A harmful green-algae bloom has triggered an emergency at Daecheong Lake, a primary drinking-water source for South Korea's Chungcheong region.

The situation is critical because the lake provides essential water for the region's population, and the surge in cyanobacteria can compromise water quality and safety.

Authorities have issued a "caution" algae alert for the Hoenam sector of the lake. The alert follows a period of prolonged heat that pushed water temperatures to approximately 30 °C [1]. These conditions have promoted the rapid growth of harmful cyanobacteria, turning the water a vivid green.

Kim Ki-su said the lake looked as if paint had been dissolved into it, leaving the water entirely green [1]. The environmental shift has led to a significant spike in algae concentrations.

According to data, the cyanobacteria cell count in the Hoenam sector exceeded 20,000 cells per millilitre [1]. This figure is about twice the standard alert threshold [1].

Kim Ki-su said the rise in water temperature to around 30 °C has made the proliferation of harmful cyanobacteria more active, raising concerns about the continued spread of the bloom [1]. Local officials are monitoring the water levels, and the density of the algae, to determine the necessary filtration and treatment responses to protect the public water supply.

“The lake looked as if paint had been dissolved into it, leaving the water entirely green.”

The emergence of a 'caution' level alert at Daecheong Lake underscores the vulnerability of regional water infrastructure to extreme weather. As heat waves increase water temperatures, the resulting cyanobacteria blooms create a dual challenge: the immediate need for advanced chemical treatment to remove toxins and the long-term necessity of managing nutrient runoff to prevent such blooms from becoming permanent features of the ecosystem.