Anthropogenic Impacts on the Distribution of Dissolved Nutrients at 4 Major River Mouths Discharging into the Inner Gulf of Thailand

Abstract

Anthropogenic activities have been identified as key contributors to nutrient loading and phytoplankton blooms along the eastern coast of the Inner Gulf of Thailand (inner GoT). This study provides a comparative analysis of dissolved nutrient levels and distributions at the river mouths of 4 major basins discharging into the inner GoT in the same period: Maeklong (MK), Thachin (TC), Chaophraya (CPY), and Bangpakong (BPK). Physicochemical parameters were measured and dissolved nutrients, in inorganic and organic forms, were examined using spectrophotometric methods. The study was conducted through monthly and seasonal monitoring (dry and wet seasons) in 2019. Monthly sampling focused on spatial variation between upstream and downstream stations of each river, while additional sites (6 - 8 stations) at the river mouths were used to monitor vertical profile changes during dry and wet seasons. Results indicated that anthropogenic influences significantly affected nutrient concentrations and their spatial distribution. TC exhibited the highest average dissolved inorganic phosphorus (9.39 ± 4.83 µM) and lowest dissolved oxygen (1.43 ± 1.06 mg L−1), reflecting high domestic waste input, organic pollution, and hypoxic conditions. Exceptionally high dissolved organic nitrogen concentrations in TC (284 ± 62.5 µM) and CPY (304 ± 124 µM) suggested metropolitan areas as major DON sources, with the potential to enhance phytoplankton growth in the inner GoT. In BPK, dam operations altered hydrological dynamics and nutrient distribution, while MK, the least disturbed basin, showed the lowest nutrient levels. A negative correlation between nutrient concentrations and salinity, along with vertical nutrient profiles, indicated that water mixing was a dominant process controlling nutrient distribution at all river mouths. This study highlighted the anthropogenic influence and the role of nutrients especially DON in the inner GoT, providing insights into eutrophication, phytoplankton blooms, and deoxygenation - essential for sustaining fisheries and informing site-specific environmental regulations. HIGHLIGHTS Anthropogenic activities influence nutrient dynamics in the Inner Gulf of Thailand (GoT). Dissolved Organic Nitrogen (DON) from urban runoff enhances primary productivity. Dam operations disrupt salinity and nutrient distributions. GRAPHICAL ABSTRACT