HYPOLIMNETIC ALKALINITY GENERATION IN 2 DILUTE, OLIGOTROPHIC LAKES INONTARIO, CANADA

It has been observed that the alkalinity concentrations in the hypolim nia of many thermally stratified lakes increase over the duration of t he summer. We have quantified the processes, both redox and exchange, that contributed to the alkalinity increases measured in two lakes tha t are situated on the Precambrian Shield of Ontario, Canada by measuri ng hypolimnetic mass balances for all substances involved in alkalinit y-generating or -consuming reactions. These include nitrate, ammonium base cations, iron, manganese, sulphate, organic anions, as well as al kalinity itself. In one lake, iron reduction was the dominant source o f alkalinity, since this process is probably reversed at fall overturn when hypolimnetic waters mix with oxygenated surface waters, the alka linity generated by this mechanism is likely temporary in nature. In t he second lake, iron reduction and sulphate reduction were both import ant; the latter should provide more permanent alkalinity.