Impact of a major flood event on riverine fish community composition.

Extreme floods are increasingly frequent disturbances in riverine ecosystems, yet their impacts on fish community structure remain variable. We assessed the effects of a major 2024 flood on riverine fish assemblages in Polish submontane and lowland streams using pre- and post-flood electrofishing data from 36 paired sites. Presence/absence-based PERMANOVA revealed no significant flood effect on overall community composition, with habitat gradients (elevation, stream order, geology, riparian land use) explaining approximately 35% of variation. NMDS ordination and dispersion tests confirmed stable compositional patterns and variability. Indicator species analysis identified only common rudd (Scardinius erythrophthalmus) as significantly associated with post-flood conditions. A Bernoulli GLMM showed a significant flood × guild interaction, with limnophilic species decreasing and rheophilic species increasing in probability of occurrence post-flood. Guild-specific richness changes supported these shifts. Results indicate resilience in taxonomic composition but subtle functional reorganisation towards lotic-tolerant taxa, highlighting the overriding role of habitat heterogeneity in structuring riverine fish communities following major disturbance.