Chemical and stable isotope analyses of the St. Clair, Detroit, Niagara and St. Lawrence rivers ("St. Lawrence" system) and their tributaries show that the chemical and isotopic compositions of the waters are strongly controlled by the geology of their drainage basins, complemented by biochemical processes in the water column. The latter controls paricularly the riverine budget of carbon species. Isotopic composition of dissolved inorganic carbon (d13CDIC) in the "St. Lawrence" system ranges from - 4.7 to + 0.7 permil (”) , considerably heavier than the values for the tributaries (- 16.5 to - 6.7 ”). The light d13CDIC values for the tributaries suggest that CO2 from bacterial respiration plays an important role in the isotopic composition of riverine DIC. However, in the main stem river(s), this bacterial signal is masked by isotopic equilibration with atmospheric CO2 due to the long residence time of water in the Great Lakes. Seasonally, the main stem river(s) have heavier d13C values in the fall than in the spring, a consequence of preferential 12C consumption by photosynthetic plants in the epilimnion of the Great Lakes during the growth season. In the down-stream portion of the St. Lawrence river, influx of isotopically light tributary waters causes progressive 13C depletion, from -1.3 to -2.0 ” and -1.4 to -3.0 ” in the fall and spring, respectively. The total DIC carbon flux of the St. Lawrence river is calculated to be 3.9 x 1011 mol/a. Mass balance calculations show that the relative contributions of the Great Lakes, tributaries, decay of organic matter, exchange with the atmosphere, and dissolution of carbonates to this total DIC flux are 81:13:2:-6:10 % in the spring, and 83:15:-2:4:0 % in the fall, respectively.
In addition to the down- river evolution, a cross section of the St. Lawrence near the city Cornwall demonstrates the existence of a biogeochemical gradient between near shore ecosystems and the main river. The main river shows d13CDIC readings close to zero ”, coincident with low concentrations of chlorophyll-A (chl-A # 5 ppb) and of dissolved organic carbon (DOC # 4 ppm). Ecosystems close to the shore show more active biogeochemical cycling and can be subdivided into wetlands and embayments. Wetlands have the lowest d13CDIC values of - 13 ”, maximal DOC readings of 40 ppm, and chl-A reading of 3 to 10 ppb. This scenario suggests a dominance of respiration processes and a stronger influence of groundwater input. Embayments that act as links between the wetlands and the main river are characterized by a balance between respiration and photosynthesis, with d13CDIC values between -2 and - 8 ”, chl-A readings of up to 30 ppb and DOC between 2 and 10 ppm.
Yang, C., Telmer, K. & Veizer, J., Geochim. Cosmochim. Acta (submitted).