Lakes play an essential role in the carbon cycle as they emit unproportional greenhouse gas despite their small surface area. However, current bottom-up estimates of CO₂ emissions from lakes have significant uncertainties due to insufficient field data. 

Satellite remote sensing approach has broad coverage and high spatiotemporal resolution, which can significantly reduce the uncertainties in CO₂ emission estimations.However, big challenges remain in developing remote sensing-based models for mapping lake CO₂ at regional or global scales. 

Recently, researchers led by Prof. DUAN Hongtao from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences developed a dissolved CO₂ concentration (cCO₂) estimation model for meso-eutrophic lakes at regional scale, and reconstructed the spatiotemporal dynamics of dissolved CO₂ concentrations for study lakes from 2016-2021 using Sentinel-3 data. 

Their findings were published in Remote Sensing of Environment on Jan. 3.

"The dissolved CO₂ in lakes is not optically active. Therefore, finding satellite-derivable variables related to CO₂ cycle processes and environmental factors in lakes is the key to realizing remote sensing estimation of lake cCO₂," said Prof. DUAN. 

For lakes with conditions ranging from mesotrophic to highly eutrophic in the middle and lower reaches of the Yangtze and Huai River basins (ML_YHR) in Eastern China, satellite-derived Chlorophyll-a, water temperature, Secchi disk depth, and photosynthetic active radiation-related variables are good predictors for lake CO₂ concentrations. 

"The stepwise quadratic polynomial model developed based on these variables showed high performance in predicting spatiotemporal dynamics of lake CO₂ concentrations for ML_YHR lakes," said Prof. DUAN.

The satellite-estimated annual mean CO₂ concentrations for ML_YHR lakes revealed that about 28% of the lakes acted as weak atmospheric CO₂ sinks (14.96 ± 1.13 μmol L^-1) while the rest were sources (19.22 ± 2.02 μmol L^-1), compared with a mean concentration of CO₂ atmospheric equilibrium (16.29 μmol L^-1). CO₂ concentrations decreased with increasing eutrophication and decreasing lake size. 

"This study advances current knowledge about CO₂ emissions from emerging meso-eutrophic lakes and shows how satellite remote sensing can expand the spatiotemporal coverage of lake CO₂,” said QI Tianci, first author of the study.