Spatial and seasonal variations of chemical oxygen demand in the Seto Inland Sea, Japan: Application of an organic matter origin model

In the Seto Inland Sea (SIS), despite reductions in chemical oxygen demand (COD) loads from terrestrial sources, the attainment rate of environmental quality standards in coastal waters remains low, representing an ongoing management challenge. Autochthonous (phytoplankton primary production) and oceanic sources also impact the COD; however, the spatial and temporal distribution of organic matter by origin and their contribution to the total COD are not clearly understood. Therefore, this study clarified the spatial distribution and seasonal variation of COD by organic matter origin in the SIS. Numerical simulations using a three-dimensional hydrodynamic and ecosystem model classified organic carbon sources as terrestrial, autochthonous, or oceanic. The results show that terrestrial sources are localized in estuarine areas, contributing less than 10% to the total COD in offshore areas. In contrast, autochthonous matter is widely distributed from estuaries to offshore areas and is the largest contributor to total COD. Oceanic COD exhibits low seasonal variation and is uniform throughout the SIS at approximately 0.65 mg L −1 but contributes more than terrestrial COD in all sea areas. Additionally, refractory COD contributes more than 50% to the total COD in all seasons. These findings indicate that the widespread distribution of autochthonous COD and the background presence of oceanic COD are the key factors severely limiting the decrease in total COD levels in the SIS despite the significant reduction in terrestrial load. The results of this study provide valuable insights for evaluating water quality management strategies and environmental standards in enclosed seas. • The chemical oxygen demand (COD) in the Seto Inland Sea was analyzed • A 3D hydrodynamic model was built and results were compared to observation data • Autochthonous and oceanic organic loads contribute significantly to the total COD • Terrestrial organic sources have a high contribution to the COD of estuarine areas • Refractory matter account for >50% of the total COD in the SIS in all seasons.