High or low pH value of water depends on several factors, namely:
a) The concentration of gases such as CO2 in water
b) The concentration of salts of carbonate and bicarbonate
c) The process of decomposition of organic matter in bottom waters.
Naturally, the pH of the waters affected by the concentration of carbon dioxide (CO2) and acidic compounds. Public waters with photosynthesis and respiration activity of organisms that live in it will form a chain reaction of carbonate – carbonate as follows:
The more CO2 generated from the results of respiration, the reaction moving to the right and gradually release of H + ions that cause water pH down. The reverse reaction occurs in photosynthesis event that requires a lot of ion CO 2, causing the pH of the water to rise. In the event of photosynthesis, phytoplankton and other aquatic plants will take up CO2 from the air during photosynthesis, resulting in increased pH of water during the day and decreases at night.
Table 3. Effect of pH on aquatic biological communities
H ion concentration in the water have an influence on the organism either directly or indirectly. There are two important things about the pH of the water to the living organisms in the water are:
1. It is a limiting factor for certain organisms can live well at low pH, while the other organisms that live at high pH or pH neutral (pH 7).
2. The pH is very closely related or a clue to its other chemical factors such as alkalinity and hardness.
The pH value in many natural waters ranging between 4-9, the presence of CO2 and strong alkaline properties of ions of sodium, potassium and calcium in seawater tends to change the status quo, so that the sea water slightly alkaline range between 7.5 to 8.4. system of carbon dioxide – carbonic acid – bicarbonate serves as a buffer that maintains the pH of sea water in a narrow range.