Coastal groundwater phosphorus drives global acceleration of algal blooms.

Coastal algal blooms (CABs) are accelerating worldwide. This intensification is particularly puzzling in regions where nitrogen inputs have been curbed, yet blooms persist or worsen. Here we reveal excess dissolved inorganic phosphorus (DIP) from coastal groundwater as a critical, previously overlooked driver. Combining a process-based physical-chemical framework with global datasets, we find decadal CAB trends are more strongly linked to coastal DIP than to nitrogen availability or water-column stability. A global meta-analysis with site-specific time series demonstrates anoxic groundwater releases substantial DIP. This mechanism explains continued CABs under nitrogen control and aligns with a shift toward phosphorus limitation in many coastal regions. Spatial and temporal variation in CAB trends reflects aquifer geochemistry, with reducing groundwater fueling blooms and oxidized systems suppressing them. Our findings establish groundwater as a missing link in the modern coastal nutrient cycle and underscore the urgency for integrated nitrogenphosphorus management to mitigate the escalating impacts of CABs.