Brief communication: Anomalous low-discharge conditions increase algal bloom risk in Central European rivers

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.