The purpose of this study is to investigate the phosphorus and nitrogen balance between coral reef and seagrass bed through their transport due to current motion (advection) in a subtropical coastal region. Field observation of phosphorus and nitrogen transport between coral reef and seagrass bed within one tidal cycle was done on a measurement line settled beside seagrass bed in a costal area in Ishigaki island, Okinawa, Japan. Results from flow measurements showed that the southwest-ward drift current constantly existed and there was an inflow from the northeast part and an outflow to the southwest part. Moreover, oscillatory current was observed in deep narrow channel.
　Phosphorus and nitrogen concentrations were high in the northeast part and low in the southwest part on average. As a result, it was found that there was a net transport of phosphorus and nitrogen from offshore to the seagrass bed (or to the mangrove forest). In addition, a sudden inflow of phosphorus and nitrogen in middle layer of deep narrow channel was also observed.