The Sound of Bubbles: Passive hydroacoustics for methane emissions monitoring in shallow artificial waters

• Applicant: Dr. Lorenzo Rovelli
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524348693

Current revisions of the global carbon cycle indicate that inland waters might contribute to nearly half of the total methane (CH4) emissions from natural and anthropogenic sources. Compared to streams, rivers, and medium to large size reservoirs and lakes, CH4 emissions from shallow small water bodies such as ponds, artificial channels and other manmade water infrastructures have been largely overlooked, despite the growing evidence of their contributions to local and large-scale CH4 emissions. In addition, little is known about the role of CH4 ebullition for the total CH4 emissions in these water bodies, also due to limitations in the associated flux measurement techniques. The proposed project aims at filling two pivotal knowledge gaps of the global carbon cycle by investigating (1) the seasonal and annual relevance of total (diffusion + ebullition) CH4 emissions from shallow (<2 m) artificial ponds (floodplain and fishponds) and stormwater retention basins, and (2) to characterize and quantify CH4 ebullition dynamics and their drivers. This will be achieved by complementing traditional flux measuring techniques (e.g., floating chambers and bubble traps) with a continuous monitoring of gas bubbles using the, in freshwaters largely unexplored, approach of passive hydroacoustics (i.e., hydrophones). Hydrophones can detect gas bubbles non-invasively, based on their size-dependent acoustic signature, and are thus very promising tools for high-resolution (several kHz) monitoring of ebullition over periods of days to years. The obtained datasets will provide new insight into ebullition dynamics and allow for more detail assessments of their short-term and long-term drivers in artificial waterbodies. The project’s secondary objective is to provide more insight into CH4 ebullition under high dissolved oxygen (O2) concentrations by investigating possible biases in volume-based CH4 ebullition estimates due to the formation of O2 bubbles. This objective will be mostly pursued via laboratory experiments under controlled settings. The project’s overarching goal is to provide much needed quantifications of ebullition and its relevance for CH4 emissions from shallow small artificial water bodies. Project data will be combined with auxiliary hydro-meteorological and biogeochemical data, to gain more insight into the drivers of short-term ebullition events and seasonal trends and to develop upscaling procedures to include these contributions into large-scale CH4 budgets.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Co-Investigator: Dr. Matthias Koschorreck
• Cooperation Partner: Professor Dr. Daniel Frank McGinnis, Ph.D.