Flow intermittence shapes the occupancy-frequency distribution of stream chironomid assemblages

Occupancy frequency distribution (OFD) approach has often been applied in community ecology to investigate species occurrence, yet the mechanisms underlying these patterns remain incompletely understood. We investigated chironomid assemblages across three European drying river networks (DRNs), to assess how stream intermittency affects OFD patterns. Our objectives were to test whether OFD patterns differ between perennial and intermittent sites, and explore the associated differences in species composition. We also examined the role of environmental heterogeneity, and analysed the gamma diversity of chironomid assemblages and correlations with OFD patterns. The perennial sections exhibited bimodal patterns, indicating that dispersal processes allow core species wide regional occupancy. In contrast, intermittent sections showed unimodal OFD, with drying acting as a strong filter, reducing core taxa and increasing the proportion of satellite taxa. No differences in environmental heterogeneity were detected between flow types, therefore, we conclude that hydrological stability itself significantly shapes OFDs. Our analyses revealed higher community similarity behind bimodal OFD in perennial sections, whereas unimodal pattern at intermittent sections exhibit turnover. These findings advance our understanding of how drying restructures community assembly and provide critical insights for forecasting species distribution dynamics and responses in drying river networks under conditions of increasing flow intermittence.