[News Terms] Algal Bloom Spreads Amid Heatwave... 'Algal Alert' Issued

Bird Warning System Operated in 29 Rivers and Lakes Nationwide
Cyanobacteria Toxins, Cause of Green Algae, Affect Human Health

The ‘Algal Bloom Warning System’ is a government system that issues warnings in stages according to the severity of green algae occurrence. It targets 29 rivers and lakes nationwide that serve as raw water sources for tap water and are used for water-related activities such as swimming and fishing. It was first implemented in 1998 for four lakes: Paldangho, Daecheongho, Chungjuho, and Juamho, and was expanded in 2016 to include areas for water-related activities.

Recently, with the continuous heatwave, green algae are spreading in rivers and lakes across the country. Green algae (녹조, Nokjo) refers to the phenomenon where harmful cyanobacteria proliferate massively in rivers and lakes, turning the water green. As of the 28th, the ‘Alert’ stage has been issued at three points along the Nakdong River: Gangjeong, Goryeong, Chilseo, and Mulgeum, Maeri. The Nakdong River is the water source for Gyeongsang Province, including Busan. The same applies to the Seoul metropolitan area and Chungcheong region. The ‘Alert’ stage has also been issued for Daecheongho and Boryeongho in the Geum River basin. The point in front of Paldang Dam within Paldangho in the Han River system is at the ‘Attention’ stage of the algal bloom warning. This is the first time in six years since 2018 that an algal bloom warning has been issued for Paldangho, the water source for the Seoul metropolitan area.

Checking the green algae in the Nakdong River by hand [Photo by Yonhap News]

Cyanobacteria are most active when water temperatures are between 20 and 30 degrees Celsius. In Korea, green algae phenomena appear from summer, when water temperatures rise above 20 degrees, through early autumn. The problem lies in the toxins produced by cyanobacteria. Some cyanobacteria that cause green algae produce toxins that affect the human body. Therefore, the Ministry of Environment designates and manages four harmful cyanobacteria species (Microcystis, Anabaena, Oscillatoria, Aphanizomenon). In particular, ‘Microcystin’ can affect the liver cells and nervous system of mammals.

Through weekly monitoring, if the number of harmful cyanobacteria cells exceeds the stage-specific criteria twice consecutively, the head of the watershed or local environmental office or the mayor/governor issues an algal bloom warning. The warning stages are ‘Attention,’ ‘Alert,’ and ‘Massive Outbreak’ in order. For water source areas, there are three stages (Attention, Alert, Massive Outbreak), and for water-related activity areas, two stages (Attention, Alert). The ‘Attention’ stage is issued when cell counts exceed 1,000 cells/mL, the ‘Alert’ stage at 10,000 cells/mL, and the ‘Massive Outbreak’ stage at 1,000,000 cells/mL. When an algal bloom warning is issued, local governments and related agencies such as the Korea Water Resources Corporation take measures according to each stage, including strengthening water treatment, removing algae, and recommending restraint or prohibition of water-related activities.

Algae removal vessel working on algae removal [Photo by Yonhap News]

The Ministry of Environment announced that the total number of days with algal bloom warnings nationwide last year was 476 days. This analysis is based on the number of days with warnings from June 8, when the first warning was issued at the Nakdong River Chilseo point, through the end of November, reflecting the nationwide green algae occurrence situation. This is a 36% decrease (267 days) compared to the same period in 2022. In particular, the number of days with algal bloom warnings in the Nakdong River basin, which had been troubled by green algae every year, significantly decreased. During this period, the number of days with algal bloom warnings in the Nakdong River basin was 267 days, about 40% of the 665 days in 2022. The number of days at the ‘Alert’ stage was only 14 days.

The Ministry of Environment analyzed that the nationwide rainfall last year increased by 1.4 times (from 1187 to 1722 mm) compared to 2022, which was favorable for responding to green algae, and that comprehensive green algae countermeasures, including preventive measures and the introduction of algae removal boats, had some effect. Kim Jong-ryul, Director of Water Environment Policy at the Ministry of Environment, stated, “The occurrence of green algae has significantly decreased due to the effects of various measures such as collecting piled-up compost and weather influences.”