When to be wary of your (freshwater) calculated pCO2 measurements
A ubiquitous method for estimating CO2 fluxes of inland freshwater bodies is by using indirect measurements (e.g. alkalinity and pH) to calculate the CO2 partial pressure (pCO2), in order to estimate the water-to-air pCO2 gradient and gas transfer velocity. This indirect approach is widely accepted and has been commonly applied across a wide range of lake, river and ocean greenhouse gas studies.
Abril et al. (2014) compared both indirect and direct methods of measuring CO2 to test the validity of indirectly measuring CO2 in order to estimate fluxes from inland waters. Understanding these inland freshwater systems and their link to the carbon cycle is a complex, necessary endeavor. Due to the complexity of these systems and their biogeochemical drivers, there is strong spatial and temporal variability in their environmental measurements.
Their team ended up collecting a dataset consisting of 761 measurements of temperature, total alkalinity (TA), pH, water pCO2, and dissolved organic carbon (DOC), sampled from various climates, latitudes, and land cover types. The results from their analysis found that at lower TA and pH, the calculated pCO2 and associated fluxes, were overestimated by 50-300% when compared with their direct pCO2 measurements.
The authors note that overestimation occurs under a few specific conditions. The first being when there is a significant contribution of organic acids to TA (Hunt et al. 2011; Abril et al. 2014). The second culprit is acidic conditions (low pH) leading to a decrease in the buffering capacity of the carbonate system. Abril et al. (2014) found that changes in TA caused the sensitivity of calculated pCO2 values to increase exponentially when pH decreased.
The important thing to take away from their study is that if you are measuring pCO2 in an inland freshwater system, indirect (calculated) methods will be most robust in neutral to basic pH waters, where TA exceeds 1 mmol l-1. Conditions with lower TA and pH values will likely result in an overestimation of calculated pCO2. In these low TA, low pH situations, direct pCO2 measurements are your best bet.
Abril et al.’s (2014) findings are important in terms of the larger picture as well. The conditions for calculated pCO2 overestimation they have identified are present in the majority of freshwater system globally, suggesting that past studies that have used calculated, instead of directly measured, pCO2 in these systems have likely overestimated the pCO2 and air-water flux values.
Abril, G., Bouillon, S., Darchambeau, F., Teodoru, C. R., Marwick, T. R.,Tamooh, F., Omengo, F. O., Geeraert, N., Deirmendjian, L., Polsenaere, P., and Borges, A.V. 2014. Overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters. Biogeosciences Discuss. 11: 11701-11725
Hunt, C. W., Salisbury, J. E., and Vandemark, D. 2011. Contribution of non-carbonate anions to total alkalinity and overestimation of pCO2 in New England and New Brunswick rivers, Biogeosciences 8: 3069–3076