Status: this preprint is open for discussion and under review for Hydrology and Earth System Sciences (HESS).
Brief communication: Anomalous low-discharge conditions increase algal bloom risk in Central European rivers
Alexander Hubig,Pia Ebeling,and Andreas Musolff
Abstract. Hydrological extremes have significant implications for riverine eutrophication as evidenced by algal (phytoplankton) blooms in European rivers during recent drought years. To assess how discharge conditions modulate nutrient-induced phytoplankton growth, we systematically analyze multi-year discharge, total phosphorus, and phytoplankton-indicating chlorophyll a data from 30 monitoring sites across Germany. We show that negative discharge anomalies consistently correspond to positive anomalies in measured chlorophyll a relative to the maximum possible chlorophyll a at the given phosphorus level. Further, we found increased algal bloom risk under below-normal discharge conditions, underlining the future challenges for water quality and eutrophication management under intensifying hydrological extremes.
Received: 04 Jun 2026 – Discussion started: 06 Jul 2026
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Reports of riverine algal blooms during recent droughts imply a link between low-flow conditions and eutrophication impacts. Here, we present evidence from observations of total phosphorus, chlorophyll a, and discharge that anomalously low discharge relates to anomalously high chlorophyll a per total phosphorus across multiple sites as well as more frequent algal blooms. The increased risk of algal blooms highlight the challenges for future river management under intensifying drought conditions.
Reports of riverine algal blooms during recent droughts imply a link between low-flow conditions...
