Particulate organic carbon potentially increases methane emissions from oxic water of eutrophic lakes.

Lakes are hot spots for methane (CH4) emissions and particulate organic carbon (POC) production, which describes the methane paradox phenomenon. However, the current understanding of the source of POC and its effect on CH4 emissions during eutrophication remains unclear. In this study, 18 shallow lakes in different trophic states were selected to investigate the POC source and its contribution to CH4 production, particularly to reveal the underlying mechanisms of the methane paradox. The carbon isotopic analysis showed that the δ13Cpoc ranged from -30.28 ‰ to -21.14 ‰, indicating that cyanobacteria-derived carbon is an important source of POC. The overlying water was aerobic but contained high concentrations of dissolved CH4. Particularly, in hyper-eutrophic lakes, such as Lakes Taihu, Chaohu, and Dianshan, the dissolved CH4 concentrations were 2.11, 1.01, and 2.44 µmol/L, while the dissolved oxygen concentrations were 3.11, 2.92, and 3.17 mg/L, respectively. The intensified eutrophication increased the POC concentration, concomitantly promoting the dissolved CH4 concentration and the CH4 flux. These correlations revealed the role of POC in CH4 production and emission fluxes, particularly as a possible cause of the methane paradox, which is crucial for accurately evaluating the carbon budget and balance in shallow freshwater lakes.

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.