Factors affecting harmful algal bloom occurrence in a river with regulated hydrology

The Geum River Basin, South Korea. In-stream structures potentially amplify harmful algal blooms (HABs) by increasing water residence time and deteriorating water quality. Removal or gate control of in-stream structures is considered a management tool to reduce HAB occurrence. Prior evaluation, however, is necessary to assess the effect of structural modifications to control HABs. This study focused on factors affecting HAB occurrence relative to the operation of 3 in-stream weirs using a 3-dimensional hydrodynamic and water quality model. Physical changes due to gate opening could not guarantee an improvement in water quality and reduction in cyanobacterial biomass. Cyanobacteria, a major component of HABs, decreased in the upstream reach but conditions deteriorated downstream during summer. Maximum cyanobacteria concentrations in the downstream reach almost doubled during summer with the open-gate scenario. Although the open-gate flow allowed less time for algal growth, (1) increased light availability with decreased water depth and (2) dissolved phosphorus, which was not depleted upstream, promoted cyanobacterial growth. Consequently, the proliferation of cyanobacterial blooms downstream was due to conditions that enhanced algal growth, despite reduced hydraulic residence time under the open-gate scenario. This analysis indicates that hydrologic regulation may not improve water quality and also may affect spatial and temporal distribution of cyanobacteria in the system. Reducing HABs will likely require a reduction in the pollution load.