Répartition verticale des communautés de plantes aquatiques en fonction des variations des niveaux d'eau du Saint-Laurent

Ce rapport decrit les resultats et la methodologie d’echantillonnage de 15 milieux humides du Saint-Laurent (entre Cornwall et le Lac Saint-Pierre) qui ont fait l’objet d’etudes en 1999, 2000, 2001 et 2002. Les bas niveaux extremes observes en 1999 et 2001 ont donne lieu a de fortes variations des communautes de plantes, dont on a tire profit dans l’analyse. Les resultats identifient six communautes de plantes situees entre les milieux plus « terrestres » (prairie humide) et les plus « aquatiques » (herbier submerge), constituant la frange de vegetation la plus sensible aux variations de niveau. Les communautes de plantes ont ensuite ete liees quantitativement aux differentes variables hydrologiques determinant leur presence, permettant de modeliser leur abondance et leur distribution, qui representent des habitats importants des especes animales. Enfin, ces connaissances permettront d’evaluer les effets environnementaux de differents scenarios de regularisation des debits sortants du Lac Ontario vers le Saint-Laurent. This report describes the results and sampling methodology of 15 St. Lawrence River wetlands (between Cornwall and Lake St. Pierre), which were surveyed in 1999, 2000 and 2001. Extreme low water levels recorded in 1999 and 2001 generated strong variations in wetlands plant communities, which were highlighted in the analyses. Results identify 6 plant communities located between the most « terrestrial » (wet meadow) to the most « aquatic » (submerged aquatic vegetation), which constitute the wetland fringe most susceptible to water level variations. Plant communities were then quantitatively linked to the different hydrologic variables determining their occurrence. These information allow to model the abundance and spatial distribution of the major plant communities comprised in wetlands, which represent key habitats for different faunal groups. This will lead to the assessment of the environmental effects of different water level scenarios resulting from the management of Lake Ontario water outflow into the St. Lawrence.