Discerning Drivers of a Coastal Karenia Brevis Bloom After Hurricane Ian (2022) on the West Florida Shelf

Tropical storms carry strong winds and heavy precipitation and are known to generate strong coastal currents and large pulses of riverine nutrient and sediment loading. These inputs can stimulate phytoplankton growth through enhanced nutrient supply but may also suppress it through dilution or light limitation from suspended sediments. Besides, wind‐driven currents may congregate or disperse phytoplankton cells, either facilitating or disintegrating algal blooms. However, the relative importance of these processes and their combined effects remains unclear. In September 2022, Category‐4 Hurricane Ian made landfall along the central West Florida coast and was shown to be related to a severe bloom of the toxic dinoflagellate Karenia brevis on the West Florida Shelf. To understand how the storm influenced the bloom development, we used a coupled hydrodynamic‐biogeochemical model to conduct five scenario experiments that can provide insights into the role of each driver associated with the storm: wind forcing, river discharge, riverine nutrient input, riverine sediment input, and wind‐driven sediment resuspension. Model results show that wind‐driven coastal circulation and riverine nutrient loading were the dominant controls on bloom initiation and intensity, respectively. In particular, the timing of wind‐driven upwelling relative to the nutrient pulse was found to be critical in determining whether offshore K. brevis cells could reach the coast while nutrient concentrations remained elevated. In contrast, light attenuation from riverine sediments and storm‐induced resuspension only temporarily suppressed growth. These findings underscore that understanding the complex interplay between hydrodynamic and biogeochemical processes is vital for predicting coastal bloom dynamics following extreme weather events.