High-frequency monitoring reveals a CO₂ source-sink shift in a subtropical eutrophic urban lake

Eutrophic urban lakes with CO₂-supersaturation represent potential carbon (C) sources; however, the drivers behind the reported C-source-sink shift remain poorly understood. This study provides a systematic assessment of daytime/seasonal pCO₂ and fCO₂ dynamics in a subtropical moderately eutrophic urban lake (Bailuwan, China), based on over a year of high-frequency monitoring, aiming to clarify the mechanisms regulating CO₂ exchange at the water-air interface in such ecosystems. Our work revealed consistent daytime declines in pCO₂ (and fCO₂) on 12 sampling days, though morning-afternoon differences were not significant (n = 24). Novel episodic undersaturation events were newly observed in October 2020 and March 2021, contrasting with the prevailing supersaturation. Annual mean values (n = 48) reached 1789 µatm (pCO₂) and 130 mmol m^-2 h^-1 (fCO₂). Critically, we identified a pronounced semi-annual divergence: pCO₂ from January to June significantly exceeded values from July to November. Both periods maintained a net source status (> 420 µatm), lacking the typical spring-sink/summer-source transition reported in previous studies. Key regulators, such as pH, chlorophyll a, and dissolved oxygen, influence C-sink-source dynamics, with eutrophication further modulating these shifts. These original findings highlight the need for targeted strategies to reduce pollutants and enhance carbon sequestration in urban lakes.