Winter blooms of Phaeocystis globosa in eutrophic semi-enclosed bays are concerning, yet the conditions supporting their persistence remain poorly understood. We investigated a prolonged P. globosa bloom in Xiamen Bay, China, from 28 November 2023 to 5 February 2024 using official bloom records, buoy observations, underway surveys, GOCI-2 remote sensing, meteorological data, and marine heatwave diagnostics.
The bloom persisted for approximately 70 days, with reported maximum cell densities ranging from 0.036 to 6 × 10⁷ cells L⁻¹. It first developed in the western bay and Jiulong River estuary, intensified in mid-December, and later showed enhanced signals in Tong’an Bay. Chlorophyll-a at Baozhu Island increased gradually with multiple peaks, whereas Tong’an Bay exhibited a rapid increase to 30.4 μg L⁻¹ on 12 January 2024. GOCI-2 Red Tide Index products captured the broad spatial evolution, with the estimated bloom area in Tong’an Bay reaching 21.06 km² on 14 January.
Marine heatwaves occurred before and during the bloom, while winds were weaker than in the corresponding periods of 2021 and 2022. Water temperature remained largely favorable for P. globosa growth, and nitrate and phosphate availability may have further supported persistence. The bloom declined rapidly in late January, coinciding with stronger northerly winds, cooling, and rainfall. These results indicate that the event was associated with an environmental window of favorable temperature, recurrent warm anomalies, weak winds, nutrient availability, and bay-scale retention rather than a single driver. The findings support incorporating thermal extremes and meteorological variability into winter harmful algal bloom monitoring and early warning.

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