Physical and chemical features of an aestival pond in western Canada

Aestival ponds are shallow, semi-permanent water bodies that retain some water throughout the year, but freeze to the bottom during the winter. They are characteristically found at high latitudes or altitudes and present special conditions as environments for aquatic life. A study of such a pond in western Canada revealed two distinct seasons: an open-water season extending from March to November, during which water volume decreased considerably through evaporation, and a winter season in which all water remaining in the basin was frozen. Transitional periods between these two seasons were short. As water volume decreased during the summer, increases were noted in the specific conductance and hardness of the remaining water, whereas bicarbonate alkalinity was found to be directly correlated with rainfall alone. Values of other chemical constituents examined exhibited no distinct patterns. The formation of ice in early winter resulted in increases in all ion concentrations in the underlying water, just as occurs during the formation of sea ice. Furthermore, as the ice cover increased in thickness, the water beneath became supersaturated with dissolved oxygen. The consequences of this cryogenic “salting out”, are discussed.