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

In this study, we compiled global CH4 emission data on 744 lakes from published studies and found a significant negative correlation (R2=0.5; 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 with respect to the various maximum depths investigated, while CH4 variation in the water column was dependent on the thickness of the hypolimnion. 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 emissions as well as the combination of two lake databases, we estimated that the annual diffusive CH4 emission from global lakes is approximately 11.3 (6.4~19.6) Tg CH4/yr, and greater than 84% is emitted from lakes with a mean depth of less than 5 m. Furthermore, two regions, namely, 40~70° N (30.4%) and 20° S~10° N (37.4%), were found to be the dominant contributors of global diffusive lake 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 and allows for the quantification of global diffusive lake CH4 emissions.