Carbon cycle in St. Lawrence aquatic ecosystems at Cornwall (Ontario), Canada: seasonal and spatial variations

Seasonal studies of a cross section through the upper St. Lawrence River in 1995 and 1996 revealed strong biogeochemical gradients between a wetland, a creek, embayments and the main river near the city of Cornwall (Ontario), Canada. These local differences are much more pronounced than over the entire length of the main river, indicating the importance of near-shore ecosystems in the riverine carbon cycle. The aqueous pCO2 ranged from atmospheric equilibrium to values as high as 5450 ppmV, reflecting a gradient from the Great Lakes-dominated main river to the peripheral ecosystems with active biological recycling. The isotopic composition of dissolved inorganic carbon fits this observation, with δ13CDIC values ranging from +2.2 to −13.7‰ from the main river to near-shore ecosystems. Warm season samples of embayments revealed active photosynthesis, with pO2 values up to 50% higher than those of the atmosphere, and pCO2 below atmospheric equilibrium. While photosynthetic activity was mostly caused by phytoplankton, as indicated by chlorophyll-a up to 63.0 mg/l, an active periphyton and macrophyte community also contributed to oxygen oversaturations in the littoral zone. In situ respiration dominated in the wetland, particularly during winter and spring when dissolved organic carbon (up to 17.3 mg/l) was transformed into CO2, leading to δ13CDIC values as low as −12.3‰. The creek was the only ecosystem with a respiratory signal inherited from near-shore baseflow, as indicated by its δ18OH2O composition that was similar to local groundwater during peak runoff periods. Its peak DOC concentrations of 38.3 mg/l, the highest amongst the studied ecosystem, may have resulted in further enhanced respiratory activity.