Particulate organic carbon (POC) is a collection of carbon-based particles from living or once-living organisms suspended in water. Operationally, scientists define POC as the organic material captured on a filter with a pore size of around 0.2 to 0.7 micrometers. This material includes a wide variety of biological matter, from microscopic cells to the decomposing remains of larger organisms.
Formation and Composition of Particulate Organic Carbon
The origins of particulate organic carbon are separated into two categories. The first, known as autochthonous sources, is material generated from within the aquatic system itself. This is primarily composed of dead phytoplankton (microscopic marine algae) and the zooplankton that graze upon them. It also includes the waste pellets excreted by these and other organisms.
The second category is allochthonous, which consists of organic material from outside the aquatic environment. This includes terrestrial plant matter, such as leaves and wood fragments, and carbon-rich soil particles washed into rivers. This land-based carbon is carried out to sea, where it mixes with the internally produced organic matter.
The chemical makeup of POC is a complex mixture of organic compounds. A significant portion consists of biomolecules such as proteins, lipids, and carbohydrates. The exact composition of POC in any given area of water can vary greatly depending on its primary source, such as a phytoplankton bloom or a river carrying heavy loads of soil.
The Role of POC in Aquatic Food Webs
Particulate organic carbon serves as an energy source for life within aquatic ecosystems. As these particles of dead organisms and waste sink, they form a phenomenon known as “marine snow.” This shower of organic material drifts from the sunlit surface waters down into the deeper parts of the ocean. This process provides a food supply to the communities living in these lower zones.
For organisms in the deep ocean (the benthos), this sinking snow is a primary source of sustenance. Since sunlight cannot penetrate these depths, photosynthesis is impossible, so deep-sea creatures depend on the rain of POC from above. Filter-feeding animals, such as certain corals and sponges, capture these particles directly from the water column.
This downward flux of organic material also nourishes microbes. Bacteria and other microorganisms colonize the sinking particles, decomposing them and releasing nutrients back into the water. In doing so, these microbes become a food source for other organisms, forming the base of a complex deep-sea food web. This transfer of energy from the surface to the deep sea supports a significant amount of the ocean’s biodiversity.
Particulate Organic Carbon and the Global Carbon Cycle
Particulate organic carbon is a component of the global carbon cycle through the biological carbon pump. This natural mechanism transports carbon from the atmosphere and surface ocean into the deep sea for long-term storage. The process begins at the ocean’s surface, where phytoplankton absorb atmospheric carbon dioxide (CO2) through photosynthesis, converting it into organic matter.
When these phytoplankton die, or are consumed and excreted as waste pellets, their carbon-containing bodies form POC particles. A portion of this POC sinks from the surface waters toward the ocean floor. As it descends, it carries its embedded carbon with it, removing that carbon from the upper ocean and atmosphere and moving it into the ocean’s interior.
Once in the deep ocean, this carbon can be sequestered for hundreds or even thousands of years. The efficiency of this biological pump is a determining factor in the ocean’s capacity to absorb atmospheric CO2. The amount of carbon that reaches the deep sea without being decomposed in the upper layers directly impacts global climate patterns. This makes the formation and sinking of POC an important climate-regulating process.
Factors Influencing POC Concentrations
The concentration and distribution of POC are influenced by a wide range of environmental variables. For instance, ocean currents can transport particles over vast distances. Seasonal changes in sunlight and temperature drive the blooms of phytoplankton that generate large quantities of fresh POC.
Human activities are also impacting POC levels. Rising sea surface temperatures can alter the composition of phytoplankton communities, sometimes favoring smaller species that are less efficient at sinking. This change can weaken the biological carbon pump, reducing the amount of carbon transported to the deep sea.
Nutrient pollution from agricultural runoff and wastewater discharge also affects POC levels. These excess nutrients, particularly nitrogen and phosphorus, can trigger large algal blooms in coastal areas. While these blooms generate a large amount of POC, their decomposition by bacteria consumes vast quantities of oxygen, creating low-oxygen or “hypoxic” zones. Ocean acidification may also interfere with the organisms that produce POC, though the full effects are still an area of active research.