Dissolved organic carbon

Dissolved organic carbon (DOC) is the portion of organic carbon that, by operational definition, slips through a filter with pores typically sized between 0.22 and 0.7 micrometers. The material caught on the filter is called particulate organic carbon (POC). A related term, dissolved organic matter (DOM), is often used interchangeably with DOC, but they are not identical: DOC refers only to the carbon mass within the dissolved material, while DOM accounts for the total mass, including other elements like nitrogen, oxygen, and hydrogen. As a result, DOM is usually about twice as heavy as DOC, and most observations about one apply to the other.

DOC is plentiful in both marine and freshwater environments and ranks among Earth’s largest active reservoirs of organic matter. It holds as much carbon as the atmosphere and makes up as much as 20% of all organic carbon. Generally, organic carbon compounds arise from the decomposition of dead organic matter, such as plants and animals. DOC can come from inside a body of water—called autochthonous DOC, typically from aquatic plants or algae—or from outside it, known as allochthonous DOC, usually from soils or terrestrial plants. When water drains from areas rich in organic soils, these compounds can wash into rivers and lakes as DOC.

The marine DOC pool is crucial for ecosystem function because it sits at the boundary between chemical and biological processes. It fuels marine food webs and is a major player in Earth’s carbon cycle.

DOC serves as a basic nutrient, supporting microorganism growth and driving the global carbon cycle via the microbial loop. For some organisms or life stages that do not feed in the usual way, dissolved matter may be their only external food source. DOC also acts as an indicator of organic loading in streams and supports the processing of organic matter in terrestrial settings like soil, forests, and wetlands. In undisturbed watersheds without extensive wetlands, bogs, or swamps, baseflow DOC concentrations typically range from about 1 to 20 mg/L of carbon. Carbon levels vary widely across ecosystems: the Everglades may be near the high end, while the open ocean may be near the low end. Occasionally, high organic carbon levels point to human influence, but most DOC comes from natural sources.