The Influence of Nutrients on Phytoplankton Growth in Three Coastal Areas with Different Freshwater Inflow Characteristics

This study examines the influence of various freshwater inflows on estuarine water quality across three regions: Wando, the Seomjin River estuary, and the Yeongsan River estuary. Wando receives minimal freshwater inflow, the Seomjin River experiences continuous input, and the Yeongsan River is regulated by embankments, leading to intermittent inflows. The study focuses on seasonal and spatial variations in nutrients, phytoplankton growth, and organic matter production using long-term data. In the Yeongsan River estuary, high Chl-a levels were recorded in June and August; consequently, the concentrations of DIN and DIP were low. In contrast, the Seomjin River estuary exhibited lower Chl-a levels during the same periods, with consistently higher DIN and DIP concentrations. A strong negative correlation between Chl-a and DIN (R² = 0.79) and DIP (R² = 0.87) was found in the Wando region, indicating nutrient availability’s key role in phytoplankton growth. In contrast, the Seomjin River estuary showed negligible correlations. Nitrogen was identified as the limiting nutrient in the Wando and Seomjin regions during specific months, while phosphorus was consistently limiting in the Yeongsan River estuary. n summer, COD levels in the Seomjin River estuary rose by 0.7 mg/L, whereas in the Yeongsan River estuary, they increased by 1.2 mg/L. These findings suggest that the estuary embankment in the Yeongsan River significantly influences nutrient concentrations and phytoplankton production, contributing to the region’s elevated productivity.