Color Dissolved Organic Matter (CDOM)
Colored (or chromophoric) dissolved organic matter (CDOM) is the portion of organic matter that absorbs light in the blue and UV part of the electromagnetic spectrum, staining water a “tea-like” color. CDOM plays a major role in controlling freshwater ecosystem processes, determining physical and chemical conditions, and water quality in freshwater ecosystems. Dissolved organic matter occurs in all natural waters and CDOM is the most abundant DOM fraction in many natural waters, especially in forested watersheds with wetlands.
Aquatic dissolved organic matter (DOM) is derived from two types of natural sources: autochthonous production by aquatic organisms and terrestrial (allochthonous) plants. DOM also is derived from anthropogenic sources, including wastewater (effluent OM, EfOM) and urban and agricultural runoff. DOM differs chemically depending on its source. CDOM is produced primarily from vascular terrestrial or wetland plants and is the most abundant DOM fraction in many natural waters.
CDOM strongly attenuates solar radiation, with major implications for physical, chemical, and biological processes in surface waters. DOM, and especially CDOM, has significant impacts on surface water quality through its ability to (i) affect pH, (ii) mobilize metals and hydrophobic organic chemicals and (iii) serve as a source of reactive intermediates in aquatic photochemistry and biogeochemical processes. CDOM also regulates heat transfer to water, controlling lake temperatures, mixing and stratification. By reducing light, CDOM decreases photosynthesis and suppresses primary productivity. UV impacts on biota are strongly affected by CDOM. In addition, CDOM increases heterotrophic activity and shifts the metabolic balance in nutrient-poor lakes toward heterotrophy, stimulating carbon burial. In addition, CDOM affects nitrogen, phosphorus, and sulfur cycling through effects on microbial metabolism and redox status.
DOM is an important source of reactive intermediates in aquatic photochemistry. UV absorption by DOM or visible light by CDOM excites molecules and transforms some into excited triplet states, 3DOM*, which react with contaminants by electron or energy transfer or hydrogen abstraction. Quenching of 3DOM* by dissolved O2 forms singlet oxygen, 1O2. Electron transfer from 3DOM* to O2 forms superoxide radical anions, O2-∙, which react further to form hydrogen peroxide, H2O2, and other 3DOM* reaction pathways form hydroxyl radicals, ∙OH. These species are important in the indirect photolysis of organic contaminants. Photobleaching of CDOM occurs on time scale of weeks to months and is important in transforming recalcitrant CDOM molecules into bioavailable forms.
Although not directly harmful to human health, CDOM has negative effects on production of safe drinking water. DOM, and especially CDOM, increase the consumption of water treatment chemicals, react with chlorine to form potentially harmful disinfection by-products, stimulate bacterial growth, and foul filtration membranes.
CDOM is easily measured in the lab on a filtered water sample as light absorbance (A) at a specific wavelength using a spectrophotometer and commonly is reported as absorptivity, aλ, at wavelength λ. There is no standard wavelength for absorbance measurements; oceanographers use 412 nm, but freshwater scientists usually use 420 or 440 nm, and less commonly, a few other wavelengths. The relationship between aλ and A is: aλ = 2.303Aλ/ℓ (units of m-1), where ℓ is light path length in meters (m). The range of a440 values we have observed in Minnesota lakes is from near zero to ~ 30 m-1. An a440 value of ~ 3 m-1 can be considered an approximate upper limit for low-colored waters. Lakes with a440 > 3 m-1 appear visibly brown-stained even to casual observers. For context, Blueberry Lake and Burntside Lake, whose absorbance spectra are shown in the graph to the right, had a440 values of 19.6 and 1.2 m-1, respectively.
CDOM can be measured by satellite imagery; a variety of retrieval equations, most based on band-ratio models, have been reported. CDOM levels in surface waters have been shown to exhibit considerable spatial and temporal variability, and as a result, remote sensing methods are likely to play an important role in improving our understanding of the spatial distribution and temporal dynamics of CDOM in coming years.
CDOM’s role in regulating biogeochemical cycles, food webs and water quality is a paradigm emerging from research of the past two decades. We now know that DOM functions as one of a small number of "master variables", including pH, redox status and phosphorus, that control important aspects of how aquatic ecosystem work and respond to environmental change, and determine the quality of the water resources. Knowledge the sources and cycling of CDOM in ecosystems thus is of great importance for management and prediction of the outcomes of ongoing environmental change.