The significant contribution of lake depth in regulating global lake diffusive methane emissions

Global lakes have been identified as an important component of natural methane (CH₄) sources. Given that lake CH₄ emissions involve multiple, complex processes influenced by various environmental factors, estimates of global lake CH₄ emissions are largely uncertain. In this study, we compiled global CH₄ emission data on 744 lakes from published studies, and found a significantly negative correlation (R² = 0.50, p < 0.01) between diffusive CH₄ flux and lake maximum depth. Further analysis indicated that no significant differences in global sediment CH₄ production were found for the different maximum depths investigated. Owing to the longer oxidation pathway, presence of oxycline layer, and the lower nutrient environment, deeper lakes yield less diffusive CH₄ efflux compared to shallower lakes. Additionally, we also found that lake area was negatively correlated (R² = 0.13, p < 0.01) to diffusive CH₄ flux. Therefore, based on empirical correlations between lake morphometry (maximum depth and area) and diffusive CH₄ emission, as well as the combination of two lake databases, we estimated that the annual diffusive CH₄ emission from global lakes is approximately 11.2 (6.2-19.5) Tg CH₄/yr, and greater than 84% is emitted from lakes with a mean depth of less than 5 m. Furthermore, two regions, 40-70° N (30.4%) and 20° S∼10° N (37.4%), were found to be the dominant contributors of global lake diffusive CH₄ emissions, resulting from the considerable total lake area and the extensive shallow lakes in these regions. This study highlights the significance of the 'depth-effect' which controls the spatial distribution of lake diffusive CH₄ flux and allows for the quantification of global lake diffusive CH₄ emissions.