Dissolved organic carbon

Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called particulate organic carbon (POC).

Dissolved organic matter (DOM) is a closely related term often used interchangeably with DOC. While DOC refers specifically to the mass of carbon in the dissolved organic material, DOM refers to the total mass of the dissolved organic matter. So DOM also includes the mass of other elements present in the organic material, such as nitrogen, oxygen and hydrogen. DOC is a component of DOM and there is typically about twice as much DOM as DOC. Many statements that can be made about DOC apply equally to DOM, and vice versa.

DOC is abundant in marine and freshwater systems and is one of the greatest cycled reservoirs of organic matter on Earth, accounting for the same amount of carbon as in the atmosphere and up to 20% of all organic carbon. In general, organic carbon compounds are the result of decomposition processes from dead organic matter including plants and animals. DOC can originate from within or outside any given body of water. DOC originating from within the body of water is known as autochthonous DOC and typically comes from aquatic plants or algae, while DOC originating outside the body of water is known as allochthonous DOC and typically comes from soils or terrestrial plants. When water originates from land areas with a high proportion of organic soils, these components can drain into rivers and lakes as DOC.

The marine DOC pool is important for the functioning of marine ecosystems because they are at the interface between the chemical and the biological worlds. DOC fuels marine food webs, and is a major component of the Earth's carbon cycling.

DOC is a basic nutrient, supporting growth of microorganisms and plays an important role in the global carbon cycle through the microbial loop. In some organisms (stages) that do not feed in the traditional sense, dissolved matter may be the only external food source. Moreover, DOC is an indicator of organic loadings in streams, as well as supporting terrestrial processing (e.g., within soil, forests, and wetlands) of organic matter. Dissolved organic carbon has a high proportion of biodegradable dissolved organic carbon (BDOC) in first order streams compared to higher order streams. In the absence of extensive wetlands, bogs, or swamps, baseflow concentrations of DOC in undisturbed watersheds generally range from approximately 1 to 20 mg/L carbon. Carbon concentrations considerably vary 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 concentrations of organic carbon indicate anthropogenic influences, but most DOC originates naturally.

The BDOC fraction consists of organic molecules that heterotrophic bacteria can use as a source of energy and carbon. Some subset of DOC constitutes the precursors of disinfection byproducts for drinking water. BDOC can contribute to undesirable biological regrowth within water distribution systems.

The dissolved fraction of total organic carbon (TOC) is an operational classification. Many researchers use the term "dissolved" for compounds that pass through a 0.45 μm filter, but 0.22 μm filters have also been used to remove higher colloidal concentrations.

A practical definition of dissolved typically used in marine chemistry is all substances that pass through a GF/F filter, which has a nominal pore size of approximately 0.7 μm (Whatman glass microfiber filter, 0.6–0.8 μm particle retention). The recommended procedure is the HTCO technique, which calls for filtration through pre-combusted glass fiber filters, typically the GF/F classification.