From drought to deluge: hydrologic extremes alter nutrient-driven harmful cyanobacterial blooms.

Globally-expanding, nutrient-driven harmful cyanobacterial blooms (CyanoHABs) are additionally modulated by climatic changes impacting freshwater-marine continua. In 2022-2023, the San Francisco Bay Delta, California's main freshwater source and important biodiversity refuge, experienced record sequential drought and flood years, which altered seasonal CyanoHABs in a hydrologically-stable Delta CyanoHAB "hot spot". We observed a severe bloom in 2022 (drought year) followed by bloom-free conditions in 2023 (flood year), despite elevated nutrient loads in 2023. In situ bioassays revealed nutrient dilution by elevated freshwater discharge in 2023 controlled CyanoHABs, independent of flushing dynamics. Findings show nutrient concentrations, not nutrient loads, impact the CyanoHAB hotspot in the short-term while nutrient loading determines long-term trophic state. Climate-modulated nutrient variability will likely affect future nutrient-bloom dynamics in waterbodies worldwide; thus nutrient reduction strategies should consider hydrology-mediated nutrient concentration patterns.