Space-for-time substitution leads to carbon emission overestimation in eutrophic lakes

Lacustrine eutrophication is generally considered as an important contributor of carbon emissions to the atmosphere; however, there is still a huge challenge in accuracy estimating carbon emissions from lakes. To test the effect of widely used space-for-time substitution on lake carbon emissions, this study monitored different processes of carbon emissions, including the carbon production potential, dissolved carbon concentrations, and carbon release fluxes in eight lakes along the trophic gradients on a spatial scale and the typical eutrophic Lake Taihu for one year on a temporal scale. Eutrophication promoted carbon production potential, dissolved carbon concentrations, and carbon release fluxes, especially for CH₄. Trophic lake index (TLI) showed positive correlations with the CH₄ production potential, dissolved CH₄ concentrations, and CH₄ release fluxes, and also positive correlations with the CO₂ production potential, dissolved CO₂ concentrations, and CO₂ release fluxes. The space-for-time substitution led to an overestimation for the influence of eutrophication on carbon emissions, especially the further intensification of lake eutrophication. On the spatial scale, the average CH₄ production potential, dissolved CH₄ concentrations and CH₄ release fluxes in eutrophic lakes were 268.6, 0.96 μmol/L, and 587.6 μmol m^-2·h^-1, respectively, while they were 215.8, 0.79 μmol/L, and 548.6 μmol m^-2·h^-1 on the temporal scale. Obviously, CH₄ and CO₂ emissions on the spatial scale were significantly higher than those on the temporal scale in eutrophic lakes. The primary influencing factors were the seasonal changes in the physicochemical environments of lake water, including dissolved oxygen (DO) and temperature. The CH₄ and CO₂ release fluxes showed negative correlations with DO, while temperature displayed positive correlations, respectively. These results suggest that the effects of DO and temperature on lake carbon emissions should be considered, which may be ignored during the accurate assessment of lake carbon budget via space-for-time substitution in eutrophic lakes.