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The population dynamics of Pisidium amnicum have been studied in one population of the St. Lawrence River over a 2-year period during which 19 samples were taken. The species can live up to 3 years and it is iteroparous; individuals generally attain sexual maturity after 1 year and they reproduce twice, once at 2 and once at 3 years of age. In adults, mortality is lower in winter than during the rest of the year and mortality rates are twice as high during the 1st year than during the 2nd year. Total dry weight (PT, in milligrams) is related to maximum length (L, in millimetres) according to the equation PT = 0.0283∙L3,80 and flesh dry weight (PC, in milligrams) is related to length as follows: PC = 0.007∙L3,18. The annual production to mean biomass (P/B) ratio is 1.4, the mean annual production being 1.298 mg/m2 of total dry weight and 107 mg/m2 of flesh weight. Comparing these results with those obtained for the gastropod Bithynia tentaculata in the same environment and at the same period of the year has shown that the growth of Pisidium amnicum is far less influenced by the temperature regime than that of the other species and that its production is at least 10 times lower.

This study investigated the effect of water hardness on the accumulation and elimination of cadmium (Cd) by two aquatic mosses, Fontinalis dalecarlica and Platyhypnidium riparioides, under laboratory conditions. The two mosses were exposed to nominal Cd concentrations of 0, 0.8, 2, and 10 microg . L-1, which includes the concentration range generally found in nature. The influence of three levels of water hardness (very soft: 11.7 mg . L-1; soft: 44.2 mg . L-1; and hard water: 92.3 mg . L-1 as CaCO3) was measured while maintaining the alkalinity and pH constant during the 28-day exposure. The Cd accumulation by the aquatic mosses was rapid, showing the potential of accumulation and the sensitivity of this biomonitor. Even if the actual Cd concentration in the water was low (concentration <0.15 microg . L-1 to 6.82 microg . L-1 of Cd), the uptake of Cd was very fast and mostly linear. This study was conducted in water hardness comparable to that found in the Canadian shield (hardness was <100 mg . L-1 as CaCO3). When the actual Cd concentration in the water was as high as 6.82 microg . L-1, the uptake of Cd was mostly linear and the steady state condition was not reach. Accumulation rates of Cd were significantly different when the mosses were in very soft (11.7 mg . L-1) as compared to hard water (92.3 mg . L-1 as CaCO3). The elimination of Cd followed a very slow process for the two species studied. The elimination rates of Cd from the mosses were not influenced by water hardness.

The life cycle of Bythinia tentaculata has been studied over a period of 2 years in five localities of the freshwater estuary of the St. Lawrence River. Sexual maturity is attained after 2 years and the species is iteroparous; this might prove to be a particularly advantageous reproductive strategy in this river. The main phases of individual growth are related to age and thermal variations of the water and they occur during the months of May, June, and July of the first 2 years. Eggs are laid once a year and growth stops during winter for about 6 months. These results on the life cycle and growth of B. tentaculata, particularly in the fluvial section of the St. Lawrence River, are more similar to those obtained in Europe than to those obtained elsewhere in North America. Numbers, biomass, and production are markedly higher here than the values found by other investigators on the same species.[Journal translation]

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