Can a eutrophic lake function as a carbon sink? Case study of a subtropical eutrophic lake in southern USA

Carbon dioxide (CO2) outgassing from inland eutrophic waters is a small but critical component of the global carbon cycle that is currently not well constrained. Eutrophic lake systems have been reported to be supersat-urated with dissolved CO2 (pCO2), therefore considered a carbon source to the atmosphere. In this study, we aimed to test two hypotheses: 1) there is a substantial diurnal fluctuation in pCO2 in eutrophic lakes, making them both a source and a sink of carbon; and 2) the diurnal fluctuation is seasonally dependent, with a pro-nounced variability during the summer. As a case study, we used a hypereutrophic lake in a highly urbanized area in subtropical southern USA. We conducted in-situ measurements on pCO2, photosynthetically active ra-diation (PAR), and other ambient parameters at 3-hour intervals for two days each in the winter and summer of 2022. The study shows that there was a clear diurnal fluctuation of pCO2 with the peak in the early morning and the lowest in the later afternoon in both seasons. In the winter, the lake water was mainly CO2-unsaturated in reference to the atmospheric CO2 of 420 ppm, ranging from a low of 128 ppm in the mid-afternoon to a high of 557 ppm in the early morning hours. In the summer, the lake water was mostly CO2-oversaturated, with pCO2 increasing drastically from 309 ppm during the early afternoon to 3895 ppm in the early morning hours. In both winter and summer, CO2 outgassing was nearly three times greater at night than in the daytime, increasing from-9.98 to -3.75 mg C m-2h-1 in the winter and from 13.17 to 45.34 mg C m -2h- 1 in the summer. Consequently, a clear diurnal trend of CO2 evasion rate (FCO2) was found, showing that the lake often acted as a carbon sink during the daylight hours but as a carbon source at night, strongly affected by PAR availability and colored dissolved organic matter (cDOM). These findings validate our initial hypotheses and suggest that the diurnal and seasonal fluctuations of CO2 evasion in eutrophic lakes should be considered in the global carbon accounting of lake systems to constrain uncertainties. Furthermore, we found thatpCO2 measurements at 10:00o'clock in the morning have the least deviation to the diurnal average and, therefore, propose standardizing this time for future studies in field pCO2 measurements in order to make data comparable over seasons, geographical regions, and lake trophic levels.

Automated summary

This study investigates the diurnal and seasonal fluctuations of carbon dioxide (CO2) outgassing from eutrophic lakes. The results show clear diurnal variations in pCO2, with peak levels in the early morning and lowest levels in the afternoon. Additionally, the study finds that CO2 outgassing is nearly three times greater at night compared to daytime. These findings emphasize the importance of considering diurnal and seasonal fluctuations in the global carbon accounting of eutrophic lake systems.