Role of phosphorus concentration and the nitrogen to phosphate ratio in the synergistic stimulation of alkaline phosphatase activity in Laizhou Bay, China, coastal waters

Abstract. In coastal ecosystems, microbial alkaline phosphatase (AP) production is primarily induced by low phosphate (PO4-P) availability but is additionally regulated by the dissolved inorganic nitrogen to phosphate (DIN : PO4-P) ratio and seasonal temperature variation. However, the dominant driver of alkaline phosphatase activity (APA) surges and potential synergistic effects among these factors remain unclear. Through integrated seasonal field surveys and enclosure experiments in Laizhou Bay, China, we demonstrate that PO4-P seawater concentration serves as the primary control for APA induction, with a consistent threshold of 0.05 µmol L−1. Significant positive correlation was observed between APA and the DIN : PO4-P ratio below this threshold (0.05 µmol L−1), as analyzed in the combined dataset of field and enclosure experiments (p<0.01, n=36). Notably, phytoplankton-dominated APA was evidenced in autumn. Genetic analysis confirms that AP-related gene expression increases only when PO4-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.