Human impacts on the carbon cycle of large river basins in boreal rivers of eastern Canada

English Abstract:

Carbon sources and cycling in boreal environments are currently the focus of a growing number of investigations in Eastern Canada, mainly beacause of the importance of hydropower and its potential in this region. Here, we investigate inorganic and organic carbon cycling in large eastern canadian river basins. Some of these rivers are impounded (La Grande 3400 m³/s, Eastmain, St. Lawrence 12 100 m³/s, Ottawa 1950 m³/s and Nelson 2370 m³/s) and some a pristine (Great Whales 680 m³/s and Koksoak) river systems. These major rivers were sampled monthly at their outlet for at least one year. The St. Lawrence River has been under investigation since June of 1997 on a bi-weekly basis. Also, synoptic surveys were undertaken in August 2008 on the La Grande and Great Whales Rivers. At each visit, physico-chemical parameters were measured and samples were collected for the analysis of i) major ions concentrations; ii) δ¹³C of dissolved inorganic and organic carbon as well as particulate organic carbon (respectively DIC, DOC and POC); iii) δ¹⁸O and δ²H of the water molecule; and iv) radio-isotopes (Uranium series and Strontium). In all the sampled river systems, POC concentrations were at least an order of magnitude smaller than the dissolved forms. As a result, the dissolved forms will be the focus of this investigation. Rivers draining carbonates (St.Lawrence and Nelson Rivers) present higher DIC concentrations and higher ẟ¹³C in DIC, in response to the dissolution of soil carbonates. DOC/DIC ratios above 2.4 are observed in rivers draining silicates; their lower ẟ¹³C-DIC content directly reflects the organic matter oxidation in soils. However, the DIC isotopic composition of impounded rivers draining silicates reflects both the organic oxidation origin of DIC and CO₂ degassing along the reservoir. The striking feature of this boreal data set is the homogeneity of the isotopic composition of 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 reveal that this most of the DOC is relatively fresh and young (A¹⁴C higher than 100% MC).