Nitrogen dynamics in a highly urbanized coastal area of western Japan: impact of sewage-derived loads

In this study, we examined the nitrogen dynamics of a highly urbanized coastal area, focusing on the impacts of sewage-derived nitrogen. High levels of dissolved inorganic nitrogen were detected in seawater near treated sewage effluent (TSE) discharge points before decreasing in the offshore direction, suggesting that the impact zone of sewage effluent is about 1–2 km from the discharge point. The stable isotope ratios of nitrate and particulate organic nitrogen suggest nitrogen uptake by phytoplankton as well as dilution by offshore seawater, which contributed to a decrease in sewage-derived nitrogen levels. However, the extent of the impact zone was controlled by tidal variations and differences in temperature between the TSE and seawater. Our results also identify nitrogen transport processes, through exchange between seawater and sediment pore water, as an additional important source of nitrogen in the study area.