Certaines observations suggèrent une influence de la vallée du Saint-Laurent sur la distribution des averses à sa surface. D'autre part, depuis quelques années, les membres du "'Stormy Weather Group" ont enregistré sur films de 35-mm les échos apparaissant sur un écran de radar doté d'un indicateur panoramique à altitude constanto (IPAC). L'utilisation de ces films permet à l'auteur d'approfondir le sujet par l'identification des premiers échos et le tracé de leur trajectoire subséquente. Des dossiers, établis pour 76 premiers échos, relient ceux-ci à la surface et au profil du terrain. [...]

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A sequential procedure for the extraction of iron hydroxides and the associated phosphate in estuarine seston is proposed. The procedure which uses a citrate-dithionite-bicarbonate (CDB) reagent has been chosen among five commonly used methods. It makes it possible to distinguish four statistically different modes of association between P and Fe based on the (Fe/P)$ sb{ rm CDB}$ values. The extraction sequence is applied to suspend particulate matter and to surface sediments of the Saint-Lawrence Upper Estuary. The intensity and the position of the core of its maximum turbidity zone is determined by sediment exchanges between the channel and large intertidal flats. These exchanges are controlling seasonal fluctuations observed in the estuarine phosphorus geochemistry: The fresh to salt water early transition zone is the main site for the flocculation and deposition of river derived colloids, mainly as surface coating on particle, a process which results in phosphate adsorption on the suspended matter; After this process has reached equilibrium in the water column, partial desorption occurs on the flats as the particles settle temporarily in spring and summer. The P-depleted solids are returned to the estuary when the marsh vegetation is destroyed in the fall by migrating snow geese. Laboratory experiments with natural materials confirm the equilibrium level of P-adsorption observed in situ, and evidence its reversibility under the low reducing conditions typical of marsh surface sediments.

Note: Some illustrations out of order in manuscript.

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Biological invasions are among the leading causes of species diversity loss; however, the impacts of invasion are context-dependent and can vary with the local environment. The mechanisms governing variation in impact and their relationship to specific abiotic and biotic factors remain largely unexplored. Recent local declines in native unionid mussels have been attributed to the invasion of North American lakes and rivers by the Eurasian zebra mussel ( Dreissena polymorpha), as a result of intense fouling of unionid shells by zebra mussels. My research investigated the role of abiotic and biotic factors in mediating the impact of zebra mussels on native mussels. I examined the impact of zebra mussels on unionids in a habitat thought to be suboptimal for zebra mussels and compared this to the impact observed in other invaded habitats. A predictive model relating fouling intensity to local environmental variables (calcium concentration, sediment particle size) was developed, and a predator-exclusion experiment was conducted to investigate the role of predation in mediating fouling intensity. Overall, I found that two abiotic factors of the local environment were significant predictors of fouling intensity and that relationships used to predict the impact of zebra mussels could be extended to a broader range of habitats.

The goal of this thesis was to examine the sources and ecological behaviour of genotoxic organics in the St. Lawrence river system. A rapid and effective version of the SOS Chromotest was developed to accomplish the task of genotoxicity assessment. The method, validated with standard reference materials, is particularly well suited to complex environmental extracts. The endpoint investigated throughout the thesis, SOS genotoxicity, is empirically related to more familiar endpoints such as mutagenicity and carcinogenicity. Analyses of literature data confirmed correlations between SOS genotoxicity and both Ames/Salmonella mutagenicity and animal carcinogenicity. Analyses of industrial and municipal wastewaters revealed that organic and inorganic chemical production facilities, metal refining and founding facilities and petroleum refineries release high potency wastes. Loading values indicate that low potency sources (e.g. municipal wastewater and pulp & paper facilities) often have exceptionally high loadings. A substantial portion (2.3-99.8%) of industrial genotoxins are associated with suspended particulates. Sorption partition efficients (K$ sb{ rm d-genotox}$) indicate that municipal wastewater and pulp & paper facilities release particularly soluble genotoxins. Extracts of suspended particulate matter collected near industrial discharges are direct-acting. Bottom sediment extracts are less potent and usually require S9 metabolic activation. Genotoxicity of both bottom sediment and suspended particulates are empirically related to the genotoxicity of regional industrial emissions. Direct-acting substances in the water column appear to be less stable and may be converted to more stable pro-genotoxic substances present in the bottom sediments. A mass balance of genotoxicity for St. Lawrence surface waters revealed that domestic, rather than industrial sources make the largest contribution to observed genotoxicity. The importance of domestic wastes was confirmed

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