Identifying the state dependent drivers associated with Florida red tide (Karenia brevis) initiation, proliferation, and termination

Harmful algal bloom (HAB) initiation, proliferation, and termination occur across highly variable spatial and temporal scales, complicating efforts to link bloom dynamics to environmental drivers. This study presents a novel approach to identify state-dependent factors associated with Karenia brevis (red tide) blooms, which occur nearly annually along Florida’s southwest coast. Bloom coverage and potential environmental drivers, including contemporaneous and antecedent conditions, were clustered simultaneously using a self-organizing map (SOM) to define distinct environmental regimes. We delineated seven red tide states based on constrained k-means clustering: no bloom, bloom initiation, expanding medium bloom, expanding widespread bloom, contracting widespread bloom, contracting medium bloom, and bloom termination, with each representing a unique combination of co-occurring environmental conditions. Results reinforce previous studies linking K. brevis to temperature and watershed loading, while also revealing that correlations with managed freshwater discharges are state dependent and shaped by a broader environmental context. During severe bloom years, a consistent bloom initiation state emerged, characterized by strong positive correlations between bloom expansion and multiple potential drivers, most notably Caloosahatchee River discharge (median r = 0.73), as well as elevated temperatures, Loop Current position, and Peace River flow. Bloom contraction and termination states generally coincided with atmospheric conditions, Loop Current position, and diminished hydrologic loads. Overall, these findings highlight the potential value of this novel methodology for identifying and interpreting not only red tide, but also other HABs and non-linear ecological dynamics by leveraging temporally clustered environmental states.