Seasonal dynamics and natural detachment of epiphytic green algae on aquaculture rafts in the Subei Shoal, China: Implications for early green tide formation.

Green tides have become a recurring ecological problem in the Yellow Sea, yet early-stage processes governing algal biomass supply remain poorly understood. Here, two years of continuous field observations (2023-2024) were conducted in the Neopyropia yezoensis aquaculture area of the Subei Shoal, China, to examine the seasonal succession of epiphytic green algal communities and their natural detachment during spring. Epiphytic communities exhibited a clear and repeatable successional pattern, shifting from dominance by cold-tolerant taxa in early spring to increased prevalence of Ulva prolifera by April, a transition closely associated with rising water temperatures. Natural detachment occurred continuously throughout the cultivation period, rather than being confined to the harvest stage. Detached biomass increased in concert with attached biomass, indicating a stable and persistent coupling between biomass accumulation and natural detachment in late spring. Spatial extrapolation suggested that natural detachment reached approximately 327.14 t and 656.96 t (wet weight) in April 2023 and 2024, respectively. Although these amounts are smaller than peak floating biomass during summer green tides, their seasonal timing coincides with the initial phase of bloom formation and may contribute to the background supply of floating algal material. Together, these findings provide field-based evidence that aquaculture-associated epiphytic algae represent a sustained early-season source of algal input to coastal waters. Incorporating epiphytic algal dynamics into monitoring, early warning, and management frameworks may improve prediction and mitigation of green tide development in aquaculture-dominated coastal systems.