Strong influence of climatic extremes on diversity of benthic algae and cyanobacteria in a lowland intermittent stream

Freshwater ecosystems are threatened by the global change‐induced extreme climatic events worldwide. The unpredictable changes in water supply create strongly disturbed environments and ultimately result in diversity changes. Here, we studied the formation of benthic algal and cyanobacterial assemblages under intermediately disturbed (IDC) and highly disturbed conditions (HDC) in a small lowland intermittent stream. We addressed our hypotheses to the Intermediate Disturbance Theory supposing that intermediate frequencies or intensities of disturbances maximise diversity independently of its level. We expected a larger influence of extremes in water supply on functional than on taxonomic and phylogenetic diversity. Our results only partially proved our first hypothesis highlighting the (i) importance of short‐time but intense disturbing effect on biofilm formation and trait composition under IDC period and (ii) the instability within the assemblages caused by large and opposing influences during HDC. Although extreme weather events caused trait extremes (supporting second hypothesis), they did not result in a decrease in functional richness (rejecting the second hypothesis). These findings in accordance with the insurance hypothesis clearly stress that a system with high functional redundancy can keep its functionality even under drastic hydrological conditions and the accompanying loss of species.