Simulation of algal bloom dynamics in a river with the ensemble Kalman filter

•The EnKF was applied for the first time to algal dynamics simulation in a river.•The effect of data assimilation of chlorophyll-a on phosphate simulation was investigated.•The effect of data assimilation in terms of spatial distribution of chlorophyll-a was examined.•Uncertainty portions attributed to flow, chlorophyll-a, phosphate were quantified.•EnKF was found applicable for water quality simulation.