The shifting pattern of CO₂ source sink in a subtropical urbanizing lightly eutrophic lake

Globally, numerous freshwater lakes exist, and rapid urbanization has impacted carbon biogeochemical cycling at the interface where water meets air in these bodies. However, there is still a limited understanding of CO₂ absorption/emission in eutrophic urbanizing lakes. This study therefore involved biweekly in-situ monitoring to evaluate fluctuations in the partial pressure (pCO₂) and flux (fCO₂) of CO₂ and associated parameters from January to September 2020 (7:00-17:00 CST) in an urbanizing lake in southwestern China. Our study revealed that during the daylight hours of the 11 sampling days, both pCO₂ and fCO₂ consistently demonstrated decreasing trends from the early morning period to the late afternoon period, with notable increases on May 7th and August 15th, respectively. Interestingly, unlike our previous findings, an nonsignificant difference (p > 0.05) in mean pCO₂ and fCO₂ was observed between the morning period and the afternoon period (n = 22). Furthermore, the mean pCO₂ in January (~105 μatm; n = 4) and April (133-212 μatm; n = 8) was below the typical atmospheric CO₂ level (C-sink), while that in the other months surpassed 410 μatm (C-source), although the average values (n = 44) of pCO₂ and fCO₂ were 960 ± 841 μatm and 57 ± 85 mmol m^-2 h^-1, respectively. Moreover, the pCO₂ concentration was significantly greater in summer (May to August, locally reaching 1087 μatm) than in spring (January to April at 112 μatm), indicating a seasonal shift between the C-sink (spring) and the C-source (summer). In addition, a significant positive correlation in pCO₂/fCO₂ with chlorophyll-a/nitrate but a negative correlation in dissolved oxygen and total phosphorus were recorded, suggesting that photosynthesis and respiration were identified as the main drivers of CO₂ absorption/emissions, while changes in nitrate and phosphorus may be attributed to urbanization. Overall, our investigations indicated that this lightly eutrophic lake demonstrated a distinct shifting pattern of CO₂ source-sink variability at daily and seasonal scales.