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 <em>a</em> data from 30 monitoring sites across Germany. We show that negative discharge anomalies consistently correspond to positive anomalies in measured chlorophyll <em>a</em> relative to the maximum possible chlorophyll <em>a</em> 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.