Levels of Carbon Dioxide and Calcium in the Water Just as the amount of oxygen contained in the water depends on several factors, so too do the levels of carbon dioxide. There is a constant exchange between the carbon dioxide in the water and the carbon dioxide in the atmosphere. In addition, all the organisms which absorb oxygen by means of respiration also create carbon dioxide (see figure 5, page 9). Plants and algae extract the carbon dioxide they require for photosynthesis from the water during the day. The consumption of carbon dioxide can be so high that there is a measurable increase in the pH value of a badly aerated pond during the hours of daylight. In such ponds pH values can rise to above 8.5. But during the night, when plants and algae produce carbon dioxide, the pH value falls again. Carbon dioxide dissolves much more freely in water than oxygen, since the combination of carbon dioxide and water produces freely soluble carbonic acid. In addition, the carbon dioxide is also present in the fixed form of calcium hydrogen carbonate. As may be seen from the above, a chemical reaction takes place between the lime (or, to use the scientific term, calcium carbonate) and the carbonated water, as a result of which the lime is dissolved. The arrows pointing in opposite directions indicate that the reaction can occur in either direction. If the carbon dioxide is removed from the water, insoluble lime and carbon dioxide will be produced until the balance is re-established. The insoluble lime is an easily recognizable white deposit, which in other contexts is generally referred to as scale. When water is heated, it loses its ability to retain lime in soluble form, so the scale that forms on kettles is precipitated lime. The amount of lime in the water will affect its pH value (i.e., whether it produces a neutral, acidic or alkaline reaction). The higher the concentration of dissolved lime, the greater the quantity of carbon dioxide there will be in the form of fixed carbonate of lime (i.e., calcium hydrogen carbonate) and the less free carbonic acid will be left. Accordingly, the pH value will then be that of a neutral or slightly alkaline solution. Conversely, if there are considerable quantities of free carbonic acid in the water, then, as the word acid indicates, the pH value will be in the region below 7.
pH Values	Water Quality Page	Nutritional Cycle