Gas bubbles in freshwater ecosystems: origin, fate, and bubble-mediated transport

在水生生态系统中，以气泡形式存在的游离气体普遍存在，气泡的大小从微米到厘米不等。气泡不仅影响水的物理性质，还促进与全球生物地球化学循环和气候有关的重要运输途径。在空气-水界面，气泡加速气体交换，从而影响全球碳循环。水生沉积物释放的气泡(沸腾)是甲烷进入大气的重要途径。此外，气泡介导的输运不仅与气体有关。上升的气泡在其表面输送颗粒、溶质和细菌。这种物质，包括碳、营养物质和污染物，来自沉积物，或在气泡上升期间从水柱中清除出来的。尽管有这种潜在的重要性，但人们对淡水生态系统中的气泡及其特征知之甚少，现有的知识主要是基于对海洋系统的观察。在这个项目中，我们调查了控制淡水生态系统中气泡丰度和特征的过程，以及对气体、溶解物质和颗粒物质运输作用的评估。我们区分了由水面空气夹带产生的气泡、由于过高的溶解气体压力而在远洋地带成核的气泡和在水生沉积物中形成的气泡。我们假设这三种不同类型的气泡具有不同的性质。在野外测量和实验室实验的基础上，我们将描述控制这三种气泡及其所输送物质的产生、老化和命运的过程和环境变量。这些观察和发现将与基于过程的模型相联系，以开发一种新的框架，用于评估水生生态系统中气泡介导的传输的相关性。这一新知识有望为水生研究、监测和管理的各个领域做出重要贡献。

The presence of free gas in form of bubbles ranging in size from micrometers to centimeters is ubiquitous in aquatic ecosystems. Bubbles not only affect the physical properties of water, they also facilitate an important transport pathway with relevance for global biogeochemical cycling and climate. At the air-water interface, bubbles accelerate gas exchange and therewith affect global carbon cycling. Bubbles released from aquatic sediments (ebullition) are an important pathway for methane to the atmosphere. Moreover, bubble-mediated transport is not only relevant for gases. Rising gas bubbles transport particles, solutes and bacteria on their surfaces. This material, including carbon, nutrients and pollutants, originates from the sediments or had been scavenged from the water column during bubble rise. Despite this potential importance, little is known about gas bubbles and their characteristics in freshwater ecosystems, existing knowledge is primarily based on observations in marine systems. In this project, we investigate the processes that control the abundance and characteristics of gas bubbles in freshwater ecosystems, along with an assessment of the role in transporting gases, dissolved and particulate matter. We distinguish between bubbles generated by air entrainment at the water surface, bubbles nucleating in the pelagic zone due to excess dissolved gas pressure and bubbles formed in aquatic sediments. We hypothesize that these three different types of bubbles have distinct properties. Based on field measurements and laboratory experiments, we will characterize the processes and environmental variables that govern the generation, aging, and fate of the three types of bubbles and the substances transported by them. The observations and findings will be linked to process-based models to develop a novel framework for assessing the relevance of bubble-mediated transport in aquatic ecosystems. This new knowledge is expected to make important contributions to various fields of aquatic research, monitoring and management.