The coastal waters of the East China Sea (ECS) are facing severe eutrophication, driven primarily by large inputs of nitrogen and phosphorus from land-based sources, which in turn accelerate harmful algal blooms. However, the understanding of the relationship between eutrophic conditions and the mechanisms of algal bloom occurrence remains insufficient. This study investigates a bloom of the toxic dinoflagellate Prorocentrum cordatum (syn. Prorocentrum minimum) in the coastal waters of Xiangshan Bay (XSB), ECS, through in situ nutrient addition experiments and DNA metabarcoding analysis. The addition of dissolved organic phosphorus (DOP) significantly promoted the growth of P. cordatum compared to the -P control (no phosphorus source: f/2-P) (p < 0.05), whereas dissolved inorganic phosphorus (DIP) had no such effect. In contrast, nitrite (NO₂⁻) and urea significantly inhibited the growth of the species relative to the -N control (no nitrogen source: f/2-N) (p < 0.05), while no significant difference was observed between nitrate and ammonium treatments when compared to the -N control. Network analysis indicated that DOP increased the topological complexity but may have reduced the stability of the community network. In contrast, NO₂⁻ and urea appeared to enhance the network stability. These findings emphasize that DOP was a critical phosphorus source for maintaining P. cordatum blooms, while the input of nitrite and urea may inhibit bloom expansion. In summary, this study suggests that DOP may serve as an important phosphorus source facilitating P. cordatum growth under phosphorus-limited conditions, thereby providing a scientific basis for refining phosphorus control and nitrogen source optimization in the context of ECS eutrophication management.