Recent increases in concentrations of dissolved organic carbon (DOC) in lakes and rivers over large regionshave been related to both changes in the climate and in atmospheric deposition chemistry. Using a data set of1041 boreal lakes along a 13u latitudinal gradient, sampled in 1995, 2000, and 2005, and an additional data set of90 lakes along a 1000-m altitudinal gradient at 68uN, we show that DOC concentrations increase in a nonlinearway along a latitudinal and altitudinal temperature gradient. The nonlinear relation of DOC to increasingtemperatures was consistent over space and time. Out of 14 meteorological, catchment, morphometric, andatmospheric deposition variables tested, the variable best explaining this kind of nonlinear pattern was thenumber of days when air temperatures exceeded 0uC, i.e., the duration of the main growing and runoff season(DT.0). Using DT.0 as an input variable, we were able to predict the nonlinear temperature response of DOCconcentrations, both spatially (R2 5 0.90, p , 0.0001) and temporally (R2 5 0.90, p , 0.0001). DT.0 has anadvantage over other variables because it includes the time factor, which is decisive for the duration thatbiogeochemical processes can take place. We suggest that DOC concentrations in lakes are influenced by climatechange and that present temperature increases over Sweden result in an accelerated DOC increase toward warmergeographical regions.
2009. Vol. 54, no 6 part 2, 2513-2519 p.