Seasonal changes in the sources and fluxes of dissolved inorganic carbon through the St. Lawrence River—isotopic and chemical constraint

Abstract

The St. Lawrence River ranks 16 th in the world rivers for its freshwater discharge into the ocean, but its particulate fluxes are relatively low due the upstream presence of the Great Lakes system. Using notably 13C measurements in total dissolved inorganic carbon (DIC) at several stations along stream, earlier studies provided information on dissolved inorganic carbon (DIC) fluxes through the St. Lawrence, with some constraints on spatial trends with respect to the in situ DIC metabolism. Here, we further document annual DIC-fluxes and seasonal changes in DIC supplies and DIC-metabolism, downstream. Chemical and isotopic measurements were performed in 1998, on a biweekly basis, at four different sites. These sites represent respectively the Great Lakes outflow into the St. Lawrence system, supplies from two tributaries (one with a silicate-rich Precambrian bedrock, the other with a carbonate-rich Paleozoic bedrock), and the St. Lawrence outflow into the maritime estuarine system. The 1998 survey yielded a total annual DIC flux of ∼6×10 12 to 7×10 12 g of carbon at the Quebec City outlet, about 40% higher than estimates from earlier studies. This represents about 1.5% of the world river DIC supplies to the ocean. Very strong seasonal variations are observed between the summer low water levels, when water supplies from the Great Lakes into the riverine system may represent up to 80% of the total outflow of the St. Lawrence River, and the spring snowmelt period, when tributaries may provide up to 80% of this outflow. δ 13C-DIC values respond to this seasonal cycle, with near isotopic-equilibrium values with atmospheric CO 2 during summer, and strongly depleted values in other seasons, notably during the spring high outflow period. This seasonal isotopic cycle is more pronounced at the Quebec City outlet, where the influence of 13C-depleted DIC supplies from tributaries is stronger than upstream. The low δ 13C-DIC values observed from fall to spring, may be due to a combination of the following factors: (i) enhanced supplies of 13C-depeleted DIC from soils and ground waters from watersheds, (ii) higher oxidation rates of dissolved or particulate 13C-depleted organic carbon, and (iii) a reduced incidence of the in situ photosynthetic activity. The outflow and isotopic seasonal cycles result notably in a strong negative correlation between δ 13C-DIC values and discharge rates. Estimates of DIC supplies from tributaries vs. those from the Great Lakes were calculated using either chemical or isotopic approaches. DIC supplies from tributaries vary between ∼5% and 60% of total DIC. Examination of pCO 2 data indicates a negligible seasonal variability at the Great Lakes outlet, with a mean annual value near equilibrium with atmospheric CO 2. In contrast, strong seasonal changes are observed downstream, at Quebec City, with minimum pCO 2 values in summer and a mean annual value of ∼1300 ppmv, suggesting that the St. Lawrence River system in its later course acts as a source for atmospheric CO 2 during most of the year.