Seasonal Fluxes of Dissolved Nutrients and Suspended Solids at the Tha Chin River Mouth, Thailand (2023–2024)

Abstract

Eutrophication in the inner Gulf of Thailand is strongly influenced by nutrient discharges from multiple river systems, with the Tha Chin River being one of the major sources. This study assessed concentrations and land–sea fluxes of dissolved nutrients and total suspended solids (TSS) at the river mouth from 2022 to 2023 across wet, dry, and transitional seasons of the Tha Chin River. Sampling was conducted every 25 h over full tidal cycles. Dissolved oxygen levels remained critically low throughout the period, ranging from 0.66 to 2.87 mg·L⁻¹. TSS peaked in the transitional season (497.41±347.44 mg·L⁻¹) and was lowest during the wet season (38.64±43.93 mg·L⁻¹). Ammonia (612.91–1,291.49 µg N·L⁻¹) and DIP (165.34–265.42 µg P·L⁻¹) reached hypertrophic levels, with nitrogen identified as the limiting nutrient based on the Redfield ratio. Water fluxes peaked in the wet season (61.26×10⁶ m³·d⁻¹), over five times higher than in the dry season. The Tha Chin River delivered substantial nutrient loads: ammonia fluxes were 38.42 t N·d⁻¹ (wet), 20.20 t N·d⁻¹ (dry), and 36.70 t N·d⁻¹ (transitional). DIP fluxes were 10.43, 4.04, and 4.78 t P·d⁻¹, respectively. TSS flux was highest during the transitional season (4,749.77 t·d⁻¹) and lowest in the dry season (1,571.86 t·d⁻¹). Normalized to watershed area, the Tha Chin River yielded higher net DIN and DIP loads than other major Thai rivers. These findings underscore the river’s key role in eutrophication and red tide formation in the inner Gulf, emphasizing the urgent need for effective watershed and nutrient management to mitigate coastal ecosystem degradation.