Bloom dynamics of Tripos muelleri in the Gulf of Maine during summer 2023 revealed through integrated biological and environmental observations

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Phytoplankton diversity is a critical indicator of ecosystem health and function, playing a central role in fueling higher trophic levels and biogeochemical cycling. The Gulf of Maine typically experiences a prominent spring bloom dominated by diatoms. However, in 2023, an extensive and intense bloom of the dinoflagellate, Tripos muelleri, was observed, with concentrations exceeding 50 000 cells L−1, approximately one order of magnitude higher than typical reported abundances,. We used an integrated observational approach combining 18S rRNA amplicon sequencing, pigment analysis and imaging data to characterize bloom structure and dynamics. These datasets were interpreted alongside concurrent oceanographic measurements to evaluate environmental conditions associated with bloom persistence and decline. Results revealed a bloom of T. muelleri that persisted until early August, when an increase in non-viable cells was detected across imaging and pigment-based indicators, coinciding with shifts in environmental conditions. Distance-based redundancy analysis indicated that key environmental variables, including nitrate, phosphate, salinity, temperature and dissolved organic nitrogen, were significantly associated with community composition, collectively explaining 46.5% of the constrained variation across the bloom period. Together, these results highlight the environmental context of an exceptional T. muelleri bloom and underscore the importance of sustained multi-method monitoring to improve understanding of phytoplankton dynamics in the Gulf of Maine.