Physico-biochemical controls on dissolved oxygen dynamics in the Eastern Upper Gulf of Thailand: field observations and numerical modeling

Hypoxia poses an increasing threat to coastal ecosystems, particularly in semi-enclosed estuarine systems such as the Upper Gulf of Thailand (UGoT). This study investigates the spatiotemporal dynamics of dissolved oxygen (DO) along the eastern UGoT, with a focus on the Siracha area, by integrating field observations with a coupled three-dimensional hydrodynamic–biochemical model. Temporal and spatial measurements in 2020 revealed pronounced seasonal and vertical variability in DO, with hypoxic events closely associated with strong salinity stratification, elevated river discharge, and phytoplankton decay. The model further identified five progressive stages of hypoxia development in the UGoT: reduced extent (November–May), onset (June), quasi-stable (July–August), expansion (September), and redistribution (October). Unusual DO profiles, characterized by minimum concentrations at mid-depth rather than near the bottom, were also observed. Model simulations indicated that these atypical profiles resulted from the intrusion of cold offshore bottom water into the study area. Nearshore DO dynamics were found to be significantly modulated by offshore circulation, stratification, water quality conditions, and tidal forcing. Scenario simulations further demonstrated that riverine nutrient loading is the primary driver controlling the hypoxia budget, whereas reaeration and tidal mixing also contribute substantially. Overall, the results underscore the critical interplay between physical and biochemical processes in shaping DO dynamics and hypoxia development in the UGoT and its eastern coast. These findings highlight the need for continuous autonomous monitoring and targeted riverine nutrient management to mitigate hypoxia in the eastern UGoT, serving as a representative example of shallow coastal systems near river mouths.