The dinoflagellate Tripos furca, known for its frequent and massive blooms in coastal waters, has been associated with significant fish mortality, particularly in aquaculture environments. In mid-May 2022, a notable bloom, characterized by intense red discoloration, was observed along the Penang Strait in the northern Malacca Strait. Our field survey revealed a high-density bloom of T. furca, which persisted for four months. To investigate the mechanisms driving the bloom dynamics, monthly sampling was conducted across 19 stations in the Penang Strait until the bloom subsided. Our results showed spatiotemporal variations in T. furca abundances, with a peak density in late June (8.2 × 10⁵ cells L^-1). Metabarcoding of the 18S rDNA revealed high read abundances of T. furca amplicon sequence variants (ASVs) during the bloom, constituting up to 88% of the total read abundances. The bloom development was triggered by elevated seawater temperatures, high photosynthetically available radiance and increased nitrogen and phosphorus availability during the warmest month of this La Niña year. High intraspecific genetic variation was observed in the bloom subpopulations. The shift in conditions favourable for growth likely caused a temporal genetic shift in T. furca subpopulations, prolonging the bloom. The bloom density began to decline with the onset of Southwest monsoon, as southward-flowing surface currents from the Andaman Sea altered environmental conditions, increasing salinity and slightly cooling the waters. These changes, along with a reduction in nitrogen and phosphorus levels, created an unfavourable niche for T. furca, leading to a shift in phytoplankton composition towards Noctiluca- and diatom-dominated populations. Additionally, an increase in potential zooplankton predators likely contributed to the bloom's cessation. Our early detection of the bloom successfully prevented severe aquaculture losses during this 2022-bloom event.
 

environmental DNA, dinoflagellates, finfish aquaculture, harmful algal blooms, metabarcoding, intraspecific genetic variation