Significance of Standardizing Carbon Dioxide Measurement Time Within Lake Systems to Constrain CO2 Outgassing Estimation Uncertainties

Recent studies have emphasized the diel nature of dissolved CO2 (pCO2) in lake systems; however, around the world, field measurements have been conducted at different times, and it is not clear how, and to what extent, this variable sampling time affects CO2 emission (FCO2). This research aimed to investigate whether 10:00 am. was an effective time for lake field sampling to capture 24 h mean pCO2 and FCO2 from subtropical eutrophic shallow lake systems. To accomplish this goal, our study compiled long-term data from three individual studies on a shallow lake system in southeast Louisiana, USA. The data comprised samples collected across three-hour intervals, seasonally, over five years. This study highlights that a lake system can function as a carbon sink during peak daylight hours and can shift to a source overnight to early morning. The findings from the longer-term data confirmed 9:00 am to 11:00 am as the time range when pCO2 deviation from the daily mean is at its lowest (111% of the daily mean). Sampling outside of this time range resulted in an increase in FCO2 (32.09 mmol m−2 h−1) of up to 271% (87.03 mmol m−2 h−1) in overnight samples, with daytime measurements of FCO2 flipping the lake from source to sink sequestering −20.17 mmol m−2 h−1. Based on our findings, we strongly recommend that future studies in similar aquatic systems utilize the 9:00–11:00 am timeframe for field sampling to increase the accuracy and compatibility of shared data in carbon emission estimation.