The riparian vegetation of about 30 islands located upstream of lake Saint-Pierre has been analysed. Eleven natural vegetation units were identified, described and defined along with five units of human origin and six agricultural types. The aquatic habitat is characterized by a Vallisneria americana plant community-type. In the semiaquatic floodland habitats, four community-types were present: a Sagittaria latifolia, a Sparganium eurycarpum, a Phalaris arundinacea, and a Scirpus pedicellatus type. The wetlands were characterized by an Acer saccharinum – Ulmus americana – Fraxinus pennsylvanica forest. Both vegetational analysis and fluctuations of the superficial water table pointed to the existence of four variants in this last type of riparian forest. Those are respectively a Matteuccia struthiopteris, a Laportea canadensis, an Onoclea sensibilis, and a Rorippa amphibia variant. Granulometric and pedologie analyses of typical soil samples are also presented and discussed.

The productivity of macrophytes has been studied in three different zones of Lake Saint-Pierre, Québec: terrestrial, semiaquatic, and aquatic. From the data collected, six vegetation units can be defined. The productivity (grams per square metre) of each unit is as follows: Calamagrostis canadensis, 528; Phalaris arundinacea, 1096; Sparganium eurycarpum, 508; Scirpusfluviatilis, 436; Scirpus acutus, 284; Vallisneria americana, 138. The vegetation distribution is described as a function of elevation and distance from the shore. The results are compared with available data in the literature.

The LG-1 area (53°46′ N; 77°30′ W) is located on La Grande Rivière, 28 km upstream of its mouth at James Bay. It is a Middle Subarctic landscape. The ecological distribution of mosquito larval populations of 21 species is analyzed. For each ecological unit the following data are taken into account: the density of the larval community (larvae per metre cubed) and relative frequency (percentage) of the species per sample.The larval productivity of snowmelt Aedes mosquitoes is evaluated for each ecological unit from the percentage of the area covered by water (percentage), the average depth of the shallow pools (in metres) and the mosquito larval density. The mean number of larvae (log x) per hectare and the standard error of mean range from 4.77 ± 0.255 (33 000 < 59 000 < 106 000) in the Carex rostrata unit with peat bog substrate up to 7.12 ± 0.413 (5.1 × 106 < 1.3 × 107 < 3.4 × 107) in the silty Carex aquatilis unit. The mapping of a typical area of 2 500 ha in which 626 ha are mosquito-productive wetlands, shows an average productivity (log x) of 5.29 ± 0.146 (140 000 < 200 000 < 280 000). These data are compared with quantitative results published from similar northern localities.