Outgassing of carbon dioxide (CO2) from inland waters has been estimated to offset approximately 20% of net uptake of carbon into the terrestrial biosphere. However, this calculation is based on estimates of source strength at the air-water interface that is highly uncertain [Ciais et al., 2013]. One of the largest unknowns is the accuracy and precision of freshwater partial pressure of CO2 (pCO2) estimates. To better understand observational uncertainty in CO2 flux from inland waters, here we conduct an analysis of historical observations and take new field observations at the North Temperate Lake Long Term Ecological Research (NTL LTER) site to quantify random errors in freshwater carbon system. We further estimate errors contribution to the precision of pCO2 derived from multiple carbonate equilibria and identify other sources of uncertainty in pCO2 calculations.
The net air-water CO2 exchange is calculated as a product of the CO2 gas transfer velocity (k), the CO2 solubility constant (K0) and the gradient between pCO2 in the atmosphere and water (ΔpCO2). The aquatic component of ΔpCO2 in current estimates of carbon evasion from inland waters relies on calculating pCO2 using thermodynamic equilibria in the carbonate system due to scarcity of direct pCO2 measurements at regional and global scales [Butman and Raymond, 2011; McDonald et al., 2013; Raymond et al., 2013]. Carbonate equilibria use temperature and the combination of two CO2-related parameters (i.e., pH,