Impact of riverine inputs on nutrient dynamics and water quality in enclosed water bodies

Abstract This study investigates the intricate dynamics of nutrient transport and stratification in Lake Biwa, highlighting the significant impact of river inflows on water quality. Utilizing a validated three-dimensional flow field model and ecosystem model including a bottom sediment model, the analysis revealed that nutrient concentrations, specifically NH₄⁺, NO₂⁻, NO₃⁻, and PO₄ 3 ⁻, exhibited pronounced seasonal variations. In the RN_double scenario, NH₄⁺ and NO₂⁻ concentrations demonstrated a slight increase of 0.1 µg/L, respectively, while NO₃⁻ concentrations rose by 0.05–0.10 µg/L in response to precipitation changes. Conversely, in the RN_half scenario, NH₄⁺ and NO₂⁻ concentrations decreased, with NO₃⁻ seeing a more substantial decline of approximately 0.1 µg/L, attributed to reduced precipitation. PO₄ 3 ⁻ levels exhibited a maximum decrease of 0.03 µg/L from summer to autumn. Furthermore, simulations limiting nutrient inflows indicated a modest reduction in concentrations: NH₄⁺ decreased by approximately 0.03 µg/L during summer, and NO₂⁻ decreased by around 0.05 µg/L from spring to summer. The results suggest that while immediate improvements in dissolved oxygen levels are limited, effective long-term nutrient management could stabilize oxygen concentrations and improve overall water quality. These findings underscore the necessity for comprehensive water management strategies to mitigate eutrophication effects and support the ecological health of Lake Biwa.