The Effect of the Sesan 2 Dam and High/Low Flow Regimes in the Limnology of the 3S System in Cambodia

Abstract:

The Mekong River is the world’s 3rd highest in fish biodiversity and home to several critically endangered fish species. It supports the largest freshwater fishery, from which local communities depend for their economy and as a primary protein food source. Recently, planning for new proposed dams has increased in Cambodia, and in January 2017 the SESAN 2 dam was built in the Sesan/Srepok confluence, two tributaries of the Mekong River. We assessed the influence of the Sesan 2 dam and flow variations on the limnology of the system and consequently on gas emissions (pCO₂ and pCH₄) throughout a hydrological year. We sampled three locations above the dam, two in the reservoir created by the dam, and four below the dam and in the main stem of the Mekong river. We found that chlorophyll a was variable but generally greater above the reservoir in the wet season and higher within the reservoir in the dry season. Above the dam sites were autotrophic, while became heterotrophic below the dam. CO₂ gas levels were greater below the dam during the wet and dry seasons but lower than the nearby Tonle Sap River. CO₂ gas emissions were highest below the reservoir during the wet and dry seasons compared to within and above the reservoir, with some locations sequestering carbon depending on the algal biomass. Within the Lower Sesan 2 Reservoir, CO₂ gas concentrations increased with depth in the dry and wet seasons. Total organic carbon was greatest below the dam and in the Mekong River near Phnom Penh than above and within the reservoir. Dam operations profoundly alter the natural flow regimes in the Sesan/Srepok confluence producing changes in water quality and gas emissions both below and above the dam.