The p CO 2 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 CO 2 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 CO 2 ( p CO 2 ). A crucial question is whether one may identify global relationships in the DOC‐ p CO 2 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 p CO 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 p CO 2 while groundwater influx has a minor contribution. Contrary to expectations, total phosphorus (P) also apparently contributed positively to p CO 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 p CO 2 . This is not reflected in power models, which imply that the p CO 2 approaches zero with zero DOC. Based on this study as well as published data on DOC‐ p CO 2 relationships, we argue that identity link gamma–generalized linear models are appropriate for predicting p CO 2 in lakes and that their application makes it possible to reach reasonably accurate global models for how p CO 2 relates to DOC and other environmental factors.