It is the basic carbon source from which plants produce sugar and more complex organic matter. Values often fluctuate, being highest late at night and lowest early in the evening. Carbon Dioxide Reactions. As carbon dioxide changes from morning to evening, so does water's pH, especially in low alkaline, productive lakes. When carbon dioxide reacts with water, it forms carbonic acid.
Acidity regulates the solubility of many minerals.
Values above 7 are alkaline or basic. Those below 7 are acidic. A change of 1 pH unit is a tenfold change in acid level. Iron may also be found in high levels in acidic water.