As soaring temperatures and a prolonged drought push water temperatures in the Nakdong River to around 32°C (90°F), toxic blue-green algae causing algal blooms are rapidly increasing in the lower sections of the river.

According to the Nakdong River Basin Environmental Office and other sources yesterday (August 4), the concentration of harmful blue-green algae measured at the Mulgeum-Maeri station between Yangsan and Gimhae on August 3 was counted at 56,949 cells per milliliter (mL).

This is an 11.5-fold increase from the 4,946 cells recorded on July 27.

During the same period, the number of harmful blue-green algae cells at the Chilseo station between Haman and Changnyeong also increased from 4,618 to 7,113 cells per mL.

Experts analyze that a favorable environment for algal blooms has been created as the river water temperature has recently reached record highs, coupled with slowed water velocities due to a lack of rainfall.

The water temperature at the Mulgeum-Maeri station 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 station since it was designated as an algae alert monitoring point, following the 31.9°C (89°F) measured on August 12, 2024.

The water temperature at the Chilseo station 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 stations.

Under the algal alert system for drinking water sources, an "Attention" stage is issued when harmful blue-green algae cells exceed 1,000 per mL in two consecutive tests, a "Warning" stage for 10,000 or more, and a "Mass Outbreak" stage for 1 million or more.

In the case of the Mulgeum-Maeri station, if the concentration of harmful blue-green algae is confirmed to be 10,000 cells per mL or more in the next survey, 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 at noon on August 2.

The water sampling for harmful blue-green algae analysis was conducted on the day following the opening of the Changnyeong-Haman Weir.

However, the Nakdong River Office explained that considering water travel time and other factors, it is difficult to judge the impact of the weir openings on the algal blooms based solely on this survey result.

Some point out that under conditions of sustained high water temperatures and low precipitation, there are limits to suppressing algal blooms through post-measures such as weir openings alone.

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 other microorganisms to grow, whereas a favorable environment is created for toxic blue-green algae to thrive. If the lack of rainfall continues, slow water velocities will be maintained, potentially worsening the algal blooms."

He added, "Without supportive environmental conditions such as rainfall, measures like opening weirs are hard to produce sufficient effects. Since options become limited after large-scale algal blooms occur, preventive-centered measures are necessary."

(Photo provided by Gyeongnam Korea Federation for Environmental Movements, Yonhap News)