Stochastic and deterministic processes influencing mountain mire diatom communities across spatial scales

Understanding how deterministic and stochastic processes influence the shape of microorganism community assembly across different spatial scales is essential for disentangling biodiversity patterns. Mires are nutrient-poor and heterogeneous wetlands that form isolated habitats supporting highly diverse diatom assemblages, particularly in mountainous areas, allowing the evaluation of dispersal and species sorting forces. Diatoms are known as ubiquitous microalgae that disperse passively and whose community can endure changing environmental conditions. While their diversity is well documented, the mechanisms driving diatom community assembly across spatial scales remain unclear. Here, we quantified the influence of ecological drift, environmental selection, and dispersal following a null-modelling procedure based on phylogenetic (βNTI) and compositional turnover (RCBray) from diatom communities in 19 mountain mires across four Iberian National parks using a hierarchical spatial design; local (within mire): ~2 km, landscape (National park): ~15–40 km, and regional (Iberian Peninsula): ~700 km. Our results reveal a spatial continuum in community organization, shifting from a predominance of homogenizing dispersal at local scales (explaining up to 44.8 ± 28.5% of community variation) to ecological drift (36.2 ± 22.0%) and heterogeneous selection (environmental filtering) (14.7 ± 12.7%) at landscape and regional scales. These findings shed light on the scale-dependent interplay of community assembly processes in shaping diatom biodiversity in aquatic ecosystems, highlighting how the balance between stochastic and deterministic processes shifts across multiple spatial scales.