Rapid intensification of Karenia brevis red tides after hurricane Milton revealed by PACE-OCI hyperspectral unmixing

Estuarine and coastal systems, shaped by transient physical–biological interactions and recurrent harmful algal blooms, often undergo episodic shifts in phytoplankton community composition (PCC). We present PhytoUnmix, a deep learning spectral unmixing framework applied to NASA’s hyperspectral PACE (Plankton, Aerosol, Cloud, Ocean Ecosystem) mission, to resolve PCC dynamics during a red tide bloom following Hurricane Milton (2024) on the West Florida Shelf, USA. PACE-derived PCC, together with upper-ocean physical observations, revealed contrasting phytoplankton community patterns across distinct post-hurricane hydrographic conditions: offshore wind-curl induced upwelling exhibited higher relative abundance of diatoms, whereas nearshore regions affected by northerly winds and Ekman transport along with enhanced upwelling were associated with coastal Karenia brevis bloom. For the first time, synergistic observations using PACE and Surface Water and Ocean Topography (SWOT) showed a rapid rise in dinoflagellates nearshore, with concurrent declines in diatoms and chlorophytes, while cryptophytes co-increased. These transient shifts likely reflected not only physical stirring but also biological responses, as freshwater inputs increased nutrient availability, and coincided with intensified red tides and spatially heterogeneous phytoplankton responses, such as, initial increases in diatoms and cyanobacteria followed by Karenia brevis dominance. Together, PACE and SWOT provide unprecedented capacity to resolve physical–biological linkages in coastal ecosystems.