La Grande Île is one of several islands at the western end of Saint-Pierre Lake. The island is divided into several sections by narrow channels. The entire island is inundated by spring floods. Some cottages exist along the edges of the island.
La Grande Île is home to one of the largest heronry in North America and the IBA also supports Black-crowned Night Heron and Great Egret. According to the ministère des Ressources naturelles et de la Faune the heronry grew from 20 to 1306 nests between 1975 and 1991. In 1997, the number of nests was essentially stable despite changes in the species composition: the number of nests used by Great Blue Herons had decreased by 10%, while the Black-crowned Night Heron had been using 10% more nests. A westward shift of the heronry was also observed. In 2001, the Black-crowned Night Heron accounted for 25% of the population and two nests of Great Egret were found. In 2006, only 1047 nests of Great Blue Heron were recorded during the five-year survey and the Black-crowned Night Heron population was accounting for nearly 33% of the colony with 524 occupied nests. The Black-crowned Night Heron colonization and the greater human disturbance are not believed to be the causes of these variations. Habitat degradation is the most probable hypothesis.
Surface water quality tests often fail for metals such as lead, chromium, aluminium, copper, and iron. Sediments in the bottom of Lac St-Pierre are frequently over the PCBs and lead safety limits set for dredging operations. There has also been some evidence of contamination from pentachlorophenol, hexachlorocenzene, and DDT, but few studies have been done on the potential effects of bio-accumulation on birds. Oil spills are a constant risk given the location of the site and the growing popularity of cruises in the archipelago may disturb the birds in the future. Wood cutting also threatens the persistence of the heronry. The site is part of the Lac Saint-Pierre Biosphere Reserve, as designated by UNESCO. It is also recognized as a wetland of international importance under the RAMSAR Convention. A large part of the island is designated a wildlife refuge and is owned by the ministère du Développement durable, de l'Environnement, de la Faune et des Parcs of Québec. The Canadian Nature Conservancy and the Canadian Wildlife Service also own some portions of the island. In 2013, the Comité ZIP du lac Saint-Pierre put up 12 nest boxes in the northern part of the island to provide additional nesting sites for tree-nesting ducks.
The Lake Saint-Pierre is essentially a widening of the St. Lawrence River that creates one of the largest wetland in Québec. Its archipelago and shores are covered by vegetation, including marshes, semi-aquatic plants, underwater plants and treed swamps visited by more than sixty species of fish. During spring floods, the floodplain became an important site for spawning and a nursery grounds for many species of fish, such the yellow perch and northern pike. Several fishes found in the lake and its tributaries are listed as endangered. This is the case of the copper redhorse, endangered specie endemic to Québec; the lake sturgeon is also considered as threatened in Québec; and the American eel, a species of least concern. The largest known population of bridle shiner, another species whose status is least concern, exploits the Lake Saint-Pierre.
The availability in habitats has decreased primarily because of wetlands draining to extend the surface available for agriculture and to control of the water levels. The shoreline is eroded by waves created by commercial shipping and pleasure crafts, while dredging of the channel alters the water flow and the structure of habitats. The implementation of an agricultural industry in the watershed, the presence of many industries upstream of Lake Saint-Pierre, the density of population and the presence of resorts are responsible for the degradation of water quality. The population of walleye, a fish very sensitive to pollution, appears to be declining in the past few years. This is also the case for the yellow perch, historically the most important commercial species in the lake.
Major species present:
Habitats in this area are characterized by high sedimentation. The contributions of many tributaries, such as Richelieu and Saint-François rivers, are largely responsible for the suspended material. This sedimentation promotes the formation of marshes and wet meadows. We found there vast submerged meadows dominated by Wild celery and Eurasian water-milfoil.The island is home to a mature silver maple forest sheltering wood-nettle and sensitive fern. IBA is home to 27 rare plant species, including Walter's barnyard grass and hop flatsedge.
Shoreline erosion, whether due to natural factors (wind, cycles of freezing and thawing, no ice to protect the banks in spring) or human (waves caused by passing ships), threatens riparian habitats. Variations in the water level in the river corridor affect the ecology of plant and animal species that live there. A significant and prolonged decrease of immersion banks could affect flora by promoting the growth of plant species over land, nature and even shrubby tree. In addition, the spread of invasive species exerts considerable pressure on the native flora of these habitats.
Major species present :
Canada wood nettle
Narrow leaf cattail
|Great Blue Heron|