Along-River and Temporal Variation of Water Column Stability in the Balikpapan Estuary During Spring Tide Based on In-Situ Observations

Balikpapan Estuary, a semi-enclosed estuarine system in East Kalimantan, is influenced by tidal forcing and freshwater discharge from surrounding rivers. These drivers regulate stratification and mixing in the water column. This study investigates how tidal phases affect salinity-driven stratification based on in-situ measurements at 13 stations along the estuary during spring tide using a CTD and a turbidity logger. Stratification, quantified by the Brunt-Väisälä Frequency, showed spatial variation with tidal phase. During low tide, water column stability was concentrated in a thin surface layer (2-3 m) from the bay mouth (0 km) to upstream regions (25 km), indicating strong freshwater influence and shallow stratification. At high tide, deeper and more stable layers formed. From 0 to 20 km, maximum stability occurred around 3-4 m depth, while in the upper bay (20-25 km), stable layers developed between 3-13 m depth due to saltwater intrusion. Temporal measurements at a fixed station showed stratification strengthened during ebb tide as surface waters freshened and weakened during flood tide due to vertical mixing. Salinity ranged from 31.64-32.52 PSU, and density from 1018.73-1019.59 kg/m³, indicating moderate but dynamic stratification. These findings emphasize the role of semidiurnal tides in modulating vertical structure and estuarine mixing in Balikpapan Bay.