Comparison of the phytoplankton community compositions between the temperate reservoir and the downstream river areas of the Han River, Korea

Phytoplankton communities serve as useful bioindicators of environmental changes in aquatic ecosystems and can be used to assess the health of freshwater systems. Their community composition responds sensitively to changes in habitat conditions. As a result, the phytoplankton dynamics of the freshwater reservoir may differ from those of the downstream river. In the present study, we conducted a comparative analysis of the phytoplankton community between a freshwater upstream reservoir (UR) and the downstream river (DR) sections of the temperate freshwater Han River (Korea), using morphological analysis. Both UR and DR exhibited similar seasonal successions of phytoplankton groups. Diatoms dominated in autumn, winter and spring, while cyanobacteria dominated during the summer. However, the composition and frequency of the dominant species varied between UR and DR. Particularly during summer, <i>Anabaena</i> sp. was found to be the most frequent at UR, while <i>Microcystis</i> sp. was the most frequently detected at DR. In addition, high abundances of the chlorophyte <i>Eudorina elegans</i> and the dinoflagellate <i>Unruhdinium penardii</i> var. <i>robustum</i> were found only at DR during the autumn. The dominance and species selection of the phytoplankton community were found to be correlated with water temperature and nutrient concentrations. These results may be helpful to understand the relationship and differences in phytoplankton dynamics in reservoirs and river sections of temperate freshwater and provide guidance for maintaining the health of freshwater systems.