Tens of thousands of fish have died in South Korea's Soyang Lake due to toxic hydrogen sulfide released from lake-bottom sediment [1].
The die-offs signal a critical environmental tipping point for the reservoir, where repeated algal blooms have fundamentally altered the water chemistry. This phenomenon threatens the local ecosystem and the livelihoods of fishermen who depend on the lake's health.
Crucian carp, carp, and eel were among the species found dead in the waters upstream of the Soyang River Dam [1]. Experts said the deaths are linked to hydrogen sulfide, a toxic gas generated when decaying green algae settle into thick layers of sediment on the lake floor [1].
This ecological shift follows three consecutive years of algal blooms between 2022 and 2024 [2]. The first major bloom occurred in the summer of 2023, marking the first such event in the 50 years since the Soyang River Dam was completed [3].
Local fishermen reported that the sediment layers have grown increasingly dense over time. Park Sang-kwon, a fisherman operating upstream in Soyang Lake, said that after the rainy season, the water becomes muddy and sediment accumulates every year, leading to a very thick state [4].
These sediment layers act as a reservoir for toxins. When the organic matter from the blooms decomposes, it creates an anaerobic environment that triggers the release of hydrogen sulfide into the water column [1]. The gas is lethal to fish, leading to the mass mortality events reported in 2024 [1].
“Tens of thousands of fish have died in South Korea's Soyang Lake”
The situation at Soyang Lake demonstrates how cumulative environmental stressors can lead to a sudden ecological collapse. While individual algal blooms are common in many reservoirs, the build-up of sediment over three years has created a 'toxic legacy' on the lake floor. This suggests that managing surface water quality is insufficient if the underlying sediment remains contaminated, as the lake-bottom can continue to release lethal gases long after a bloom has subsided.
