Eutrophication reshaped the dependence of carbon emission on lake size: More attention to global upscaling

Accurate estimation of carbon gas (CO2 and CH4) emissions from lakes is crucial for understanding the global carbon budget. Carbon gas emissions have been considered greatest in small lakes, however, this conclusion may be biased due to limited samples of diverse lake types and sizes. Remote sensing would allow detailed mapping of regional emissions but has hitherto not been developed. Here, we demonstrate the high accuracy, continuity, and large-scale observation capabilities of optical satellite data in mapping CO2 and CH4 dynamics. Using this innovative approach, we investigated 113 meso-eutrophic lakes in eastern China and estimated diffusive carbon gas fluxes. Our findings indicate that eutrophication tends to shift small productive lakes from being CO2 sources to sinks. However, the sampled lakes acted zonally as carbon sources due to significant CO2 emissions from larger lakes. Furthermore, CH4 emissions offset CO2 uptake, accounting for 56% of the total annual carbon efflux (in CO2-equivalents). We found that large lakes (>100 km2, comprising 23% of the total abundance) dominated the carbon emissions (92% of total efflux) due to their larger surface area and less eutrophication. Moreover, eutrophication affected the relationship between lake size and carbon emissions. Neglecting these effects may result in significant overestimation of CO2 emissions (by one order of magnitude) and underestimation of CH4 emissions (by three times). This study highlights the remarkable potential of satellite-based observations in addressing biases in lake carbon emission estimation. It emphasizes the necessity of incorporating large lakes to enhance global-scale carbon upscaling estimates.