Organic carbon sources and cycling in the St. Lawrence River: Stable isotopes and elemental approaches

The general aim of the present study is to establish a comprehensive carbon budget in the St. Lawrence River, with special attention to seasonal and interannual variations. Complementary information on organic carbon fluxes and cycling is provided. Added to concentration measurements, isotopic approaches combined with C/N analyses were used to document sources and cycling of DOC and POC in the river. Four sampling sites have been monitored since 1997, the Montreal area, the Quebec City area, the Ottawa River and the Mascouche River. All sampling locations were visited on a two-weekly basis since mid-1997 and June 1998 to November 2001. In situ measurements included water temperature, pH, redox potential, conductivity, alkalinity and dissolved oxygen concentration. Samples were then analyzed for δ 13C-values in DOC, POC and in dissolved inorganic carbon (DIC), δ 18O-values of dissolved oxygen, as well as for the C/N ratios and concentrations of dissolved and particulate organic matter (respectively DOM and POM). The limit between DOM and POM was set using pre-combusted 0,3 μm glass fiber filters. Mean annual isotopic compositions of DOC, POC and DIC as well as their concentration and C/N ratios are summarized. Elemental analysis of POC revealed low C/N ratios suggesting a prominent planktonic and/or bacterial origin for POM. C/N ratios of DOM are also relatively low, except in the Ottawa River which yielded higher values suggesting a more important contribution of terrestrial organic matter. Moreover, δ 13C-values in POC from the St. Lawrence River show a strong seasonal pattern not unlike that depicted by δ 13C-values in dissolved CO2. This suggests that organic matter in the St. Lawrence River is mainly produced within the river or the Great Lakes, at its head. Based on our monitoring program, total annual discharge rates could be calculated for DOC, POC and DIC. They were respectively 1.38.10+1+2 , 0.10.10+1+2 and 5.25.10+1+2 gC in year 2000