Geographical and environmental drivers of regional differences in the lake pCO2 versus DOC relationship across northern landscapes

Several recent studies have identified dissolved organic carbon (DOC) as playing a key role in determining surface water partial pressure of CO2 (pCO(2)) in northern lakes and, in particular, in shaping the commonly observed patterns of CO2 supersaturation. The nature of this role is unclear, however, and appears to vary regionally, as evidenced by the contrasting strength and shape of the diverse pCO(2) versus DOC (pCO(2)-DOC) relationships. Here we combine original data on lake pCO(2) from six boreal and temperate regions of Quebec (Canada) with 13 studies from northern temperate and boreal aquatic landscapes published in the past 15 years to explore the factors that explain the differences in regional pCO(2) baselines (pCO(2) at low DOC) and in the slopes of the pCO(2)-DOC relationships. Mean elevation was the best predictor of the regional pCO(2) baselines, suggesting that lake position in the landscape determines the contribution of terrestrially derived CO2 to lake pCO(2). In contrast, the slope of the pCO(2)-DOC relationships was strongly negatively correlated to the mean regional TP:DOC ratio. The relationship between DOC and TP varied at the cross-regional scale, and there was a large increase in the TP:DOC ratio at TP > 20 mu g L-1, resulting in negative slopes of the pCO(2)-DOC relationships for regions situated in that part of the TP gradient. These results highlight the interplay that exists between geographical gradients, large-scale biogeochemical patterns in regional lake trophic status, and the associated metabolic balance in determining pCO(2) dynamics in northern lakes.