Fluorescent Dissolved Organic Matter (FDOM) is a naturally occurring optical tracer used by scientists to quickly assess the condition of aquatic environments. FDOM is the fraction of dissolved organic molecules that can fluoresce, or emit light, when stimulated by specific wavelengths of energy. This measurable property makes FDOM an effective proxy for tracking the movement and composition of the entire dissolved organic matter pool in rivers, lakes, and oceans. FDOM measurements are now a standard component in water quality monitoring, offering near real-time insights into pollution events, ecosystem health, and drinking water safety.
Defining Dissolved Organic Matter
Dissolved Organic Matter (DOM) is the non-living, carbon-based material in water small enough to pass through a fine filter, typically 0.45 micrometers. This mixture includes compounds like humic acids, fulvic acids, amino acids, and carbohydrates. DOM originates from the breakdown of terrestrial sources, such as leaf litter and soil, and aquatic sources, including algae and microbial cells.
DOM is a factor in aquatic chemistry because it influences nutrient cycling, light penetration, and the transport of contaminants. It is distinct from particulate organic matter, which consists of larger suspended solids and living organisms. The specific composition of the DOM pool—whether it contains large, stable molecules or smaller, recently produced components—provides clues about the processes occurring within the watershed.
How FDOM is Measured
FDOM measurement relies on the principle of fluorescence: molecules absorb light energy at a short wavelength and then re-emit that energy at a longer wavelength. FDOM is a subset of Colored Dissolved Organic Matter (CDOM), which is the portion of DOM that absorbs light and can color the water yellow or brown. Only a small fraction of CDOM, however, exhibits fluorescence.
Scientists use specialized instruments called fluorometers to stimulate the water sample with an excitation light beam and measure the resulting emission light. For detailed characterization, researchers employ Excitation-Emission Matrix (EEM) spectroscopy. This method systematically scans the water across a wide range of wavelengths, generating a unique spectral “fingerprint” of the FDOM present. Analyzing this matrix allows for the identification and quantification of different FDOM components, such as humic-like substances from soil and protein-like substances from microbial activity.
What FDOM Reveals About Water Health
FDOM serves as a sensitive indicator of water health, often revealing changes faster than traditional chemical tests. High levels of humic-like FDOM signal a substantial input of terrestrial material, such as soil runoff during heavy rainfall events. Conversely, an increase in protein-like FDOM, specifically the tryptophan-like component, is a direct sign of recent microbial activity or contamination, frequently indicating wastewater or sewage effluent.
The ratio of protein-like to humic-like FDOM, known as the T/C ratio, is useful for detecting episodic pollution events from sources like sewage treatment works. A sudden spike in this ratio can reveal contamination that a traditional, infrequent sampling program might otherwise miss. In drinking water treatment, FDOM is monitored because these organic molecules are precursors for Disinfection Byproducts (DBPs) when water is chlorinated. Elevated FDOM levels in source water predict a higher potential for DBP formation, guiding operators to adjust processes to minimize these compounds.
FDOM also affects light penetration. High concentrations of the colored fraction can block sunlight from reaching aquatic plants like seagrasses, limiting their photosynthesis. The microbial breakdown of organic matter can deplete dissolved oxygen, potentially leading to hypoxic conditions that harm aquatic life. Continuous FDOM measurement allows environmental managers to track these changes in real-time and assess the ecological balance of a water body.
Sources and Cycling of FDOM
The origin of FDOM is categorized into two types: allochthonous and autochthonous, each having distinct chemical signatures.
Allochthonous FDOM
Allochthonous FDOM is derived from outside the aquatic system, primarily from the surrounding land, where it is leached from decaying plant matter, leaf litter, and soil organic matter. These terrestrial sources are rich in large, complex humic and fulvic acids, which contribute the majority of the humic-like FDOM component.
Autochthonous FDOM
Autochthonous FDOM is produced within the water body itself, originating from the metabolic activities of algae, phytoplankton, and bacteria. This fraction is characterized by smaller, protein-like molecules, such as tryptophan and tyrosine. These molecules are readily biodegradable and indicate recent biological productivity or decomposition.
Both types of FDOM represent a major mobile pool of organic carbon in aquatic systems, playing a role in the global carbon cycle as they are transported from land to the ocean. Once in the water, FDOM undergoes transformation processes, including photodegradation by sunlight in surface waters, which breaks down the complex molecules. Microbial communities also consume FDOM, rapidly degrading the protein-like components and slowly processing the more recalcitrant humic-like material. Monitoring the shift in FDOM composition and intensity over time helps scientists understand the rate at which organic carbon is being processed and removed from the water column.