In August 2022, San Francisco Bay (SFB) experienced an overgrowth of the raphidophyte Heterosigma akashiwo, resulting in the most extensive harmful algal bloom (HAB) on record. The progression of the bloom, and associated parameters including concentrations of dissolved oxygen (DO), chlorophyll a (chl a), and nitrate + nitrite (NOx), were monitored using in situ high-frequency (15 min) moored sensors and boat-based high-speed surface mapping. The mooring data – with high temporal resolution – were used to estimate temporal changes in organic matter concentration (𝑑𝑂𝐶 𝑑𝑡 ), while the mapping data – with high spatial resolution – were used for basin scale estimates of NOx drawdown and chl a production. Both measurements show that substantial pre-bloom NOx (25 µM) was fully converted to phytoplankton biomass, with peak bloom chl a levels > 100 µg L-1 . From the mooring data we identified three bloom phases tied to DIN supply: Phase 1 (rapid growth) – rapid increase in DO and chl a as DIN is drawn down to sensor detection limits; Phase 2 (slowed growth) –chl a slowly increases while DO decreases due to atmospheric transfer, Phase 3 (bloom collapse) – DO and chl a levels plummet, resulting in widespread hypoxia. 𝑑𝑂𝐶 𝑑𝑡 calculated for Phase 2 suggests that the bloom was sustained by ongoing DIN loading from publicly owned treatment works (POTWs), while close agreement between chl a- and DO-based 𝑑𝑂𝐶 𝑑𝑡 during Phase 3 suggests a top down pressure such as a marine virus likely precipitated the bloom collapse. Our results indicate that DIN loading from POTWs substantially contributed to the severity of the HAB, and support the need for upcoming load reductions