According to a 1963 study at Harvard University, carbon dioxide, bicarbonate and carbonate act as buffers to fluctuating hydrogen ion concentrations in natural sources of unprocessed water, helping the water maintain a relatively constant pH. Since the concentrations of buffer chemicals are low, outside forces also influence water pH.
The three carbon molecules are collectively referred to as the "carbonate system." This system can decrease the pH of the water when necessary by donating a proton from bicarbonate to form carbonate. If the water then becomes too acidic, both carbonate and bicarbonate are capable of accepting a proton, thereby lowering the concentration of hydrogen ions and raising the pH.
Since each of these molecule types are present in a dynamic equilibrium, any small change in the concentration of one molecule will have an effect on the concentration of all the others. This phenomenon is described by LeChatelier's Principle, which states that if any dynamic equilibrium between multiple related chemical species is disturbed by changing the conditions of that equilibrium, the position of equilibrium will change in order to counteract the change. The changes described by LeChatelier are regular for any chemical and can be predicted based on the chemistry of each component.