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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. While 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.

International audience; Research on rivers has traditionally involved concentration and flux measurements to better understand weathering, transport and cycling of materials from land to ocean. As a relatively new tool, stable isotope measurements complement this type of research by providing an extra label to characterize origin of the transported material, its transfer mechanisms, and natural versus anthropogenic influences. These new stable isotope techniques are scalable across a wide range of geographic and temporal scales. This review focuses on three aspects of hydrological and geochemical river research that are of prime importance to the policy issues of climate change and include utilization of stable water and carbon isotopes: (i) silicate and carbonate weathering in river basins, (ii) the riverine carbon and oxygen cycles, and (iii) water balances at the catchment scale. Most studies at watershed scales currently focus on water and carbon balances but future applications hold promise to integrate sediment fluxes and turnover, ground and surface water interactions, as well as the understanding of contaminant sources and their effects in river systems.