Aquatic carbon occurs in different forms. Firstly, a division is made between organic and inorganic carbon. Organic carbon is a mixture of organic compounds originating from detritus or primary producers. It can be divided into POC (particulate organic carbon; particles > 0.45 μm) and DOC (dissolved organic carbon; particles .
Likewise, inorganic carbon also consists of a particulate (PIC) and a dissolved phase (DIC). PIC mainly consists of carbonates (e.g., CaCO), DIC consists of carbonate (CO), bicarbonate (HCO), CO and a negligibly small fraction of carbonic acid (HCO). The inorganic carbon compounds exist in equilibrium that depends on the pH of the water. DIC concentrations in freshwater range from about zero in acidic waters to 60 mg C L in areas with carbonate-rich sediments.
POC can be degraded to form DOC; DOC can become POC by flocculation. Inorganic and organic carbon are linked through aquatic organisms. CO is used in photosynthesis (P) by for instance macrophytes, produced by respiration (R), and exchanged with the atmosphere. Organic carbon is produced by organisms and is released during and after their life; e.g., in rivers, 1–20% of the total amount of DOC is produced by macrophytes. Carbon can enter the system from the catchment and is transported to the oceans by rivers and streams. There is also exchange with carbon in the sediments, e.g., burial of organic carbon, which is important for carbon sequestration in aquatic habitats.
Aquatic systems are very important in global carbon sequestration; e.g., when different European ecosystems are compared, inland aquatic systems form the second largest carbon sink (19–41 Tg C y); only forests take up more carbon (125–223 Tg C y).