August 4, 2026 | 06:53 pm
TEMPO.CO, Jakarta - The National Research and Innovation Agency (BRIN) has revealed the results of an analysis on the suspected cause of the mass death of tens of tons of fish in Lake Batur, Bangli Regency, Bali in late July. Preliminary findings suggest the event is linked to a natural process where water masses from the lake's bottom layer rise and mix with surface water—a common occurrence during the dry season.
Sulung Nomosatryo, a biogeochemistry researcher at BRIN’s Research Center for Limnology and Water Resources, explained that the lake's bottom layer contains lower oxygen levels than the surface water. This condition is driven by the long-term decomposition of organic matter and the accumulation of sulfide compounds.
He explained that when water masses mix, oxygen-poor water from the depths rises to the surface, carrying sulfides into the fish's habitat. "Under certain conditions, sulfides can transform into hydrogen sulfide, which is the most toxic form for fish," Sulung said, as cited in a written statement on Monday, August 3, 2026.
Sulung noted that during the incident, the research team found dissolved oxygen levels at the lake's surface dropping to near zero milligrams per liter. Concurrently, a pungent odor of hydrogen sulfide was detected.
"However, we cannot immediately conclude that sulfide was the sole cause of the fish deaths," he said. "Laboratory analysis and an assessment of all water quality parameters are required to fully understand what caused this to happen."
Located in a Volcanic Area
Arianto Budi Santoso, a lake hydrodynamics researcher at BRIN, explained that Lake Batur's nature as a volcanic lake situated near Mount Batur results in relatively high levels of sulfur compounds in the area. According to Arianto, under oxygen-depleted (anoxic) conditions, sulfate compounds at the lake bottom convert into sulfides.
When water mixing occurs, these sulfides rise to the oxygen-rich layer and undergo oxidation, converting back into sulfates. Arianto noted that this process also consumes the lake's dissolved oxygen reserves. “Based on research findings, the mixing period in Lake Batur typically occurs from June to August, in late December, and in February,” Arianto stated in the same release.
Published studies indicate that Lake Batur is a polymictic lake—meaning it undergoes water mass mixing multiple times a year. In this area, processes such as sulfide oxidation, organism respiration, and the decomposition of organic matter can accelerate the depletion of oxygen levels during the mixing process.
Read: Microplastics Threaten Fish Most in Mating Season, Study Finds
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