Different forms and interactions of aquatic carbon. Inland waters primarily receive C from terrestrial ecosystems (Thomas, 1997). This C (1.9 Pg C y-1) is transported to the oceans (0.9 Pg C y-1), buried in the sediments (0.2 Pg C y-1) or emitted as CO2 (0.8 Pg C y-1) (Cole et al., 2007). More recent estimations are different: Raymond et al. (2013) claims that CO2 emission from inland waters can be as high as 2.1 Pg C y-1. Aquatic C occurs in different forms. Firstly, a division is made between organic and inorganic C. Organic C 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 < 0.45 μm). DOC usually makes up 90% of the total amount of aquatic organic C. Its concentration ranges from 0.1 to >300 mg L-1 (Sobek et al., 2007). Likewise, inorganic C also consists of a particulate (PIC) and a dissolved phase (DIC). PIC mainly consists of carbonates (e.g., CaCO3), DIC consists of carbonate (CO32-), bicarbonate (HCO3-), CO2 and a negligibly small fraction of carbonic acid (H2CO3). The inorganic C compounds exist in equilibrium that depends on the pH of the water (Stumm and Morgan, 1996). DIC concentrations in freshwater range from about zero in acidic waters to 60 mg C L-1 in areas with carbonate-rich sediments (Madsen and Sand-Jensen, 1991). POC can be degraded to form DOC; DOC can become POC by flocculation. Inorganic and organic C are linked through aquatic organisms. CO2 is used in photosynthesis (P) by for instance macrophytes, produced by respiration (R), and exchanged with the atmosphere. Organic C 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 (Thomas, 1997). Carbon can enter the system from the catchment and is transported to the oceans by rivers and streams. There is also exchange with C in the sediments, e.g., burial of organic carbon, which is important for C sequestration in aquatic habitats (Regnier et al., 2013). Aquatic systems are very important in global C sequestration; e.g., when different European ecosystems are compared, inland aquatic systems form the second largest C sink (19–41 Tg C y-1); only forests take up more C (125–223 Tg C y-1) (Luyssaert et al., 2012).

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