In many waters, calcium ion is associated with bicarbonate and carbonate ions, so when carbonate increases to an appreciable concentration, calcium carbonate will precipitate because this compound is relatively insoluble. The precipitation of calcium carbonate tends to moderate pH, but pH values of 9 or above may occur during photosynthesis in waters with high concentrations of calcium. In some waters, sodium and potassium are associated with bicarbonate and carbonate. The pH of these waters may rise to especially high levels, even 10 or 12, during periods of rapid photosynthesis. This phenomenon results because sodium carbonate and potassium carbonate are more soluble than calcium carbonate, allowing for a greater accumulation of carbonate ion, which through hydrolysis is the source of hydroxylion (Dall et al., 1990).