Numerical Modeling of Long-Term Biogeochemical Processes and Its Application to Sedimentary Bed Formation in Tokyo Bay

Abstract

Even though models of water quality have become increasingly detailed over time, their applicability to analyze long-term effect on sedimentary bed formation is yet to be clarified. Hence, an integrated, layer-resolved, process-based, sediment-water coupled, long-term robust, three-dimensional (3D) ecosystem model, including realistic sedimentary and pelagic processes, was developed. The constituents of the integrated model included a multi layered ecosystem model, a quasi-three dimensional hydrodynamic model, a wave hindcasting model, and a sediment deposition and erosion model. Because numerical modeling difficulties arise in setting the initial conditions, especially for a sedimentary bed, this novel modeling approach suggests to initiate the model with the simplest initial conditions of no particulate organic carbon content (POCC) and uniform inorganic sediment distribution. The model was applied to Tokyo Bay and quasi-steady state POCC in the sediment was obtained through long-term computation with realistic sedimentary and pelagic processes. Wave induced bed shear stress (WBSS) is critical for sediment erosion and spatial movement of sediment pollution, while a stable morphology is determined by the balance between the current induced bed shear stress (CBSS) and WBSS in Tokyo Bay. This novel modeling approach, with the simplest sedimentary initial conditions and realistic sedimentary and pelagic processes, provides a great tool for long-term ecosystem modeling in future studies.

A practical pCO2 estimation and carbonate dynamics at an event of hypoxic water upwelling in Tokyo Bay

“…Appearance of hypoxia and anoxia in the bottom waters between spring and autumn has been a serious water quality problem causing mortality of fishes and benthic animals in tidal flats and shallow water areas. The bottom anoxic waters often contain hydrogen sulfide generated by sulfate reduction (Kodama and Horiguchi, 2011;Furukawa, 2015;Amunugama and Sasaki, 2018). In addition, navigation channels and borrow pits (dredged for the foreshore reclamation) exist at the bay head, in which anoxic waters with a high concentration of hydrogen sulfide often appear (Sasaki et al, 2009a;Sasaki et al, 2009b).…”

“…Tokyo Bay, Japan, is a typical eutrophic urban bay with the Tokyo metropolitan area as its watershed. During the period of rapid economic growth from the 1960s, water pollution due to eutrophication became more serious, resulting in sediment organic pollution; the formation of bottom hypoxic and anoxic waters in summer and coastal upwelling of these waters lead to the mortality of benthic animals (Kodama and Horiguchi, 2011;Furukawa, 2015;Amunugama and Sasaki, 2018). To alleviate the problem, the Environment Agency (now the Ministry of the Environment) introduced a Total Pollutant Load Control System policy (TPLCS) in 1979, with COD as the target water quality item.…”