Hydrologic support of carbon dioxide flux revealed by whole‐lake carbon budgets

Freshwater lakes are an important component of the global carbon cycle through both organic carbon (OC) sequestration and carbon dioxide (CO 2 ) emission. Most lakes have a net annual loss of CO 2 to the atmosphere and substantial current evidence suggests thatbiologic mineralization of allochthonous OC maintains this flux. Because net CO 2 flux to the atmosphere implies net mineralization of OC within the lake ecosystem, it is also commonly assumed that net annual CO 2 emission indicates negative net ecosystem production (NEP). We explored the relationship between atmospheric CO 2 emission and NEP in two lakes known to have contrasting hydrologic characteristics and net CO 2 emission. We calculated NEP for calendar year 2004 using whole‐lake OC and inorganic carbon (IC) budgets, NEP OC and NEP IC , respectively, and compared the resulting values to measured annual CO 2 flux from the lakes. In both lakes, NEP OC and NEP IC were positive, indicating net autotrophy. Therefore CO 2 emission from these lakes was apparently not supported by mineralization of allochthonous organic material. In both lakes, hydrologic CO 2 inputs, as well as CO 2 evolved from net calcite precipitation, could account for the net CO 2 emission. NEP calculated from diel CO 2 measurements was also affected by hydrologic inputs of CO 2 . These results indicate that CO 2 emission and positive NEP may coincide in lakes, especially in carbonate terrain, and that all potential geologic, biogeochemical, and hydrologic sources of CO 2 need to be accounted for when using CO 2 concentrations to infer lake NEP.