Isotopic constraints on the transpiration, evaporation, energy, and gross primary production budgets of a large boreal watershed: Ottawa River basin,Canada

The residual signals of the physical and chemical processes undergone in a river basin are stored in the isotopic composition of river water and are u sed here to isolate and quantify fluxes of water, energy and carbon for a l arge boreal river basin. The integrated nature of the river signal is explo ited to provide meaningful basin-wide annual averages for fluxes difficult to quantify and extrapolate by studying highly variable interface exchanges at discrete locations. The slope of the linear regression of deuterium (de lta D) and oxygen (delta(18)O) isotopes for the Ottawa River is similar to 6.0, considerably less than the slope of the local meteoric water line (7.7 ). This discrepancy is a consequence of evaporative loss from open water bo dies and soils and, through a new method, is calculated to be 8.1% of annua l precipitation. As well, on the basis of thirty years of daily meteorologi cal and discharge data, annual evapotranspiration for the Ottawa River basi n is calculated to be 53.1%. Combining the evaporation and evapotranspirati on calculations apportions 45% of the water losses to transpiration. The en ergy required to drive these cycles is calculated to be 8% of annual solar radiation for total evapotranspiration and 13% of growing season solar radi ation for transpiration. These energies are transformed into latent heat. T he water use efficiency ratio is used to estimate total fixation of carbon (gross primary production (GPP)) for the basin at 15.6 mol C m(-2) yr(-1). This rate is substantially greater than the export of carbon via rivers plu s rates estimated for carbon respiration in the literature, indicating that the boreal forest is a plausible component of the postulated "missing" car bon sink. Comparison of accumulation rates of C in peatlands and the rates required to account for the missing sink suggest that peat accumulation rat es are similar to 20 times too slow to account for the missing sink flux. S peculatively, the living biomass of the boreal forest is the dominant sink. Accepting this, the respiration rate needed for a steady state balance bet ween the calculated boreal forest GPP and the missing global carbon sink is found to be around 5.6 mol C m(-2) yr(-1).