Carbon and oxygen dynamics in the Laurentian Great Lakes: Implications for the CO2 flux from terrestrial aquatic systems to the atmosphere

We collected samples in lakes Superior, Erie and Huron, including Georgian Bay, in mid-summer during the stratified period to determine the relative importance of production, respiration and atmospheric exchange to carbon dynamics in the lakes. In all lakes sampled, oxygen saturation averaged 107 ± 2%, significantly greater than the atmospheric equilibrium, and the isotopic composition of dissolved oxygen, δ18ODO, averaged 22.8 ± 0.1‰, significantly less than the isotopic composition of dissolved atmospheric oxygen, suggesting that biological production was greater than respiration in the epilimnion during the summer months. lakes worldwide are commonly oversaturated in carbon dioxide, average pCO2 in the epilimnion of the lake studied was 342 ± 42 μatm and the isotopic composition of dissolved inorganic carbon, δ13CDIC, was 0.2 ± 0.2‰, both values not significantly different from expected atmospheric CO2 equilibration (n = 65, p = 0.89, n = 64, p = 0.81). This trend is atypical; approximately ninety percent of lakes globally are oversaturated in carbon dioxide. The atmospheric equilibrium observed in the Laurentian Great Lakes and other studies suggest that pCO2 in large lakes is depleted relative to smaller lakes. This is also the case for rivers that are sourced from or flow though large lakes and they can be expected to have lower pCO2 downstream from the lakes and lower per unit area carbon fluxes than other rivers. An improved understanding of the global carbon cycle mandates that this be taken into account when considering CO2 fluxes from aquatic ecosystems.