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

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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.