Autumn blooms and seasonality of the dinoflagellate Unruhdinium penardii in the Han River (Korea) as tracked by morphological and molecular techniques

Abstract

Abstract Dinoflagellate blooms mainly occur in marine environments, and also occur in freshwater. This study reported the autumn blooms of the freshwater dinoflagellate Unruhdinium penardii var. robustum from the Paldang Reservoir in the Han River, South Korea. Morphological and molecular methods were used to examine 2 years of water samples collected monthly in 2012 and 2019. Environmental factors such as temperature, dissolved oxygen and nutrients showed similar seasonal patterns in 2-year comparisons. The chlorophyll-a and phytoplankton cell concentrations were higher in 2019 than in 2012. Dinoflagellate cells were abundantly detected in autumn, with a maximum of 5.1 × 106 cells·L−1 in 2012 and 3.1 × 106 cells·L−1 in 2019. Of these, the dinoflagellate U. penardii var. robustum was predominant in October 2019. Quantitative polymerase chain reaction assays targeting 28S rDNA of Unruhdinium detected the highest copy numbers (5.1 × 108 copies·L−1 in November 2012 and 8.6 × 108 copies·L−1 in October 2019) in autumn. These patterns matched well with the microscopic cell counts. Canonical correspondence analysis showed that autumn samples were correlated with 28S rDNA of Unruhdinium. These results suggest autumn blooms and seasonality of the dinoflagellate U. penardii var. robustum in the Paldang Reservoir.