Dissolved organic carbon (DOC) is a broad classification for organic molecules of varied origin and composition within aquatic systems. The "dissolved" fraction of organic carbon is an operational classification. Many researchers place the dissolved/colloidal cutoff at 0.45 micrometers, but 0.22 micrometers is also typical. The DOC in marine and freshwater systems are one of the greatest cycled reservoir of organic matter on Earth. The majority of the DOC is caused by different sources, depending on the focused body of water. In general, organic carbon compounds are a result of decomposition processes from higher organic matter such as plants. When water gets in touch with high organic soils, a certain amount of these components can be drained into rivers and lakes as DOC. DOC is also extremely important in the transport of metals in aquatic systems. Metals form extremely strong complexes with DOC, enhancing metal solubility while also reducing metal bioavailability.
Significance
DOC causes an increased growth of microorganisms, as it is a food supplement. Moreover it is an indicator of organic loadings to streams, as well as terrestrial processing (e.g., within the soil, forests, and wetlands) of organic matter. In the absence of extensive wetlands, bogs, or swamps, baseflow concentrations of DOC in undisturbed watersheds generally range from approximately 1 to 20 mg carbon/L. There is a high degree of variability in carbon concentrations across ecosystems. For example, the Everglades may be near the top of the range and the middle of oceans may be near the bottom. Occasionally, high levels of organic carbon indicate anthropogenic influences, but most DOC is natural in origin. The biodegradable DOC fraction (BDOC) consists of organic molecules that heterotrophic bacteria can utilize as a source of energy and carbon. Within the context of drinking water quality, some subset of DOC constitutes the precursors of disinfection byproducts, and the BDOC constitutes the nutritional resources that can contribute to biological regrowth within water distribution systems.
