Biotic interactions in a changing world – how canopy-forming species modulate biodiversity and temperature from micro to latitudinal scalesClimate Change Refugia

Cátia Monteiro ^1,2, Gabriela Borer¹, Rita da Silva¹, Rocío Nieto-Vilela¹, Trine Bekkby³, Thomas Burel⁴, Anaëlle Bizien⁴, Øystein Varpe⁵, Jonas Collén⁶, Glenn Philippe⁶, Markus Mollis⁷, Juan Vicente de Miguel⁷, Marta Prieto⁷, Ulrike Dietrich⁷, Fernando P. Lima¹

¹CIBIO/BIOPOLIS, Portugal - ²University of Porto, Portugal - ³Norwegian Institute for Water Research (NIVA), Norway - ⁴ Univ Brest, CNRS, IRD, Ifremer, LEMAR, IUEM, France - ⁵University of Bergen, Norway - ⁶Sorbonne Université, CNRS, Station Biologique de Roscoff, France - ⁷The Arctic University of Norway, Norway

Canopy-forming seaweeds are often referred to foundation species as they promote the development and persistence of ecological communities by modifying the structure and physical conditions of the habitat. However, this assumption has rarely been thoroughly tested and the ecological functions they provide are hardly ever quantified. This is particularly concerning considering that a multitude of anthropogenic and natural stressors currently threatens these canopy-forming species. The direct effects of warming on the physiological performance of these marine forests have been described and linked to recent shifts in distribution range. However, how biological interactions act as local modulators of the effects of climate change and, conversely, how climate change is modifying those same biological interactions has been largely overlooked. Thus, understanding the magnitude and direction of these biological interactions is key to assessing and predicting foundation species’ resilience to climate change and the impact on ecosystem functioning and food webs of these interactions. To investigate the ecological functions of canopy-forming species we experimentally manipulated canopy cover and measured thermal buffer and habitat provision offered by the brown macroalgae Ascophyllum nodosum. To measure how the ecological functions provided by A. nodosum vary along its latitudinal range, we set up canopy and canopy removal experimental plots in a standardized way in six regions along the NE Atlantic, from Norway to Portugal. To account for small-scale thermal heterogeneity in the intertidal zone, we selected “hot” and “cold” microhabitats at the top and mid-intertidal levels. Preliminary data indicates that the magnitude of thermal buffer provided by A. nodosum canopies often surpasses thermal gradients driven by latitude and exposure to solar radiation. Additionally, these canopies act as local refuges of biodiversity, highlighting their key role in intertidal ecosystems.