Estuarine dam water discharge enhances summertime primary productivity near the southwestern Korean coast

The gate opening of estuarine dams discharge a large amount of freshwater into coastal zones during the summer monsoon in northeast Asia. We investigated seasonal and spatial variations in primary productivity (PP) and community structures of phytoplankton on a downstream macrotidal flat and examined the dam discharge effects. Our harmonic analysis of the PP revealed a clear seasonality with a unimodal peak in summer, possibly driven by high ammonium loading through internal recycling. External nitrate supply from the dam-water discharge promoted PP during July–August, generating conspicuous blooms near the discharging site. Phytoplankton community was characterized by a predominance of diatoms all year round and seasonal dominances from dinoflagellates, cryptophytes, and prasinophytes in spring to chlorophytes in summer, reflecting spatiotemporal patterns in ammonium and nitrate supply. Our findings provide new insights into the summer phytoplankton bloom linked to monsoonal rainfall in the shallow coastal seas along the Northeast Asian coast.