Greenhouse gas (CO2, CH4) exchange between the atmosphere and freshwater bodies in the south of Vietnam

Abstract

The role of lakes/reservoirs in the global carbon cycle remains controversial due to the coexistence of methanogens and methanotrophs in this ecosystem type. The study aims to compare greenhouse gas (CH4 and CO2) emissions between three reservoirs (Trị An, Dầu Tiếng, and Buôn Tua Srah) and two natural lakes (Lắk and Bàu Trắng), associated with the influence of water physico chemical properties on the emission quantities. Field data were collected in October 2020 and April 2021, coinciding with the wet and dry seasons, respectively. An autoanalyzer ultraportable gas analyzer was used to measure the rates of greenhouse gas flux es exchanged between the water bodies and the atmosphere in a transparent floating chamber at one Hz frequency within five to eleven hours, depending on the weather condition. Water quality parameters, including pH, temperature, conductivity, turbidity, OR P, and DO were measured hourly. The results indicated that CO2 dominated the total carbon emission quantities in the lakes and reservoirs. The CH4 and CO2 amounts released hourly were higher in the natural lakes relative to the reservoirs and remarkably lo wer during the dry season compared to the wet season. Trị An reservoir functioned as a carbon sink in both seasons, with significantly higher influxes in the wet season. The CO2:CH4 ratios ranged drastically between the water bodies, and the low values of these ratios suggested a higher contribution of CH4 to the total carbon emissions in the natural lakes. The DO and ORP values were low, implying contamination in these water bodies. Oxygen competition probably resulted in the decreased population size of p hotosynthetic organisms, which in turn, could lead to a reduced rate of hourly carbon emission during the dry season. These findings may affirm the contribution of microalgae and other photosynthetic microorganisms to the role of carbon sink/source of a fr eshwater body.