Geochemistry and fluxes of organic and inorganic carbon in aquatic systems of eastern Canada: Examples of the St. Lawrence River and Robert-Bourassa Reservoir. Isotopic approach (French and English text)

Many aspects of the global carbon cycle remain unclear. Carbon fluxes on the continental scale are particularly imprecise. In this thesis, we document two types of fluxes, those of a major river, namely the St. Lawrence River, towards its estuary and those at the water-air interface of a water body (here, the Robert-Bourassa reservoir).; The St. Lawrence River exports annually from 0.10 to 0.13 · 10 12 g of POC, about 1.38 · 1012 g of DOC and from 5.24 to 5.54 · 1012 g of DIC to its estuary. POC in the St. Lawrence River is mainly of aquatic origin and is produced within the river. This is not necessarily the case of POC in its tributaries. While POC exported from the Ottawa River to the St. Lawrence is mainly of terrestrial origin, POC in the Mascouche River is aquatically produced in summer and terrestrial in origin in winter. Conversely, DOC, which is ten times more abundant than POC, seems to be mostly terrestrial in origin throughout the year in the St. Lawrence River as well as in its tributaries. In addition, DOC is heavily degraded and oxidized in the St. Lawrence River before its export to the estuary. In summer and fall, about 25% of all entering DOC in the St. Lawrence River is oxidized. Annually, this DOC loss represents about 10% of the St. Lawrence's DOC export to its estuary. Nevertheless, instantaneous carbon fluxes from the St. Lawrence to its estuary can be estimated from polynomial relationships with water discharge at Quebec City.; In the Robert-Bourassa reservoir, CO2 emitted at the surface essentially originates from dissolved organic matter (DOM) degradation and oxidation within the water column of the reservoir. This DOM mainly comes from the watershed of the reservoir and is relatively "young". Primary production in the reservoir does not seem to markedly influence DIC and DOC isotopic signatures. As a result, isotopic signatures observed at a given time of the day, across the reservoir, are considered to be representative of the whole day. Data collected across the surface of the reservoir indicate that CO2 production is relatively intense on half the surface and relatively low on the other half. In winter, DOM degradation appears to be strongly reduced, then, in spring, fresh DOM accumulated during winter seems to be intensely degraded during ice break-up. Also, it is now clear that the Robert-Bourassa reservoir acts isotopically and physico-chemically as a northern Quebec deep lake. The isotopic model of CO2 flux calculation gives an average annual flux across the water-air interface of 1.04 +/- 0.21 · 1011 g of CO2 in 2000 and 1.89 +/- 0.40 · 1011 g of CO2 in 2001. Annual CO2 loss at the turbines of the power stations is estimated to be 3.3 +/- 2.2 · 1011 g of CO2, representing 50 to 80% of total annual emissions which are estimated at 4.64 to 5.79 · 1011 g of CO2. CO2 loss upon ice break up in the spring seems to be negligible here (5 to 10% of total annual emissions).* (Abstract shortened by UMI.); *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office.