CO₂ dynamic of Lake Donghu highlights the need for long-term monitoring

Inland freshwater lakes have been widely considered as significant sources of CO₂ to the atmosphere. However, long-term measurements of CO₂ dynamics in lakes are still lacking, but are necessary due to their large temporal variations. Herein, we present the long-term dynamics of water parameters in Lake Donghu from 2002 to 2016, and further calculate the partial pressure of CO₂ (pCO₂) based on the measurements of pH, water temperature, and alkalinity from 2008 to 2016. The results revealed that a significantly high pCO₂ occurred during the winter in Lake Donghu (p < 0.01), whereas no significant spatial difference was observed (p = 0.37). Statistical analysis indicated that the pCO₂ in the lake was only positively correlated with the total phosphorus (TP) concentration (p < 0.05). A multilinear regression model provided the best predictors for the pCO₂; however, it only explained 16% of the observed pCO₂ variability. This indicates the complex factors that influenced the pCO₂ in Lake Donghu between 2008 and 2016. Our estimated CO₂ flux revealed that Lake Donghu acted as a small CO₂ source to the atmosphere during this period, with a mean CO₂ flux of 10.8 ± 37.4 mg m^-2 day^-1 corresponding to a mean CO₂ emission of 0.13 ± 0.43 Gg year^-1. The CO₂ emission fluxes in Lake Donghu were much lower than the mean CO₂ fluxes reported for other lakes in China and globally. Furthermore, the long-term evolution of the CO₂ flux indicated that Lake Donghu has shifted between acting as a CO₂ source and sink, which highlights the need for long-term monitoring to accurately evaluate CO₂ emissions from lakes.