▲ Harmful algae bloom near Changnyeong-Haman Weir on the 3rd
As heatwaves and drought drive water temperatures in the Nakdong River to around 32°C (90°F), harmful blue-green algae causing toxic blooms are rapidly increasing in the lower reaches.
According to the Nakdong River Basin Environmental Office and other sources yesterday (August 4), the concentration of harmful blue-green algae cells measured at the Mulgeum-Meri point between Yangsan and Gimhae on August 3 was tallied at 56,949 cells per 1 milliliter.
This is an 11.5-fold increase from the 4,946 cells measured on July 27.
Over the same period, at the Chilseo point between Haman and Changnyeong, the number of harmful blue-green algae cells also increased from 4,618 to 7,113 cells per 1 milliliter.
Experts analyze that a favorable environment for algae proliferation has been created as recent water temperatures in the Nakdong River have reached record highs, coupled with slow water velocity due to a lack of rainfall.
The water temperature at the Mulgeum-Meri point the previous day was 31.8°C (89°F), which is 2.2°C higher than the 29.6°C (85°F) recorded on July 27.
This is the second-highest water temperature recorded at this point since it was designated as an algae alert observation site, following the 31.9°C (89°F) measured on August 12, 2024.
The water temperature at the Chilseo point also rose from 30.4°C (87°F) to 32°C (90°F) over the same period.
Currently, an algae alert "Attention" stage is in effect for both points.
Under the algae alert system for public water supply sources, an "Attention" stage is issued when the number of harmful blue-green algae cells exceeds 1,000 per 1 milliliter in two consecutive tests, a "Warning" stage for 10,000 or more, and a "Major Outbreak" stage for 1 million or more.
In the case of the Mulgeum-Meri point, if the next survey also confirms 10,000 or more harmful blue-green algae cells per 1 milliliter, the algae alert will be upgraded to the "Warning" stage.
The Ministry of Climate, Energy and Environment has been sequentially opening four weirs in the lower Nakdong River from July 30 to August 5 to reduce algal blooms.
This marks the first opening of weirs under the seasonal algae management system introduced this year, with the Changnyeong-Haman Weir opened starting at noon on August 2.
The water sampling for harmful blue-green algae analysis was conducted the day after the opening of the Changnyeong-Haman Weir.
However, the Nakdong River Office explained that considering the travel time of water, it is difficult to judge the impact of the weir opening on algal blooms based solely on the results of this survey.
Some point out that under conditions of sustained high water temperatures and low rainfall, there are limitations to suppressing algal blooms solely through post-hoc measures such as opening weirs.
Lee Seung-jun, a professor in the Department of Applied Biosciences at Kyungpook National University, explained, "Generally, when water temperatures exceed 28°C (82°F), conditions become unfavorable for the growth of other microorganisms, whereas an environment favorable for the proliferation of harmful blue-green algae is created." He added, "If the lack of rainfall continues, water velocities will remain slow, potentially worsening the algal blooms."
He continued, "Unless environmental conditions such as rainfall are met, it is difficult for measures like opening weirs to produce sufficient effects," and noted, "Since options become limited after large-scale algal blooms occur, proactive, prevention-focused measures are necessary."
(Photo provided by Gyeongnam Federation for Environmental Movements, Yonhap News)
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