Dissolved organic carbon (DOC) is a widely distributed component of the global carbon cycle, found in all natural waters. This complex mixture of organic molecules plays a fundamental role in various environmental processes, influencing aquatic ecosystems and broader biogeochemical cycles. Understanding DOC provides insights into how carbon moves through different environments and impacts water body health.
What is Dissolved Organic Carbon
Dissolved organic carbon refers to carbon-containing compounds dissolved in water, able to pass through a fine filter, typically with a pore size of 0.45 micrometers. These compounds derive from living or once-living organisms, such as plants, animals, and microbes. DOC is not a single chemical substance but a highly diverse and heterogeneous mixture of organic molecules, including simple sugars, amino acids, humic substances, and proteins.
The precise composition of DOC varies significantly depending on its origin and the specific aquatic environment. It exists in various forms within oceans, freshwater lakes, rivers, and groundwater systems. This diverse nature reflects the biological and geological history of the water.
Sources of Dissolved Organic Carbon
Dissolved organic carbon originates from a variety of sources, both natural and human-influenced. A significant natural source is the decomposition of organic matter from plants and animals in terrestrial and aquatic settings. As leaves, wood, and other organic debris break down, they release soluble carbon compounds into surrounding waters. Leaching from soils also contributes as water percolates through the ground, picking up organic substances.
Aquatic organisms themselves contribute to DOC through excretion and the release of cellular material during their life cycles. For instance, algae and aquatic plants release organic compounds into the water during photosynthesis. Beyond these natural processes, human activities also introduce DOC into water bodies. Agricultural runoff, laden with decomposed organic material from fields, can elevate DOC levels in nearby streams and rivers. Wastewater discharge from municipal and industrial sources also contains a range of dissolved organic compounds.
Role in Aquatic Ecosystems
Dissolved organic carbon serves as a dynamic link in the global carbon cycle, connecting terrestrial and aquatic environments. It represents a significant pool of carbon that can be transported over long distances, influencing carbon budgets across different biomes.
Beyond its role in carbon transport, DOC acts as a primary energy and nutrient source for various aquatic microbes, including bacteria and fungi. These microorganisms metabolize DOC, driving microbial food webs and influencing nutrient cycling within water bodies. The availability and composition of DOC directly impact the growth and activity of these microbial communities, which form the base of many aquatic ecosystems. Consequently, changes in DOC can cascade through the food web, affecting larger organisms.
Furthermore, DOC impacts several aspects of water quality, including its color and the penetration of light. High concentrations of certain DOC compounds can impart a brownish tint to water, reducing light availability for submerged aquatic plants and algae. DOC also possesses the ability to bind with and transport various substances, such as nutrients, metals, and pollutants. This binding capacity influences the mobility and bioavailability of these substances, affecting their ecological impact within aquatic systems.
Measuring Dissolved Organic Carbon
Scientists quantify dissolved organic carbon to understand its abundance and dynamics within different aquatic environments. DOC is typically measured as the total concentration of organic carbon after particulate matter has been removed. This filtration step ensures that only the truly dissolved fraction of organic carbon is analyzed. The most common analytical technique involves high-temperature combustion.
In this method, a water sample is first acidified to remove inorganic carbon, such as carbonates. The sample is then injected into a high-temperature furnace, where the organic carbon compounds are completely oxidized. This oxidation converts all organic carbon into carbon dioxide (CO2) gas. The amount of CO2 produced is then measured using an infrared detector, which directly correlates to the original concentration of dissolved organic carbon in the sample. Studying DOC remains challenging due to its often low concentrations, requiring sensitive and precise instrumentation.