Bubble sizes inferred from bubble gas composition in a temperate freshwater fish pond

Abstract

Rising bubbles play a fundamental role in emitting greenhouse gases from shallow waters. Their size is crucial for bubble dissolution, gas exchange with the surrounding water, and the release of gases into the atmosphere. However, little is known about bubble sizes in shallow waters. To address this, we investigated bubble diameters in a 1.2 m deep fish pond, employing two methods: firstly, the bubble size distributions were measured by optical bubble sensors. Secondly, we used an existing single bubble dissolution model to determine diameters representative for the respective bubble size distributions at the water surface based on measured bubble oxygen contents and dissolved oxygen concentrations. Results from optical bubble sensors were relatively similar at all sites; however, subsequent analysis revealed problems particularly in detecting small bubbles under the turbid, shallow water conditions. Model-derived bubble diameters ranged from 0.5 to 10.5 mm, varied spatially within the pond, and displayed diurnal fluctuations. With increasing bubble flux, bubble diameters increased - bubbles at feeding sites were larger than in the open water area. A detailed sensitivity analysis revealed that, depending on the bubble size distribution, the uncertainty of the model increases with increasing water depth. For a typical bubble diameter of 5 mm, the simple method can provide robust estimates of representative bubble size in waters shallower than 50 m.