Source and Decomposition of Organic Matter in the Estuary of Yodo River

The inner part of Osaka Bay in Japan is affected by eutrophication, and the western part of the bay is affected by oligotrophic conditions. In this study, the distribution, sources, and decomposition rate of organic nitrogen （ Org-N ） in the estuarine region of Osaka Bay, defined as the region from the mouth of the Yodo River to the bay head, were studied to elucidate the cause of this uneven distribution of nutrients in the bay. Concentrations of Org-N are much higher in the estuary region than in freshwater areas or in the bay itself. Particulate organic nitrogen, which potentially sinks toward the bottom, accounted for 74 ± 11% of Org-N. The decomposition rate of dissolved organic nitrogen over 100 days, which is potentially supplied to the sea with low-salinity water, was 29 ± 4%. These results regarding the decomposition rate of organic matter in the estuarine region of Osaka Bay may improve the accuracy of models of the impact of land-sourced nutrient load on nutrient conditions in the bay.