Spatio-temporal dynamics of riverine cyanobacteria and selected water quality indicators under two hydrological regimes

Global climate change increases the likelihood of hydrological extremes in rivers such as floods and low flow conditions, with profound consequences on ecology and human wellbeing. To study how water quality parameters including phytoplankton respond to hydrological extremes, we analysed two distinct years with high (2021) and low discharge (2022), in a regulated Central European river and using a multidisciplinary approach: weekly in situ biochemical data, longitudinal profiles, hydrological modelling and remote sensing of chlorophyll. The drought summer 2022 in comparison to the wet and cold summer 2021 was on average characterized by lower discharge (39 vs. 328 m³/s), longer flow time (31.8 vs. 9.9 days), warmer water (24.1 vs. 20.0 °C), lower total nitrogen (TN) (1.6 vs. 3.4 mg/L) and a lower TN/TP ratio (11 vs. 29). An intensive cyanobacterial bloom developed in 2022 (maximum chlorophyll-a 177 µg/L), dominated by the scum-forming genus Microcystis (maximum 4.8 × 10⁸ cells/L). Our findings provide evidence that low flow, combined with other stressors, deteriorate some water quality indicators and promote toxic cyanobacterial blooms in regulated rivers. Ongoing hydrological and climatic changes may increase such risks in rivers.