Seasonality remained significant after accounting for hydrographic conditions in the GLM.  Not only nitrate concentrations but also nitrogen sources have a seasonality in the SOJ. The mixed layer deepens in the winter, and hence the nitrate originated in deep-sea water or local Japanese rivers in this season. The utilization of nitrate provided in winter occurs in spring, and the Tsushima Warm Current remains weak (Yabe et al., 2021).  Kodama et al. (2015) also showed that subsurface nutrient maximum induced by the horizontal advective transport of the Tsushima Warm Current could be observed from the beginning of June. Therefore, the seasonality of δ15NPOM may be associated with horizontal advective transport, albeit additional research is required to understand the seasonality.