Abtract

  Since the impact of global warming and climate change due to emissions of greenhouse gas is increasingly serious, freshwater aquatic ecosystems are considered to be one of the most important natural carbon sinks. The key process of the carbon cycle and energy flow in the system is photosynthesis where CO₂ is fixed to produce organic compounds by aquatic producers or phytoplankton. The concept of net primary productivity (NPP) is generally used to describe the net amount of energy and CO₂ stored in producer biomass. Nonetheless, phytoplankton primary production depends directly on various physicochemical as well as biological factors. This study investigated the variation of NPP to estimate CO₂ absorption in relation to the influence of physicochemical parameters in tropical freshwater ecosystems by using Srakaew reservoir as a case study. Water samples were collected in three consecutive seasons during September 2018 to April 2019. The results revealed that CO₂ captured by phytoplankton in the reservoir to produce their biomass or NPP range from 350 to 5,777 mg m^-2 day^-1 (mean = 2,813 mg m^-2 day^-1). CO₂ absorption displayed a significant linear relationship with light intensity and water temperature. Seasonal variation can affect NPP and CO₂ absorption. In the hot season, NPP and CO₂ absorption in the water were significantly higher than the cool season while there was no significant difference in the rainy season compared with the other seasons. According to the trophic state assessment, Srakaew reservoir was classified as eutrophic and hypereutrophic due to its low Secchi transparency coupled with high nutrient levels in the water.

Keywords: Tropical reservoir, Primary production, CO₂ absorption, Trophic state