Variation in Environmental Conditions and Biodiversity Between Contrasting Intertidal Habitats Across Multiple Spatial Scales on European Rocky Shores

Aim: Foundation species sustain biodiversity by providing habitat, resources, and protection from environmental stress. However, their influence on environmental severity and variability and their role in shaping diversity across spatial scales remain poorly understood. We conducted a large-scale study to assess how environmental conditions and community diversity vary between areas with and without canopy-forming macroalgae across local to regional spatial scales. Location: Latitudinal range from 28°34′N to 54°11′N, encompassing both the Northeast Atlantic and Western Mediterranean regions. Time Period: 2019–2020. Major Taxa Studied: Marine macroalgae and benthic invertebrates. Methods: Using a standardised nested sampling design across 13 sites in two marine regions (the Northeast Atlantic and Western Mediterranean) we compared the intensity and variability of temperature and light intensity, as well as community α- and β-diversity, between intertidal rocky areas with and without canopy-forming macroalgae. Data were analysed by means of linear mixed-effects models. Results: There were regional differences in variations of environmental conditions between habitats: NE Atlantic macroalgal canopies were characterised by lower environmental extremes and variability—especially in summer—than open rock areas, whereas differences between habitats were small in the W Mediterranean. Likewise, differences in α- and β-diversity between habitats were region-specific and scale-dependent: in the NE Atlantic, α-diversity was higher in the presence of canopies than in open rock areas from local to regional scales, while β-diversity decreased in canopy stands with increasing spatial scale. In the W Mediterranean, α-diversity was higher under canopies than on open rock at the regional scale, whereas β-diversity was higher under canopies at small to intermediate scales. Main Conclusions: Our results support the view that key environmental conditions and biodiversity on intertidal rocky shores differ between areas characterised by the presence or absence of canopy-forming macroalgae acting as foundation species, across a hierarchy of spatial scales.