Observed variability of Lake Superior pCO₂

We present and compare direct and indirect pCO₂ observations taken in Lake Superior in the last decade and use them to understand temporal and spatial variability in lake carbon cycle processes. In situ observations from 2001 and biannual survey data for 1996-2006 indicate that Lake Superior was, on average, supersaturated (annual mean = 46.7 ± 17.3 Pa [461 ± 171 μatm]) with respect to atmospheric pCO₂ (mean 5 38.3 6 0.6 Pa) in April and close to equilibrium (mean = 37.5 ± 6.7 Pa) with respect to atmospheric pCO₂ (mean = 36.4 ± 0.7 Pa) in August. Both data sets indicate that temporal variability in surface lake pCO₂ from weekly to interannual timescales was predominantly controlled by changing dissolved inorganic carbon and associated changes in pH. An unstratified water column appears to have limited pCO₂ fluctuations in spring. Through summer and into early fall, pCO₂ variability on a daily timescale at 12 m increased with time to a maximum amplitude of 19 Pa, likely as a result of internal waves on the thermocline. Year-to-year changes in mean surface lake pCO₂ and temperature were of the same sign and approximate magnitude at all observed points, consistent with the lake's small size relative to the synoptic-scale meteorological systems that force it. Variability in pCO₂ was not correlated with major climate indices. While these data provide a first large-scale overview of Lake Superior's pCO₂ and its temporal variability, their time-space resolution and accuracy are not sufficient to further refine previously imbalanced lake-wide carbon budgets.