Origin of particulate organic carbon in the upper St. Lawrence: isotopic constraints

Seven sampling locations in the upper St. Lawrence River near the city of Cornwall (Ontario, Canada), including the main river and six near-shore ecosystems (a creek, embayments and a wetland) were studied in order to determine the origin of particulate organic carbon. Parameters studied included chlorophyll-a (chl-a), particulate organic carbon (POC) and dissolved inorganic carbon (DIC), as well as the isotopic compositions of the latter two (δ13CPOC, δ13CDIC). The results show that in situ 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 μg/l and chl-a concentrations up to 26.1 μg/l. Near-shore ecosystems have a wide range of δ13CPOC values (−31.5 to −16.3‰), but this variability is not reflected in the `Main Channelʹ. There, the δ13CPOC signal is uniformly close to −27‰, in accord with estimates from earlier studies on the riverʹs estuary. This suggests that the POC contribution from near-shore ecosystems is minor. Although the `Main Channelʹ has low chl-a concentrations, model calculations suggest that most of its POC originates from photosynthetic activity, probably within the Great Lakes.