Dissolved carbon dynamics in large boreal rivers; what happens when large boreal rivers are impounded?

Hélie J.-F.

É. Rosa

A. Lalonde & C. Hillaire-Marcel
GEOTOP – Université du Québec à Montréal, C.P. 8888 succursale Centre-ville, Montréal (QC), Canada, H3C 3P8

Correspondance : helie.jean-francois@uqam.ca

The carbon cycling in boreal environments is currently the focus of a growing number of investigations in Eastern Canada, mainly because of the importance of hydropower and its potential in the area. Here, we investigate dissolved inorganic and organic carbon behaviour in 5 impounded (La Grande 3400 m³/s, Eastmain, St. Lawrence 12 100 m³/s, Ottawa 1950 m³/s and Nelson 2370 m³/s) and 2 pristine (Great Whales 680 m³/s and Koksoak) river systems. These major rivers were sampled monthly at their outlet for one year except at the St. Lawrence River that has been sampled since June of 1997 on a bi-weekly basis. Complementary synoptic surveys were undertaken in August 2008 on the La Grande and Great Whales Rivers. At each visit, water temperature, pH, alkalinity and specific conductivity were measured. Samples were collected for the analysis of i) major ions concentrations; ii) δ¹³C of dissolved inorganic and organic carbon as well as for the measurement of concentrations of particulate organic carbon (respectively DIC, DOC and POC); iii) δ¹⁸O and δ²H of the water molecule; and iv) U series and Sr isotopes. In all the sampled river systems, POC concentrations were at least an order of magnitude smaller than the dissolved forms. Rivers draining carbonates bedrocks (St.Lawrence and Nelson Rivers) present higher concentrations and δ¹³C-values of DIC linked to carbonate dissolution in soils. Conversely, rivers draining silicate-rich watersheds present lower DIC δ¹³C-values linked to the production of an isotopically light CO₂ through oxidation of organic matter in soils and that of soil-derived DOC along river courses. However, isotopic composition of DIC in impounded rivers draining silicate catchments indicate significant CO₂ degassing and isotopic exchanges with atmospheric CO₂ in reservoir. A relatively strong relationship is observed between pCO₂ and δ¹³C-values of DIC across the studied river systems suggesting a continuum between the production of CO₂ through DOM oxidation and CO₂ degassing. A relationship is also observed between DIC δ¹³C-values and DOC/DIC suggesting here that the more DOC is available for degradation, the more it will be oxidized to dissolved CO₂. The striking feature of this boreal data set is the homogeneity of the isotopic composition of its DOC (-27.4±0.2‰ vs. V-PDB). Moreover, C:N ratios and 14C activity (A¹⁴C) of bulk dissolved organic matter (MOD) measured in the La Grande River (impounded) reveal that this DOC is relatively fresh and young (A¹⁴C higher than 100% MC).

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