Nutrient distribution mapping model of the Bera River basin in Saleh Bay, West Nusa Tenggara

Saleh Bay is a semi-enclosed marine ecosystem that is vulnerable to nutrient inputs from surrounding watersheds. This study aimed to map the spatial distribution of watershed-derived nutrients and assess their implications for coastal water quality using numerical modeling. Simulations incorporated wind, tides, bathymetry, river discharge, and in- situ concentrations of dissolved oxygen (DO), nitrate (NO₃), ammonia (NH₄), and phosphate (PO₄). Hydrodynamic model validation against the current Copernicus Marine Service data showed strong performance, with RMSE values of 0.0316–0.0340 and correlation coefficients exceeding 0.97. Nutrient simulations indicated generally low NH₄ (<0.003 mg/L) and PO₄ (<0.010 mg/L) concentrations, with localized coastal increases of up to 0.04 mg/L for NH₄ and ≥0.014 mg/L for PO₄. NO₃ concentrations were relatively higher, ranging from <0.03 mg/L offshore to >0.055 mg/L near the Bera River mouth. DO concentrations decreased to below 3 mg/L in the southern coastal areas during the low tide. Although direct nutrient validation is limited by field data availability, the validated hydrodynamic model provides a reliable basis for nutrient transport simulations. These results highlight the dominant role of tidal dynamics and watershed inputs, and demonstrate the usefulness of numerical modeling for sustainable coastal water quality management.