Harmful algal bloom dynamics in a tidal river influenced by hydraulic control structures

The Singok weir, an instream structure in the Han River estuary constructed in the 1980s for multiple purposes, has interfered with natural river flow. Removal of this structure has been considered to restore water quality, including reducing algal blooms. Spatiotemporal changes in water quality, hydrodynamics, hydrology, and algal growth resulting from the removal of the Singok weir were analyzed using 3-D hydrodynamic, water quality, and sediment transport modules in the Environmental Fluid Dynamics Code model for a five-year period (2013 ∼ 2017) with field data. Weir removal decreased water depth and thus water volume by >10%, which increased average nutrient concentrations given fixed pollutant inputs. With the weir removal, average cyanobacteria decreased in the upper region during summer but increased in the lower region. However, differences in cyanobacteria biomass were less than 1.0 mg Chl-a/m3 except during 2015, the driest summer, where peak cyanobacteria increased by 20% (11.8 mg/m3). Factors limiting summer algal growth tended to increase modestly during normal flow (<3.1%), which suggests the weir removal can potentially enhance algal growth. The role of hydrology stood out as the major factor determining algal growth, with greater relative importance than nutrients, light, and temperature. Simulations showed minimum discharge of 150 m3/s from the Paldang dam was required to avoid excessive cyanobacteria growth downstream. Hydraulic residence time during summer determined the growth potential of algae; low discharge provided sufficient residence time for algae to proliferate, and conversely. This result also emphasizes the importance of seasonal and hydrodynamic assessments when analyzing cyanobacteria in response to external changes, which can be masked in annual average values by numerous factors.