Beyond the flow: ecological insights from diatom communities of a Mediterranean intermittent river

Intermittent rivers (IR) are among the most widespread watercourses globally, characterized by alternating dry and wet phases. Despite these fluctuations, IRs support diverse ecosystems and considerable biodiversity. Among their biotic communities found in these rivers, diatoms serve as key primary producers and bioindicators. Due to the limited knowledge of how droughts affect IR biodiversity, this study investigates diatom responses to seasonal hydrological changes and water flow. Specifically, it focuses on taxonomic and functional composition and community structure of diatoms in a naturally occurring Mediterranean IR. Overall, species richness appeared to be unaffected by droughts and water flow, with similar values observed at both lentic and lotic sites. Taxonomic and functional α-diversity were generally higher in lotic waters (TRic = 12.9; Fric = 9.79) compared to lentic (TRic = 8.7; Fric = 3.23) sites across all diversity indices, though no significant differences were found in α-diversity between sites. While the environmental variables considered did not significantly affect variability between sampling sites, permutational analysis identified temperature (F = 1.49, p = 0.09) and dissolved oxygen (F = 2.11, p = 0.01) as important predictors for taxonomic and functional α-diversity. However, seasonality significantly influenced taxonomic (F = 2.28, p = 0.009) and functional (F = 8.86, p < 0.01) β-diversity, with greater variability (1.33 times) observed in August. Findings also indicate significant differences in taxonomic dispersion between site types (F = 2.21, p = 0.04) but no significant differences in functional dispersion. These results suggest that whereas IR conditions may not definitely influence diatom α-diversity, seasonal changes play a key role for community turnover, highlighting the importance of the hydroperiod for shaping IR biodiversity. In the context of ongoing climate change, these findings are particularly relevant, as shifts in hydrological regimes may increasingly affect ecological dynamics and community structure in IR systems.