Abstract 15 The partial pressure of CO 2 (pCO 2 ) in surface waters is an indicator of the state of a 16 lake’s carbon cycle and net metabolic balance. In this paper, we present and compare 17 pCO 2 observations taken in Lake Superior in the last decade with the goal of reconciling 18 the independent,observations,and,using them,to understand,temporal,and,spatial 19 variability in lake carbon processes. Using direct in situ observations in 2001 and bi-20 annual EPA survey data for 1996-2006, we find that Lake Superior is, on average, 21 supersaturated (mean = 412.7 ∝atm) with respect to atmospheric,pCO 2 (mean = 380.3 22 ∝atm) in April and approximately,at equilibrium (mean = 364.8 ∝atm) with respect to 23 atmospheric pCO 2 (mean = 359.3 ∝atm) in August. Both datasets indicate that temporal 24 variability in surface lake pCO 2 is predominantly,controlled by changing,dissolved 25 inorganic carbon (DIC). A well mixed water column,prevents large fluctuations in pCO 2 26 in spring, but through summer and early fall, short-term variability in pCO 2 (100-200 27 µatm) increases over the summer at 12m, likely due to oscillations of the thermocline in 28 the increasingly stratified lake. Year to year changes,in surface lake pCO 2 and 29