A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Environmental Engineering and Management

Thesis (M. Sc.) - Asian Institute of Technology, 2025

Small artificial waterbodies (SAWs), such as ponds, canals, and ditches, are widespread and increasingly recognized as significant contributors to methane (CH₄) emissions. Their high organic matter loading, susceptibility to eutrophication, and limited water flow create ideal conditions for methane production. Despite their ecological importance, the roles of key biotic and abiotic factors in methane flux dynamics remain poorly understood. This study investigated the influence of electron acceptors (oxygen, nitrate, nitrite, acetate, and sulfate) and macrophyte plants (free-floating fern and water lily) on methane flux in a small artificial canal. Methane fluxes were measured using static chambers over a 96-hour period under controlled conditions, including substrate amendments and the presence of macrophytes. Water quality parameters such as pH, dissolved oxygen (DO), temperature and nutrient concentrations (e.g., nitrate, nitrate, nitrite, sulfate, and total phosphorus) were monitored across all experiments to determine their correlation with methane flux. The study also evaluated the role of macrophytes, which may either enhance or reduce methane emissions through various mechanisms. By elucidating the relationships between biotic and abiotic factors influencing methane flux, this research provides critical insights into methane emissions from small artificial waterbodies. The study found that acetate significantly increased both methane (CH₄) and carbon dioxide (CO₂) emissions, while sulfate treatment moderately reduced CH₄ but unexpectedly lowered CO₂ levels, likely due to microbial inhibition by sulfide. In contrast, nitrate/nitrite amendments sustained longer activity by supporting denitrifying bacteria and anaerobic methane oxidation (DAMO/n-DAMO). Macrophyte treatments achieved the strongest CH₄ suppression, at times reducing emissions below detection, attributed to physical barriers, rhizospheric oxidation, and prolonged gas residence times. Oxygen, which is commonly deficient in SAWs, can effectively mitigate methane in the form of aeration. Water quality analysis revealed inverse relationships between CH₄ and dissolved oxygen/nitrate/nitrite, while sulfate negatively correlated with CO₂, highlighting the role of electron acceptor availability, redox conditions, and microbial processes in regulating greenhouse gas emissions from freshwater systems. These findings will contribute to improving greenhouse gas inventories and developing climate change mitigation strategies for urban and agricultural water management.