Fluorescent Dissolved Organic Matter, or FDOM, is a naturally occurring fraction of dissolved organic matter (DOM) found in aquatic environments. It consists of a complex mixture of organic compounds that fluoresce when exposed to specific wavelengths of light. FDOM is present in various water bodies, from rivers and lakes to oceans, and its characteristics provide valuable insights into water quality and ecosystem health.
Understanding FDOM: What It Is and Where It Comes From
FDOM is a complex mixture of organic compounds originating from both natural and human-influenced sources. Its chemical composition is diverse, including humic-like substances, characterized by complex molecular structures derived from the breakdown of terrestrial plant and soil material, and protein-like substances, which often originate from microbial activity or biological production.
The decomposition of plants and animals in terrestrial environments releases organic compounds into soils, which can then leach into rivers and streams through runoff. Microbial activity within aquatic environments also contributes to FDOM, as microorganisms like bacteria and algae produce and release organic compounds. River discharge is a major source of humic-like FDOM in coastal waters, while protein-like FDOM can stem from biological production.
Human activities also introduce FDOM into aquatic systems. Wastewater discharge can contribute protein-like and detergent-like FDOM components, indicating sewage pollution. Agricultural runoff, containing decomposed plant matter and fertilizers, can alter the composition of DOM in water bodies. Industrial effluents introduce specific fluorescent compounds that can act as indicators of pollution.
The Role of FDOM in Aquatic Ecosystems
FDOM plays a multifaceted role within aquatic ecosystems, influencing various biological and physical processes. It impacts light penetration in water bodies by absorbing light, particularly in the ultraviolet and blue regions of the spectrum. This reduces the sunlight available for photosynthesis by aquatic plants and algae, which can affect primary productivity, the foundation of aquatic food webs.
FDOM is also a component of the global carbon cycle. Dissolved organic carbon (DOC), which includes FDOM, is transported by rivers, moving carbon from land to oceans. FDOM can serve as a nutrient source for microorganisms, providing energy and organic nutrients for bacterial metabolism. Its availability influences microbial community structure and activity within the water column.
FDOM contributes to the color of water, often imparting a yellow or brown hue, especially in systems rich in terrestrial organic matter. This coloration can influence the thermal properties of water bodies by affecting how much solar radiation is absorbed, potentially leading to warmer surface waters. The presence and characteristics of FDOM are intertwined with the overall functioning and health of aquatic environments.
FDOM as an Environmental Indicator
Scientists use FDOM as a natural “fingerprint” to gain insights into environmental conditions and processes. Its unique fluorescent properties allow researchers to identify different organic compounds and their origins, such as terrestrial plant material or algae. This makes FDOM a valuable tool for tracking water sources and understanding mixing patterns between water bodies, like river water and seawater.
FDOM can also serve as an indicator of pollution. Specific fluorescent components within FDOM can be linked to anthropogenic sources like wastewater contamination or agricultural runoff. For example, the presence of certain protein-like or detergent-like FDOM components can signal sewage pollution or industrial discharge. Monitoring FDOM helps assess aquatic ecosystem health by revealing the presence and types of organic pollutants.
Changes in FDOM composition can also indicate broader environmental shifts. Scientists can detect alterations in natural organic matter composition due to factors like climate change or land use changes by tracking FDOM characteristics over time. Increased river discharge due to changing climate patterns, for instance, can lead to higher levels of humic-like FDOM in coastal areas. This allows researchers to monitor the impact of human activities and natural variability on water quality.
How Scientists Study FDOM
Scientists primarily study FDOM using fluorescence spectroscopy, a technique that exploits its light-emitting properties. This method involves exciting a water sample with light at specific wavelengths and then measuring the light emitted back as fluorescence. Different types of FDOM, due to their distinct chemical structures, absorb and emit light at different characteristic wavelengths.
By analyzing the excitation and emission spectra, scientists can identify various FDOM components, such as humic-like or protein-like substances. Humic-like FDOM, for instance, generally has broader emission and excitation spectra compared to protein-like FDOM. This allows for the characterization of the FDOM mixture in a water sample. Advanced techniques like excitation-emission matrix (EEM) spectroscopy, often coupled with parallel factor analysis (PARAFAC), distinguish between allochthonous (externally derived) and autochthonous (internally produced) sources of dissolved organic matter.