[1] In this study, we examine the magnitude and temporal variability of surface water pCO 2 in a set of lakes in boreal Québec, and explore the links between lake and catchment properties. The study lakes were consistently supersaturated in CO 2 , with the mean lake pCO 2 ranging from 400 to over 1800 matm. There was significant interannual variability in pCO 2 , apparently driven by regional patterns in precipitation. The best multivariate model of average pCO 2 included dissolved organic carbon (DOC), lake area and chlorophyll as independent variables, suggesting that external carbon (C) loading to lakes plays a central role in lake CO 2 dynamics and that lake trophic status may modulate the influence of external C loading. We show that even if the key drivers of lake pCO 2 are similar, they interact differently among regions and the resulting models may be dramatically different. In particular, we show that although pCO 2 is invariably correlated to DOC, the shape of this relationship varies greatly among regions, suggesting large-scale regional differences in C delivery, quality, and in-lake processing. As a consequence, current models cannot be extrapolated across regions unless we apply region-specific variables.

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