The pCO₂ in boreal lakes: Organic carbon as a universal predictor?

During the past two decades, it has become evident that a majority of lakes are net conduits of CO₂ to the atmosphere. This insight has implications both for lake metabolism per se and for assessing the role of lakes in the global C cycle. The concentration of dissolved organic carbon (DOC), which constitutes >90% of the total organic carbon (TOC), has been identified as a key driver of partial pressure of CO₂ (pCO₂). A crucial question is whether one may identify global relationships in the DOC-pCO₂ relationship in lakes or whether this has to be determined regionally or locally. A second major aspect is how to best predict CO ₂ as a function of DOC. Based on a survey of pCO₂ and a range of lake and catchment variables in 112 lakes, we support the view that DOC is by far the most important determinant of pCO₂ while groundwater influx has a minor contribution. Contrary to expectations, total phosphorus (P) also apparently contributed positively to pCO₂, owing to the fact that most P in these lakes is on the form of allochthonously organic P, and thus correlates strongly with DOC. Physical principles dictate that even a lake completely devoid of DOC should have a nonzero pCO₂. This is not reflected in power models, which imply that the pCO₂ approaches zero with zero DOC. Based on this study as well as published data on DOC-pCO ₂ relationships, we argue that identity link gamma-generalized linear models are appropriate for predicting pCO₂ in lakes and that their application makes it possible to reach reasonably accurate global models for how pCO₂ relates to DOC and other environmental factors.