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 o rder to determine the origin of particulate organic carbon. Parameters studied included chlorophyll-a (chl-a), particulate organic carbon (P OC) and dissolved inorganic carbon (DIC), as well as the isotopic comp ositions of the latter two (delta(13)C(POC), delta(13)C(DIC)). The res ults show that in situ photosynthesis and detrital inputs are both sig nificant contributors to the POC pool in the isolated embayments. The former dominates during warm seasons, with POC concentrations up to 26 63 mu g/l and chi-a concentrations up to 26.1 mu g/l. Near-shore ecosy stems have a wide range of delta(13)C(POC) values (-31.5 to -16.3 part s per thousand), but this variability is not reflected in the 'Main Ch annel'. There, the delta(13)C(POC) Signal is uniformly close to -27 pa rts per thousand, in accord with estimates from earlier studies on the river's estuary This suggests that the POC contribution from near-sho re ecosystems is minor. Although the 'Main Channel' has low chl-a conc entrations, model calculations suggest that most of its POC originates from photosynthetic activity, probably within the Great Lakes. (C) 19 98 Elsevier Science B.V. All rights reserved.