Influence of tidal phenomena on nutrient variations in water column of the Chao Phraya River estuary

¹ Department of Environmental Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
² Program in Industrial Toxicology and Risk Assessment, Department of Environmental Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
³ Research Unit (RU) of Waste Utilization and Ecological Risk Assessment, Chulalongkorn University, Bangkok 10330, Thailand

^* Corresponding author: sarawut.sr@chula.ac.th

Abstract

The Chao Phraya River estuary is a vital ecosystem characterized by its dynamic interactions between freshwater and saltwater, driven by tidal cycles. The research aims to elucidate how tidal cycles impact concentrations of ammonium (NH₄⁺), nitrate (NO₃^-), and phosphate (PO₄^3-), and their subsequent effect on phytoplankton biomass, indicated by chlorophyll a (Chla) concentrations. Water samples were collected over a 24-hour period during different tidal phases—high tide, low tide, ebb tide, and flood tide—from multiple depths in the estuary. The analysis revealed significant variations in nutrient concentrations corresponding to tidal changes. Ammonium levels ranged from 0.10 to 0.48 mg/L, with the lowest concentrations observed during high tide and the highest during flood tide. Nitrate concentrations varied between 2.10 and 2.82 mg/L, showing the highest values during flood tide. Phosphate concentrations fluctuated from 0.013 to 0.117 mg/L, with higher levels during flood and low tides. Despite these fluctuations in nutrient concentrations, Chla levels, which ranged from 0.460 to 9.192 µg/L, did not exhibit significant variation in response to tidal changes, indicating a minimal impact on phytoplankton biomass. These findings highlight the critical role of tidal dynamics in influencing nutrient distribution within the estuary, with lower nutrient concentrations typically observed during high tide due to dilution effects. The lack of significant changes in Chla concentrations suggests that while nutrient levels are affected by tidal cycles, phytoplankton growth remains relatively stable. Understanding the interactions between tidal cycles and nutrient dynamics is essential for effective estuarine management. This study underscores the importance of incorporating tidal influences into strategies aimed at mitigating nutrient pollution and preserving the ecological health of estuarine environments.