Dissolved organic carbon
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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 use the term "dissolved" for compounds below 0.45 micrometers, but 0.22 micrometers is also common, saving colloidal for higher concentrations. A practical definition of dissolved typically used in marine chemistry is all substances that pass through a GF/F filter. The recommended measure technique is the HTCO technique after filtration on precombusted glass fiber filters, typically GF/F filters.
DOC in marine and freshwater systems is one of the greatest cycled reservoirs of organic matter on Earth, accounting for the same amount of carbon as the atmosphere and up to 20% of all organic carbon. The source of DOC depends on the body of water. In general, organic carbon compounds are a result of decomposition processes from dead organic matter such as plants or marine organisms. When water contacts highly organic soils, these components can drain into rivers and lakes as DOC.
DOC is a food supplement, supporting growth of microorganisms and plays an important role in the global carbon cycle through the microbial loop. Moreover it 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.
More precise measurement techniques developed in the late 1990s have allowed for a good understanding of how DOC is distributed in marine environments both vertically and across the surface. It is now understood that DOC in the ocean spans a range from very labile to very refractory. The labile DOC is mainly produced by marine organisms and is consumed in the surface ocean, and consists of sugars, proteins, and other compounds that are easily used by marine bacteria. The refractory DOC is evenly spread throughout the water column and consists of high molecular weight and structurally complex compounds that are difficult for marine organisms to use such as the lignin, pollen, or humic acids. Therefore, the observed vertical distribution consists of high concentrations in the upper water column and low concentrations at depth.
In addition to vertical distributions, horizontal distributions have been modeled and sampled as well. In the surface ocean at a depth of 30 meters, the higher DOC concentrations are found in the South Pacific Gyre, the South Atlantic Gyre, and the Indian Ocean. At a depth of 3000 meters, highest concentrations are in the North Atlantic Deep Water where DOC from the high concentration surface ocean is removed to depth. Since the time scales of horizontal motion along the ocean bottom are in the thousands of years, the refractory DOC is slowly consumed on its way from the North Atlantic and reaches a minimum in the North Pacific.
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