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.