Macroalgal canopies as thermal refuges in coastal systems of the NW Iberian PeninsulaClimate Change Refugia

Gestoso Ignacio^1,2,3, Cacabelos Eva^4,2, Olabarria Celia⁵, Gestal Camino⁶, Bernal Alejandro^7,2

¹University of Cádiz, Spain - ²MARE – Marine and Environmental Sciences Centre, Portugal - ³Smithsonian Environmental Research Center, USA - ⁴Instituto Español de Oceanografía (IEO) - ⁵Universidade de Vigo, Spain - ⁶Instituto de Investigaciones Marinas IIM-CSIC, Spain - ⁷Instituto de investigacion y Formacion Agraria y Pesquera, Spain

In the NE Atlantic, seawater temperatures have steadily risen in recent decades, with more intense and frequent heatwaves. Ocean warming and extreme heat events linked to climate change pose significant threats to coastal ecosystems. Several processes influence the vulnerability of these ecosystems, but the intrinsic resilience of natural communities is a key factor in shaping their responses. The role of biogenic habitats, particularly canopy-forming macroalgae forests, in buffering warming effects remains underexplored, despite evidence from terrestrial forests where canopy cover has been shown to reduce thermal stress. This microclimatic regulation benefits forest-dwelling organisms by mitigating the impacts of global warming. This study aimed to explore the role of macroalgal forests in coastal ecosystems, focusing on the genus Cystoseira sensu lato, which was selected from different coastal areas of the Galician Rías Baixas (NW Iberian Peninsula). Additionally, a mesocosm experiment exposed synthetic macroalgal forests to a simulated marine heatwave (MHW) during low tide to assess their buffering capacity. Results revealed a strong correlation between the structural complexity of macroalgal forests (i.e. thallus density and size, algal biomass, patch-size), and the composition of associated macroinvertebrate communities. Moreover, the mesocosm experiment confirmed the heat-stress buffering capacity of macroalgal forests, though this effect was highly dependent on forest integrity, i.e. patchiness. These findings contribute to our understanding of the ecological role of submerged macroalgal forests and their potential as refuges in the face of warming, highlighting their importance in maintaining biodiversity and ecosystem function in benthic systems.