In coastal ecosystems, microbial alkaline phosphatase (AP) production is primarily induced by low phosphate (PO<sub>4</sub>-P) availability but is additionally regulated by the dissolved inorganic nitrogen to phosphate (DIN:PO<sub>4</sub>-P) ratio and seasonal temperature variation. However, the dominant driver of APA surges and potential synergistic effects among these factors remain unclear. Through integrated seasonal field surveys and an enclosure experiments in Laizhou Bay, China, we demonstrate that PO<sub>4</sub>-P seawater concentration serves as the primary control for APA induction, with a consistent threshold of 0.05 . Below this threshold, APA exhibits a significant positive correlation with the DIN:PO<sub>4</sub>-P ratio in both the field and an enclosure experiment under P limitation (combined analysis of field and experimental data, p＜0.01; n=36). Notably, phytoplankton-dominated APA is evidenced in autumn. Genetic analysis confirms that AP-related gene expression increases only when PO<sub>4</sub>-P falls below the identified threshold. These findings refine the conceptual framework for AP regulation in coastal ecosystems, highlighting the hierarchical control of phosphorus limitation over stoichiometric effects.