Role of Sedimentary Nitrogen Regeneration in the Tonegawa River Estuary

Nitrogen is one of the key nutrients limiting marine primary productivity and plays a central role in controlling phytoplankton growth and ecosystem structure. In this study, we focus on the Tonegawa River estuary along the Pacific coast of Japan. In addition to direct nutrient inputs from the Tonegawa River, this region is influenced by the interaction between the Kuroshio and Oyashio currents, leading to strong variability in water masses and elevated biological productivity. Owing to these characteristics, the area has long been recognized as one of Japan’s major fishing grounds.

 

In the estuary and adjacent coastal waters, the sediment–water interface also represents an important site for inorganic nitrogen regeneration. Organic nitrogen deposited on the seafloor can be transformed into dissolved inorganic nitrogen through mineralization and nitrification processes and subsequently released into the overlying water column. Using bottom-water observations together with simulations from a sediment biogeochemical model, we show that sedimentary processes provide a substantial source of inorganic nitrogen to the water column in the study area. More than 80% of the regenerated nitrogen is released as ammonium, while approximately 16% is returned as nitrate, highlighting the dominant role of ammonium regeneration in sustaining nitrogen availability in the estuarine system.

 

Previous studies using hydrodynamic models (MRI.COM) coupled with ecosystem models (NPZD), without explicitly including sediment processes, have emphasized river inputs as the primary nitrogen source in this region. Furthermore, our results indicate that benthic biogeochemical processes at the sediment–water interface make an important contribution to the nitrogen budget of the Tonegawa estuary. These findings highlight the need to explicitly include sedimentary nitrogen regeneration when studying nitrogen cycling and ecosystem dynamics in marginal seas and estuarine environments.