The influence of opening river weir gates on algal biomass – nutrient relationships on a large scale has been understudied in the fluvial ecosystem. The 5 waterbodies (weir impoundments and dam reservoirs) formed by 3 middle-stream weirs, 1 estuary dam, and 1 upstream dam, all of which are in the same main stem of the third largest river (Geum River) in South Korea, were compared quantitatively between the closed and opened periods of weir gates in chlorophyll a (Chl) responses to total phosphorus (TP). Regardless of whether weir gates were opened or closed, weir impoundments had a higher Chl:TP ratio than dam reservoirs did. However, the opened gate period, irrespective of waterbody type, had a lower Chl:TP ratio than the closed gate period did. These results corroborated that, after weir gate openings, algal biomass accruals were somewhat limited because of increases in flow rates and reductions in hydraulic residence time, but a significant reduction in the Chl:TP ratio was not displayed. Furthermore, the Chl:TP ratio differed among weir impoundments, likely because of bottom topographical heterogeneity caused by declines in water levels. Long-term and periodic monitoring of the algal responses to P varying with river hydrology as well as basin morphometry is required to verify whether the possibility of algal biomass accruals following weir gate openings has significantly become attenuated.