Evaluating the Microbubble Hypothesis: Apparent Air−Water CH4 and CO2 Gas Transfer Velocities in Relation to Microbubble Concentrations in a Meso-Oligotrophic Lake

Aquatic systems exchange large amounts of carbon dioxide (CO2) and methane (CH4) with the atmosphere. The microbubble hypothesis suggests that microbubbles preferentially enhance air−water transfer of sparingly soluble gases like CH4 over CO2, explaining elevated normalized gas transfer velocity ratios (k600,CH4:k600,CO2) observed in many inland waters. This hypothesis remains to be empirically tested. We tested this hypothesis in meso-oligotrophic Lake Stechlin (NE Germany) using acoustic bubble spectroscopy to quantify microbubble concentrations, floating chambers to measure CH4 and CO2 fluxes, and dissolved gas concentrations to estimate k600 for CH4 and CO2. Contrary to our hypothesis, k600,CH4:k600,CO2 averaged near unity and was independent of microbubble concentration. Theoretical microbubble-mediated CH4 fluxes contributed ∼0.05% to total CH4 fluxes. Thus, microbubbles affected gas transfer velocities weakly under the relatively low-wind, posteutrophication conditions of our sampling campaign. Instead, k600,CH4:k600,CO2 was more likely explained by gas-specific near-surface concentration gradients, surfactant-induced surface immobilization, and chemical enhancement of CO2 exchange. Microbubble concentrations were mainly linked to wind-induced turbulence and were elevated in littoral areas. Our results limit the applicability of the microbubble hypothesis and suggest that larger microbubble gas fluxes likely require stronger wind-induced forcing than observed here, or site-specific sources, such as littoral vegetation.