Chromophoric Dissolved Organic Matter, or CDOM, is a widespread group of organic molecules found in natural waters worldwide. It is present in oceans, lakes, rivers, and groundwater. Understanding CDOM is key to comprehending the functioning and health of aquatic ecosystems.
Understanding CDOM
The term “chromophoric” in CDOM refers to the light-absorbing properties of these organic molecules. Chromophores are specific parts of molecules that absorb light, particularly in the ultraviolet (UV) and visible regions of the electromagnetic spectrum. Dissolved organic matter, on the other hand, comprises a diverse mixture of carbon-based compounds that are soluble in water.
CDOM is not a single chemical entity but a highly heterogeneous collection of organic substances. This mixture includes humic and fulvic acids, lignin-derived compounds, and breakdown products from various biological processes. Its characteristic yellowish-brown color results from this light absorption.
Where CDOM Comes From
CDOM originates from both terrestrial and aquatic environments through natural processes. Terrestrial sources contribute significantly, as water runoff from soils, decaying plant matter, and the degradation of humic substances in forests and wetlands transport organic compounds into rivers and lakes. These carbon-rich substances are then carried downstream into estuaries and coastal oceans.
Within aquatic systems, CDOM is also produced through the decomposition of phytoplankton, algae, and other aquatic organisms. Microbial breakdown of these biological materials releases soluble organic compounds into the water column. Additionally, human activities like wastewater discharge and agricultural runoff introduce more organic matter, influencing CDOM levels.
CDOM’s Role in Aquatic Environments
CDOM plays multiple roles within aquatic environments, influencing light conditions, biological processes, and ecosystem health. A primary function is light attenuation, where CDOM absorbs sunlight, especially in the UV and blue light portions of the spectrum. This absorption reduces the depth to which light can penetrate the water column.
The reduction in light penetration directly impacts primary productivity, the rate at which aquatic plants and algae convert light energy into organic matter through photosynthesis. In waters with high CDOM concentrations, photosynthetic organisms are limited to shallower depths. This can alter the distribution and abundance of phytoplankton and submerged aquatic vegetation.
CDOM also acts as a natural “sunscreen” for aquatic organisms, protecting them from harmful ultraviolet radiation. UV radiation can damage DNA and proteins, impairing growth and reproduction in aquatic life. By absorbing incoming UV light, CDOM creates a safer environment for many species, particularly those living near the surface.
CDOM is a significant component of the global carbon cycle, serving as a reservoir and transport mechanism for organic carbon in aquatic systems. It moves carbon from land to oceans and influences carbon bioavailability, which is the ease with which carbon can be utilized by microbes. The availability of CDOM impacts microbial activity, as bacteria break down these organic compounds, releasing carbon dioxide or incorporating carbon into their biomass.
Levels of CDOM can also serve as an indicator of water quality and ecosystem health. Changes in CDOM concentrations can reflect shifts in land-use patterns, such as deforestation or urbanization, which alter runoff characteristics. Elevated CDOM might also signal increased organic pollution from sources like agricultural runoff or wastewater discharge, providing insights into environmental disturbances.
How Scientists Study CDOM
Scientists employ various methods to measure and analyze CDOM, primarily focusing on its optical properties. Spectroscopic techniques are widely used, with absorption spectroscopy being a common approach. This method measures how much light at different wavelengths is absorbed by the water sample, providing a quantitative assessment of CDOM concentration and its spectral characteristics.
Fluorescence spectroscopy is another powerful tool, where scientists excite CDOM molecules with specific wavelengths of light and then measure the light emitted back. The intensity and spectral patterns of this emitted light can reveal information about the quality, origin, and molecular complexity of the CDOM present. These spectroscopic signatures help researchers understand the sources and transformations of organic matter in aquatic environments.