Daily pCO2 and CO2 flux variations in a subtropical mesotrophic shallow lake.

Carbon dioxide (CO2) emissions from lakes have been proven to be an important component of the continental carbon balance, but most CO2 evasion estimates ignore daily variability of partial pressure of CO2 (pCO2). To discern the variability of pCO2 and its effect on CO2 outgassing estimations, we conducted in-situ biweekly pCO2 measurements during daylight from November 2017 to June 2018 at 7:00, 10:00, 14:00, and 17:00 Central Standard Time of the United States (CST) in a subtropical shallow lake in Louisiana, USA. Daytime pCO2 varied largely from 154 to 1698 μatm with an average of 736 μatm, while daytime CO2 flux ranged from -43 to 284 mmol m2 h-1 averaging at 44 mmol m2 h-1. Significant decreases in pCO2 and CO2 fluxes from mornings to late afternoons were observed throughout the study period. Specifically, in morning hours average pCO2 and CO2 fluxes were 940 μatm and 83 mmol m2 h-1, respectively, while in afternoon hours average pCO2 and CO2 fluxes were only 410 μatm and 6 mmol m2 h-1, respectively. Relationships between pCO2 and ambient factors reveal that solar radiation, lake trophic status and water temperature play a major role in pCO2 and CO2 dynamics in lake water. We argue that uncertainties may exist in the current regional and global CO2 evasion estimates due to this high daily pCO2 variability.