Low-temperature dominance and species succession shape Alexandrium bloom dynamics in Jinhae Bay, Korea.

Paralytic shellfish poisoning (PSP) has recurred almost every spring for decades in Jinhae Bay, Korea, yet the environmental factors governing its interannual variability remain poorly understood. To investigate the ecological drivers of Alexandrium blooms, we combined long-term field observations with species-specific molecular monitoring. Thirty Alexandrium strains were isolated and identified using LSU rDNA sequences, revealing four co-occurring species (A. catenella, A. pacificum, A. affine, and A. fraterculus). Species-specific TaqMan quantitative PCR (qPCR) assays were developed and applied to a 12-year monitoring dataset (2011-2022). The analyses revealed clear temperature-driven seasonal succession, with A. catenella dominating in spring, A. pacificum in late spring to early summer, and A. affine and A. fraterculus in summer to autumn. Alexandrium abundance was consistently higher under relatively low temperatures, which also coincided with increased PSP intensity. During the spring bloom period, dissolved inorganic phosphate (DIP), but not dissolved inorganic nitrogen (DIN), showed a significant positive association with A. catenella abundance, suggesting that phosphate availability may further promote bloom development. Species composition was closely associated with PSP dynamics, with A. catenella linked to severe toxicity events and A. pacificum contributing to prolonged toxicity periods. Overall, these findings demonstrate that temperature-driven niche differentiation is the primary mechanism governing Alexandrium bloom dynamics in Jinhae Bay, while phosphate availability acts as an additional environmental factor influencing bloom intensity and PSP severity.