Environmental and elevational drivers of diatom diversity in alpine temporary ponds

High-elevation temporary ponds (TPs) represent habitats of high ecological value, yet they are particularly vulnerable to ongoing climate change. Diatoms are excellent bioindicators, useful for detecting ecological shifts driven by natural fluctuations or anthropogenic impacts. This study investigates whether differences in the floristic composition, measured as diversity and uniqueness (i.e., the proportion of taxa occurring exclusively in one pond), of diatom communities at small spatial scales among alpine TPs (Central Apennines, Italy) are mainly driven by isolation, elevation, or environmental factors. We found that diatom diversity in alpine biomes primarily responds to local geographical gradients, which tend to obscure any detectable effects of environmental variables. Elevation plays a key role in shaping the uniqueness of diatom communities in TPs, as higher ponds host a greater proportion of exclusive taxa. Our results indicate that local environmental conditions associated with elevation override the effects of isolation (distance between ponds), leading to the development of functionally distinct diatom communities at higher altitudes. The diversity and uniqueness of diatom assemblages are influenced by a combination of geographic and, to a lesser extent, environmental factors, with elevation acting as a central driver, both enriching diversity and promoting floristically unique communities, especially in interaction with other variables. Furthermore, electrical conductivity appears to favour communities composed of similarly adapted species, likely reflecting specific ecological conditions or adaptive responses to environmental gradients. These findings highlight the importance of elevation and spatial gradients in shaping diatom community patterns in high-altitude ephemeral freshwater habitats.