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 (pCO(2)). A crucial question is whether one may identify global relationships in the DOC-pCO(2) relationship in lakes or whether this has to be determined regionally or locally. A second major aspect is how to best predict CO2 as a function of DOC. Based on a survey of pCO(2) 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 pCO(2) while groundwater influx has a minor contribution. Contrary to expectations, total phosphorus (P) also apparently contributed positively to pCO(2), 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 pCO(2). This is not reflected in power models, which imply that the pCO(2) 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 pCO(2) in lakes and that their application makes it possible to reach reasonably accurate global models for how pCO(2) relates to DOC and other environmental factors.