| Temporal and spatial investigations of seven ecosystems in the St. Lawrence River (the 'Main Channel', a creek, a wetland and embayments) near the city of Cornwall, Ontario revealed more intense biogeochemical activity in near-shore ecosystems. The resulting gradients between near-shore ecosystems and the 'Main Channel' exist despite the Great Lakes' strong buffering effect, which is reflected in the well mixed water masses of the 'Main Channel', with consistent average d 18OH2O and d DH2O values of -6.9 and -50‰ VSMOW, respectively. These isotopic compositions also reflect lake-surface evaporation. The evaporated water is then added to atmospheric vapor and produces a deuterium excess of 11.8 +/- 1.4‰ in local rain at Cornwall that indicates admixture of evaporated Great Lakes water to the local precipitation.; Precipitation over the Great Lakes catchment area contributes 3.9% of Mg2+ and 31.2% of SO42- to the major ion flux of the `Main Channel', with other major elements (Ca 2+, Na+, K+, Cl-, SiO 2) falling between these two end-members.; The pCO2 of waters ranges from values close to atmospheric equilibrium, 358 ppmV in the 'Main Channel', to as high as ∼5450 ppmV in near-shore ecosystems indicating their higher groundwater influx and more active decomposition of organic matter. This gradient is also manifest in isotopic compositions of dissolved inorganic carbon, with d 13CDIC values ranging from +2.2 to -13.7‰ VPDB for the 'Main Channel' and near-shore ecosystems, respectively.; Photosynthesis and detrital inputs are both significant contributors to the POC pool in the isolated embayments. The former dominates during warm seasons, with POC concentrations up to 2663 m g/L. Near-shore ecosystems have a wide range of d 13CPOC values (-31.5 to -16.3‰), but this variability is not reflected in the 'Main Channel.'; The carbon cycle studies are the most essential part of this work and outline three major ecosystems for the St. Lawrence River: (1) The 'Main Channel' and related ecosystems that are dominated by the Great Lakes and exchange processes with the atmosphere; (2) nearshore ecosystems (creeks and wetlands) that are dominated by groundwater and decomposition of organic matter; (3) isolated embayments that, during the warm season, also have a strong photosynthetic component. The latter two types of ecosystems are the most active in carbon recycling, indicating their importance in terms of net primary productivity. They also mark the locations with the highest vulnerability to anthropogenic influences. (Abstract shortened by UMI.)... |
