Seasonal Fluxes of Dissolved Inorganic Nutrients and Suspended Solids at the Mouth of the Prasae River, Eastern Thailand
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Abstract
This study investigates the seasonal fluxes of freshwater, suspended solids, and dissolved inorganic nutrients (ammonia, nitrite, nitrate, phosphate, and silicate) at the Prasae River mouth in eastern Thailand. Water sampling and flux measurements were conducted over eight time points between 2016 and 2017, covering both dry and wet seasons. Water quality and suspended solids showed pronounced seasonal and vertical variations driven by monsoon rainfall and tides, with strong stratification in October 2016 and September 2017. Ammonia spiked to >600 µg N·L-1 in June 2016 and ~280 µg N·L-1 in September 2017, while nitrite (~10 µg N·L-1) and nitrate (~350 µg N·L-1) peaked in October 2016, reflecting rapid terrestrial input. Phosphate reached ~100 µg P·L-1 during peak runoff, and silicate remained high (>1,000 µg Si·L-1), peaking near 3,000 µg Si·L-1. Suspended solids were low at the surface (<40 mg·L-1) but elevated near the bottom, indicating sediment resuspension. River fluxes showed strong monsoonal influence, with peak wet-season exports of nitrate (>2,200 kg N·d-1), phosphate (~480 kg P·d-1), and silicate (~10,000 kg Si·d-1); ammonia and nitrite fluxes were more variable. During the wet season, nutrient exports were mainly discharge-driven, whereas internal processes were more influential in the dry season. TSS fluxes were episodic, indicating alternation between source and temporary sink. Compared with the Rayong, Trat, and Chanthaburi Rivers, the Prasae River shows elevated inorganic nutrient levels.
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