Colored dissolved organic matter
Colored dissolved organic matter (CDOM) is the optically measurable component of the dissolved organic matter in water. Also known as chromophoric dissolved organic matter, yellow substance, and gelbstoff, CDOM occurs naturally in aquatic environments primarily as a result of tannins released from decaying detritus. CDOM most strongly absorbs short wavelength light ranging from blue to ultraviolet, whereas pure water absorbs longer wavelength red light. Therefore, non-turbid water with little or no CDOM appears blue. The color of water will range through green, yellow-green, and brown as CDOM increases.
CDOM can have a significant effect on biological activity in aquatic systems. CDOM diminishes light as it penetrates water. This has a limiting effect on photosynthesis and can inhibit the growth of phytoplankton populations, which form the basis of oceanic food chains and are a primary source of atmospheric oxygen. CDOM also absorbs harmful UVA/B radiation, protecting organisms from DNA damage.
Absorption of UV radiation causes CDOM to "bleach", reducing its optical density and absorptive capacity. This bleaching (photodegradation) of CDOM produces both low-molecular-weight organic compounds which may be utilized by microbes, and reactive oxygen species, which may damage tissues and alter the bioavailability of limiting trace metals.
CDOM also interferes with the use of satellite spectrometers to remotely estimate phytoplankton population distribution. As a pigment necessary for photosynthesis, chlorophyll is a key indicator of the phytoplankton abundance. However, CDOM and chlorophyll both absorb light in the same spectral range so it is difficult to differentiate between the two.
Although variations in CDOM are primarily the result of natural processes, human activities such as logging, agriculture, effluent discharge, and wetland drainage can affect CDOM levels in fresh water and estuarine systems. In general, CDOM concentrations are much higher in fresh waters and estuaries than in the open ocean, though concentrations are highly variable.