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

目前对全球碳循环的修订表明，内陆沃茨可能占自然和人为源甲烷排放总量的近一半。与溪流、河流和大中型水库和湖泊相比，池塘、人工渠道和其他人造水利设施等小型浅水水体的CH4排放量在很大程度上被忽视，尽管越来越多的证据表明，这些水体对当地和大规模的CH4排放有贡献。此外，鲜为人知的是，在这些水体中的总CH4排放量的CH4沸腾的作用，也由于相关的通量测量技术的限制。拟议的项目旨在填补全球碳循环的两个关键知识空白，调查（1）浅（<2米）人工池塘（洪泛区和鱼塘）和暴雨蓄水池的甲烷总排放量（扩散+沸腾）的季节性和年度相关性，（2）描述和量化甲烷沸腾动态及其驱动因素。这将通过补充传统的通量测量技术（例如，浮室和气泡捕集器），在很大程度上未被开发的淡水中，使用被动水声学方法（即，水听器）。水听器可以根据气泡的尺寸依赖性声学特征，非侵入性地检测气泡，因此是非常有前途的工具，可用于在数天至数年的时间内对沸腾进行高分辨率（几kHz）监测。所获得的数据集将为沸腾动力学提供新的见解，并允许对其在人工水体中的短期和长期驱动因素进行更详细的评估。该项目的第二个目标是通过调查由于O2气泡的形成而导致的基于体积的CH4沸腾估计中的可能偏差，提供对高溶解氧（O2）浓度下CH4沸腾的更多了解。这一目标将主要通过受控环境下的实验室实验来实现。该项目的总体目标是提供急需的沸腾量化及其与小型浅水人工水体CH4排放量的相关性。项目数据将与辅助水文气象和地球化学数据相结合，以更深入地了解短期沸腾事件和季节趋势的驱动因素，并制定升级程序，将这些贡献纳入大规模CH4预算。

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.