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Climate change fuels record algal blooms in major Korean water sources

Published: 16 Sep. 2025, 07:00

Green algae removal vessels work on green algae from the upper reaches of Soyang Lake in Chuncheon, Gangwon [CHON KWON PIL]

Korea's major rivers and reservoirs — including the Han River, which supplies drinking water to 20 million people — are seeing record algal blooms fueled by stronger heat waves and torrential rains linked to climate change, raising concerns over water safety and even greenhouse gas emissions.

Soyang Lake in the upper reaches of the Han River, a key water source for the Seoul metropolitan area, was blanketed in green algae resembling spilled paint late last month. Boats were deployed to suction up the blooms, marking the third consecutive year the reservoir has been plagued by large-scale outbreaks.

"Farmers around here use enormous amounts of pesticides and fertilizer for highland crops, and when the summer rains come, it all flows into Soyang Lake," said Shim Young-in, a 71-year-old fisherman. "That water eventually travels down the Han River to Seoul, and the algae problem is getting worse."

Climate change is erasing zones that were safe from algae. Stronger summer heat waves and heavier downpours are creating ideal conditions for blooms to thrive in rivers and lakes.

As of late August, algae warnings had been issued for a combined 412 days at 29 points across the country's four major rivers — the Han, Nakdong, Geum and Yeongsan, according to data from the Ministry of Environment.

That is more than double the figure for the same period in 2022 and 1.4 times higher than last year, which was marked by extreme heat waves. It is the highest number since the alert system was expanded in 2020.

Green algae is sucked up by a removal vessel from the upper reaches of Soyang Lake in Chuncheon, Gangwon [CHON KWON PIL]

Even the Han River and the Yeongsan River, once regarded as relatively algae-free, saw alerts this summer, with 14 days for the Han River and 15 days for the Yeongsan River. In fact, the Yeongsan's Juam Reservoir, a water source for Gwangju and South Jeolla, was placed under algae alert for the first time in 14 years.

The Korea Water Resources Corporation, which manages Juam Dam, said, "Water is being drawn from deeper, algae-free layers and treated before supply," assuring there is no impact on tap water quality.

Still, conditions along the Han River, which supplies drinking water to the capital region, are deteriorating. Analysis of water quality at Seoul points along the river since 2010 shows rising concentrations of cyanobacteria, the harmful blue-green algae responsible for summer blooms.

Excluding unusual drought-related spikes in 2015 and 2018, cyanobacteria concentrations in the 2010s never exceeded 3,000 cells per milliliter. But since 2021, levels have surpassed 5,000, and in both 2023 and 2024 came close to 9,000. On Sept.4, levels at Hannam Bridge reached 8,829 cells per milliliter.

A view of the Sinpyeong Bridge area near Juam Lake in Suncheon, South Jeolla on Aug. 26. This is the first time in 14 years since 2011 that an algae alert has been issued for Juam Lake, a water source for Gwangju and South Jeolla. [YONHAP]

Fishing boats mobilized to suppress algal blooms in the upper reaches of Soyang Lake are seen causing waves on the riverbank [CHON KWON PIL]

Experts say stronger summer storms and heat waves are the main drivers. Over the past decade, the number of hot days in Seoul rose from nine in 2014 to 27 this year. The proportion of days with heavy rainfall of at least 30 millimeters also grew, from 15.4 percent in 2014 to 20.6 percent this year.

Normally, steady monsoon rains suppress algae blooms, but extreme downpours wash phosphorus and nitrogen into rivers — key nutrients for algae growth. With abundant nutrients and strong sunlight, blooms thrive.

"Over the past decade, summer rainfall has correlated strongly with rising total phosphorus levels in the Han River," said Kim Ja-yeon, a postdoctoral researcher at Ohio State University's College of Public Health. "The trend of phosphorus increasing with rainfall is particularly clear in the Han River."

Choi Geun-jae — a researcher at Seoul National University's Climate Research Lab who studied the relationship between climate change and algal blooms — analyzed that when the water level of the Han River suddenly rose due to heavy rains and then a heat wave arrived, water temperatures rose, leading to a marked increase in algae, including cyanobacteria.

A research team from Ewha Womans University conducts water collection at the Hangang River's Haengju Bridge [JEONG EUN-HYE]

The problem extends beyond water quality. Proliferating algae are turning the Han River into a source of greenhouse gases like methane, worsening climate change.

A research team at Ewha Womans University analyzed Han River water samples collected at Haengju Bridge on July 25. They found carbon dioxide at 3,783.55 µatm, methane at 1,497.83 nanomoles per liter and nitrous oxide at 48.56 nanomoles per liter at levels far above natural river averages, indicating the river itself is emitting greenhouse gases.

If climate change drives more frequent algae outbreaks, risks to water safety could intensify. In the Nakdong River, the main water source for the Yeongnam region, the number of algae alert days could rise by as much as 45 with continued warming.

"In this climate, preventing pollutants such as untreated sewage from flowing into rivers is critical," said Chung Se-Woong, a professor of environmental engineering at Chungbuk National University. "Comprehensive basin-wide management of phosphorus and nitrogen is needed to respond to worsening algae conditions."

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.
BY CHON KWON-PIL, JEONG EUN-HYE [[email protected]]