Dynamics and scale variations of blooms of Phaeocystis globosa “giant colony” ecotype and key environmental factors in the Beibu Gulf, China

The Beibu Gulf has experienced blooms of Phaeocystis globosa “giant colony” ecotype (PGGCE), with noticeable variations in bloom scale across years. However, driving environmental factors and their roles remain poorly understood. In this study, we quantified dynamics of PGGCE cells in 2016–2017 and 2018–2019, and analyzed their correlations with environment factors. The results revealed that PGGCE blooms primarily occurred in Guangxi coast and western waters of Leizhou Peninsula during winter months, exhibiting distinct developmental processes. Bloom intensity, duration, and distribution differed significantly between two bloom events. In 2016–2017, peak PGGCE density exceeded 2.0 × 105 cells L−1 nearly double that of 2018–2019. Furthermore, bloom sustained five months during 2016–2017, compared to three months during 2018–2019. Prolonged period of low temperatures and elevated nitrate concentrations favored PGGCE growth and colony formation, resulting in a larger scale bloom during winter 2016 as opposed to winter 2018.