Phenotypic plasticity supports green tide persistence before bioenergetic failure underlies terminal decline

Trans‐regional green tides dominated by Ulva prolifera exert significant impacts on coastal ecosystems. While the mechanisms of early‐stage proliferation are well documented, the specific physiological and molecular drivers governing the transition from bloom persistence to terminal decline during this trans‐regional drift remain unresolved. Using in situ physiological and transcriptomic data collected across a 300‐km latitudinal gradient in the Yellow Sea, we found that a dominant molecular type (5S‐II) within the sampled 2021 U. prolifera population showed phenotypic plasticity, with its physiological and transcriptional traits varying across environmental conditions. Physiological observations showed declines in photosynthetic performance ( F v /F m and rETR) and pigment content along the northward drift path, marking a transition from active growth to physiological decline. Transcriptomic and physiological evidence further indicated that the intermediate drifting phase was characterized by a compensatory photoprotective state, potentially involving cyclic electron flow. By contrast, terminal‐phase stress was associated with reduced expression of G‐protein‐signaling‐related genes and patterns suggesting that PSII repair could no longer compensate for photodamage as energy metabolism declined. This bioenergetic failure coincided with a shift toward endogenous catabolism and a transition from vegetative growth toward sporulation. Together, these findings provide insight into bloom termination and suggest that ocean warming may accelerate late‐stage senescence, whereas acidification may intensify photosynthetic inhibition under multiple stressors, potentially increasing coastal hypoxia risk.