@I 1983, by the American Society of Limnology and Oceanography, Inc. Freeze-out of salts in hard-water lakes1

A 1Syear series of daily measurements of pH and total alkalinity recorded at the water intake of a municipal water-treatment plant on Clear Lake, Iowa, demonstrated that rapid chemical changes occur annually as the ice cover melts. Additional measurements of total and calcium hardness on this and three other eutrophic, hard-water lakes showed that total and calcium hardness concentrations declined rapidly as the main ice tiass was destroyed. This decline was not caused by chemical or biological precipitation of carbonates, but by a dilution effect of melting ice. The magnitude of the declines could be predicted from mea-surements of hardness under the ice, lake volume, and volume of ice. Studies of natural lakes and farm ponds have shown that water hardness and al-kalinity levels are generally higher in winter under an ice cover than in spring or summer and that the magnitude of the winter to spring-summer decline is great-er in eutrophic than in oligotrophic water