affinity species. The Cantabrian Sea, a region acutely impacted by climate change, is also witnessing this shift. This study delved into the impacts of increasing sea surface temperature on the subtidal macroalgal communities in the southeastern Bay of Biscay over the last four decades, by using data from the years 1982, 2007, 2014, and 2020. We found that temperature has shaped the community structure, with warm-affinity species steadily displacing their cold-affinity counterparts. Notably, new communities exhibited a profusion of smaller algal species, explaining the observed increased biodiversity within the area. In the last period investigated (2014-2020), we observed a partial recovery of the communities, coinciding with cooler sea surface temperatures. Shallow algal communities were more reactive to temperature variations than deeper communities, possibly associated with higher exposure to increased temperatures. Our study offered insights into the intricate relationship between the changes in ocean temperature and algal species in the southeastern Bay of Biscay, shedding light on the ongoing ecological shifts in this region.

Split-plot marine experiment to assess ecophysiological responses of Gelidium corneum assemblages.

Canopy-forming macroalgae are facing large declines due to climate change worldwide. The foundation species Gelidium corneum (Hudson) J.V. Lamouroux has shown a long-term decline in the Southeastern Bay of Biscay. We conducted an in situ experiment to investigate the combined effect of solar radiation and nutrient availability on the photosynthetic acclimation and growth of this macrophyte, and on the species richness and diversity of the assemblages that it forms. Photochemical stress in G. corneum was found to be greater at the end of the study, probably as a result of a prolonged exposure to high irradiance (PAR and UVR) and due to high temperatures during summer. We found an acclimation of G. corneum specimens to summer light and thermal conditions through dynamic/reversible photoinhibition and a decrease in photosynthetic efficiency. Nutrients may also have had a positive effect in dealing with the negative effects of these stressors. Under ongoing global climate change and projections for the future, further research will be needed to better understand the effects not only on canopy forming species but also on the whole community and thus on the functioning of the ecosystem.

Short-term response of macroalgal communities to ocean warming in the Southern Bay of Biscay.

Climate change is causing significant shifts in biological communities worldwide, including the degradation of marine communities. Previous research has predicted that southern Bay of Biscay canopy-forming subtidal macroalgal communities will shift into turf-forming Mediterranean-like communities by the end of the century. These predictions were based on a community-environment relationship model that used macroalgal abundance data and IPCC environmental projections. We have tested the short-term accuracy of that model by resampling the same communities and locations four years later and found the short-term predictions to be consistent with the observed communities. Changes in sea surface temperature were positively correlated with changes in the Community Temperature Index, suggesting that macroalgal communities had responded quickly to global warming. The changes over four years were significant, but canopy-forming macroalgae were more resilient in local sites with favourable temperature conditions. Our study demonstrated that updating predictive models with new data has the potential to yield reliable predictions and inform effective conservation strategies.

A spatially-modelled snapshot of future marine macroalgal assemblages in southern Europe: Towards a broader Mediterranean region?

The effect of climate change on species distribution has been the focus of much recent research, but the community-level approach remains poorly studied. Our investigation applies a present assemblage-environment relationship model for the first time to the predict changes in subtidal macroalgal assemblages in the northern Iberian Peninsula under the RCP 4.5 and RCP 8.5 climate scenarios by 2100. Water temperature is the most relevant factor in shaping assemblage distribution, whilst nutrient availability plays a secondary role. The results partially support our hypothesis that there may well be a potential meridionalisation of northern Iberian assemblages in the future. Under the most pessimistic scenario, the model projects that the north-western assemblages will remain distinct from the rest, whereas the central and eastern assemblages of the north coast of the Iberian Peninsula will come to resemble those of the Mediterranean region more closely than those of the northwest coast. This research may help predict how the biodiversity of the coastal ecosystem will respond to new environmental conditions. This is essential information for developing proper management and conservation policies.