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 (CH) sources. Given that lake CH emissions involve multiple, complex processes influenced by various environmental factors, estimates of global lake CH emissions are largely uncertain. In this study, we compiled global CH emission data on 744 lakes from published studies, and found a significantly negative correlation (R = 0.50, p < 0.01) between diffusive CH flux and lake maximum depth. Further analysis indicated that no significant differences in global sediment CH 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 CH efflux compared to shallower lakes. Additionally, we also found that lake area was negatively correlated (R = 0.13, p < 0.01) to diffusive CH flux. Therefore, based on empirical correlations between lake morphometry (maximum depth and area) and diffusive CH emission, as well as the combination of two lake databases, we estimated that the annual diffusive CH emission from global lakes is approximately 11.2 (6.2-19.5) Tg CH/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 CH 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 CH flux and allows for the quantification of global lake diffusive CH emissions.