Lower trophic ecosystem dynamics in the eastern part of the Seto Inland Sea and their response to changes in nutrient supply from the rivers

• Three-dimensional physical model with an advanced biogeochemical model. • Osaka Bay and Harima-Nada differ in lower trophic ecosystem dynamics. • The Yodo River contributes the most freshwater and nutrients. • Increased riverine nutrients lead to elevated nutrient concentrations. • Lower trophic ecosystems respond to changes in nutrient supplies. The eastern part of the Seto Inland Sea is significantly influenced by anthropogenic and territorial nutrient sources, exacerbated by high population and industrial activities. Several rivers, with the Yodo River in Osaka Bay as the largest contributor, play a vital role by providing freshwater, substantial nutrients, and pollution loads to this coastal region. We aimed to understand the responses of ecosystem and coastal dynamics to the changes in nutrient supply from the rivers. Accordingly, a comprehensive three-dimensional physical model coupled with a complicated biogeochemical model was developed to replicate processes in this area. The findings reveal significant differences in nutrient dynamics, and phytoplankton community structures and biomass between Osaka Bay and Harima-Nada. Osaka bay exhibits higher nutrient concentrations, leading to elevated phytoplankton biomass, primarily dominated by a large micro-phytoplankton. Contrastingly, Harima-Nada exhibits lower concentrations, resulting in diminished phytoplankton biomass, with nano-phytoplankton prevailing. The experiment with heightened nutrient inputs from rivers significantly elevated nutrient concentrations for the entire areas in both Osaka Bay and Harima-Nada. Contrastingly, phytoplankton biomass remarkably increased only in the nearshore areas. Furthermore, the response of phytoplankton community structures differs between the two regions. Specifically, in Harima-Nada, nano-phytoplankton shifted primarily to large micro-phytoplankton, while large micro-phytoplankton remains predominant in Osaka Bay. This emphasizes the vital role of nutrient availability in influencing the structure of phytoplankton communities in these regions. The interplay between nutrient availability and phytoplankton dynamics stands as a key factor in comprehending and effectively managing these coastal ecosystems.