Key Factors Controlling Primary Production and Cyanobacterial Harmful Algal Blooms (cHABs) in a Continuous Weir System in the Nakdong River, Korea

To identify key factors that control primary production (P.P.) and trigger cyanobacterial harmful algal blooms (cHABs), we investigated spatio-temporal variations in P.P. in a continuous weir system in the Nakdong River once or twice a month from April to October 2018. P.P. was measured through an in-situ incubation experiment using a(13)C tracer. Relative proportion of pigment-based phytoplankton composition was calculated by the CHEMTAX program based on pigment analysis using a high-performance liquid chromatography (HPLC). P.P. was higher in spring (1130 +/- 1140 mg C m(-2)d(-1)) and summer (1060 +/- 814 mg C m(-2)d(-1)) than autumn (180 +/- 220 mg C m(-2)d(-1)), and tended to increase downstream. P.P. was negatively related to PO43-(r = -0.41,p< 0.01) due to utilization by phytoplankton during the spring and summer when it was high. The relative proportion of pigment-based cyanobacteria (mainlyMicrocystissp.) was positively correlated with water temperature (r = 0.79,p< 0.01) and hydraulic retention time (HRT, r = 0.67,p< 0.01), suggesting that these two factors should affect cHABs in summer. Therefore, to control HRT could be one of the solutions for reducing cHABs in a continuous weir system.