Dissolved Inorganic Carbon DIC in freshwater occurs as four different species in equilibrium with one another. The four species of DIC are; carbon dioxide CO2, carbonic acid H2CO3, bicarbonate HCO3, and carbonate CO3=. The total amount of DIC largely determines the buffering capacity of freshwater, and the ratio of these species with one another largely determines the pH. Carbon dioxide dissolves readily in water. At air equilibrium, the concentration of CO2 in air and water is approximately equal at about 0.5 mgL. Unfortunately, CO2 diffuses about ten thousand times slower in water than in air. This problem is compounded by the relatively thick unstirred layer or Prandtl boundary that surrounds aquatic plant leaves. The unstirred layer in aquatic plants is a layer of still water through which gases and nutrients must diffuse to reach the plant leaf. It is about 0.5 mm thick, which is ten times thicker than in terrestrial plants. The result is that approximately 30 mgL free CO2 is required to saturate photosynthesis in submerged aquatic plants.

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