Occurrence of Harmful Blue-green Algae at Algae Alert System and Water Quality Forecast System Sites in Daegu and Gyeongsangbuk-do between 2012 and 2019

Objectives:We analyzed the occurrence of blue-green algae at Algae Alert System and Water Quality Forecast System sites in Daegu and Gyeonsangbuk-do between 2012 and 2019. Methods:Data from 17 sites in Daegu and Gyeonsangbuk-do were collected and reclassified to 8 Nakdong river (ND) sites and 9 water source (WS) sites. Influencing factors on the occurrence of blue-green algae were investigated. Results and Discussion:At the ND sites, blue-green algae were observed in the range of 0-495,360 cells/mL. Between 2012 and 2019, the average number of blue-green algae increased as the sites go downstream from ND-1 (Sangju weir) to ND-8 (Dalseong weir), while the number of blue-green algae was lower at the site in-between weirs than the site adjacent to weir. At the WS sites, blue-green algae were observed in the range of 0-112,000 cells/mL. The average numbers of blue-green algae in 2014, 2015 and 2017 were higher than those of other years. The dominant species of blue-green algae was Microcystis at all the sites during the summer when the water temperature was high; when the water temperature was low Aphanizomenon had higher rates of dominance. Water temperature was positively correlated with the number of blue-green algae grown, while the dissolved oxygen concentration was found to have a negative correlation with it. pH and chlorophyll-a were less correlated. Conclusions:In Daegu and Gyeongsangbuk-do, the main stream of the Nakdong river showed higher blue-green algae occurrence than the water source sites. In most cases, Microcystis was dominant species. In the main stream of Nakdong river, blue-green algae tended to occur more downstream, and the number of blue-green algae was higher at sites close to weir than sites in-between weirs. The occurrence of blue-green algae was highly related to water temperature.