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Longitudinal variations of phytoplankton biomass and composition were assessed in a 250 km-long section of the St.Lawrence River, which alternately runs through narrow (< 2 km) river cross sections and wide (up to 10 km) fluvial lakes . In the main river stem, concentrations of suspended matter and total phosphorus increased with distance downstream, whereas light penetration decreased . Seasonal changes in plankton composition and biomass were more important than those resulting from differences in water mass (tributary) of origin . Sampling at three cross river sections and in two fluvial lakes showed a progressive downstream decrease in phytoplankton biomass and changes in size structure and taxonomic composition. River plankton was primarily composed of small (< 10 µm equivalent spherical diameter), truly planktonic cells belonging to Cryptophyceae and diatoms, with Chlorophyceae in summer. Plankton sampled in summer among rooted macrophytes in fluvial lakes exhibited a higher biomass of resuspended periphytic algae than in the main river stem, which contributed slightly to downstream phytoplankton biomass.Successive river cross sections always shared about 50% of their taxa, indicating a rapid downstream transport of algae within the main water mass . However, the proportion of species common to all cross sections was highest during the spring freshet, and lowest during summer low discharge, likely resulting from the development of a distinct flora in fluvial lakes during summer . Conversely, about 30% of the identified taxa were exclusive to a cross section and were replaced by others occurring downstream . Overall, phytoplankton composition along the St .Lawrence River is primarily controlled by advective forces, which result in a homogeneous flora in the main river stem, with a local contribution of resuspended periphyton from fluvial lakes .
Urban cycling is increasingly common in many North American cities and has the potential to address key challenges of urban mobility, congestion, air pollution and health. However, lack of safety is often cited by potential bike users as the most important deterrent to cycling. This study aimed to evaluate the effect of cycling facility type on traffic conflict likelihood. Four participants recorded a total of 87 h (1199 km) of video, which was reviewed by trained observers to identify and characterize traffic conflicts, yielding 465 conflicts with vehicles and 209 conflicts with pedestrians. Bootstrapped generalized additive logit regressions (GAM) were built to predict traffic conflict likelihood. Results show that while cycling on an off-street bike path effectively reduces the likelihood of conflict with a vehicle, it increases the probability of conflict with a pedestrian. Bike lanes were associated with an increase in the likelihood of a conflict with a vehicle. Decision makers should favor physically segregated and clearly marked cyclist-only facilities to ensure safe and efficient conditions for commuter cyclists.
Les sédiments de 39 stations du lac St-Louis ont été échantillonnés et analysés pendant l'été 1984 pour la granulométrie, dix éléments majeurs et les contaminants (Ni, Co, Cr, V, Pb, Zn, Hg, As, HCB, pp'-DDE, Aroclor1242™, Aroclorl254™, Aroclorl260™). Dans la plupart des stations les teneurs sont plus élevées que les normes établies par le ministère de l'Environnement de l'Ontario pour le dépôt de déchets de dragage en eau libre. Avec ces données, nous avons réalisé des cartographies du lac, ce qui nous a permis de discerner trois zones où les teneurs en contaminants sont plus élevées : entre la baie de Valois (Dorval) et la Grande Anse (île Perrot), au sud des îles de la Paix (Beauharnois) et au sud de l'île Perrot (Anse au Sable). D'autre part, nous avons calculé des corrélations entre les variables, ce qui nous a permis de constater qu'il existe de fortes corrélations entre les métaux lourds et entre les contaminants organiques entre eux, mais beaucoup moins entre ces deux groupes de contaminants. Finalement, des méthodes statistiques multidimensionnelles ont été utilisées pour regrouper les stations en fonction de leur composition chimique, et cartographier le lac St-Louis. Il en ressort un groupe principal pour les éléments majeurs comprenant les 3 zones de sédimentation. Cependant, ces trois zones semblent avoir des caractéristiques différentes quant à leur composition en contaminants.
A sediment study, involving both chemical and biological analyses, was carried out in the St. Lawrence River near Montreal (Lake St. Louis). About 60 stations were sampled during 1984-85, and the sediments were analyzed for support variables, heavy metals, and organochlorinated compounds. Subsamples were elutriated using a 1 to 4 sediment/water ratio. The resulting elutriates were analyzed for several chemical variables, while toxicity was measured using the Microtox test, algal 4 C assimilation, and lethality/developmental inhibition in cladocerans, rotifers, and nematodes. The results showed a great variety of responses and sensitivity, and correlations between the tests were non-significant. In terms of toxic responses, the algal and Microtox tests were the most sensitive. Toxic responses could not be explained in simple terms of contaminant concentrations. Therefore, the chemistry of elutriates is not predictive of the toxic potential of contaminated sediments. Biotests can give an insight into the hazard assessment of sediments, but no single test will be sufficient; the use of a battery of standardized biotests, representing different levels of organization/food chain, including representative natural species, is highly recommended.
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