Summertime sadness? Not for diatoms

In the context of ongoing climate change, mountain freshwater ecosystems are increasingly exposed to altered hydroclimatic regimes, which may affect the structure and functioning of biological communities. However, the responses of diatom assemblages to seasonal variability in high-elevation temporary ponds remain poorly understood. Here, we investigated seasonal variation in water chemistry and the taxonomic and functional diversity of diatom communities in temporary ponds of a mountain plateau, comparing summer and autumn conditions. We assessed α and β diversity patterns and explored relationships between community structure, functional traits, and environmental drivers. Overall, no significant seasonal differences were detected in taxonomic (R2 = 0.039, p = 0.774) or functional (R2 = 0.104, p = 0.159) β diversity, and no significant differences were observed in most α diversity indices. Functional evenness was significantly higher in autumn than in summer (W = 69, p = 0.0018), while other functional diversity components remained stable across seasons. Trait-based analyses highlighted a significant seasonal shift in intermediate cell volume (BioVol_c2; p = 0.004), which was more abundant in summer. Multivariate analyses (RDA) showed that environmental variables explained up to 90% of the variation in taxonomic α diversity in autumn (F = 9.27, p = 0.025), with temperature (F = 19.68, p = 0.015) and resistivity (F = 8.02, p = 0.049) identified as significant drivers. In summer, however, these relationships were weaker or not significant. These results indicate a relatively stable diatom community structure across seasons, with limited temporal turnover in both taxonomic and functional composition. Environmental filtering appears more pronounced during summer conditions, while autumn assemblages show weaker associations with measured variables. Spatial and environmental heterogeneity appears to play a stronger role than seasonal variation in structuring diatom communities in high-elevation temporary ponds.