Net ecosystem production in the great lakes basin and its implications for the North American missing carbon sink: A hydrologic and stable isotope approach

This study resolves the Great Lakes Basin (GLB) hydrologic cycle into its components using stable isotopes of hydrogen and oxygen together with long term meteorological data. The results are used to calculate basinwide Net Primary Production (NPP). Stoichiometric relations of carbon and water during photosynthesis form the basis of computing NPP. Mean annual discharge from the GLB is 29% of the precipitation flux and in delta D-delta O-18 space it has a slope of 5.6, distinctly lower than the 7.5 average slope for precipitation in the basin. Mean annual evaporation flux to the atmosphere from water bodies, soils, and wet canopies is 24% of the precipitation flux. Transpiration however, is the strongest pathway for loss of water from the basin, annually returning 342 Pg (petagram = 10(15) g) or 47% of the precipitation flux to the atmosphere. Transpiration and carbon assimilation during photosynthesis are coupled processes. For the GLB vegetation, approximately every mole of CO2 fixation requires 850 mol of H2O loss by transpiration. Therefore, basinwide annual NPP is 0.268 Pg C. Heterotrophic respiration in soils and herbivory annually release 0.138 Pg C to the atmosphere. The surplus NPP or 0.130 Pg C year(-1) Net Ecosystem Productivity (NEP) is stored in GLB vegetation, consistent with the postulated missing North American carbon sink. (c) 2007 Elsevier B.V. All rights reserved.