Abstract: Phosphorus (P) is a limiting nutrient for food production, yet we face an overabundance of P entering waterbodies, from agricultural production and municipal wastewater. This impairs water quality, undermines the health of aquatic ecosystems, and threatens water security. This seminar explores the challenges we face in managing P and eutrophication. Changing patterns in P use and management have led to a conundrum of simultaneous P excesses and deficiencies, and the build-up of P legacies. In seeking solutions to eutrophication, we need to better quantify P sinks and sources in watersheds, and ensure realistic expectations for mitigation strategies and timescales for recovery.