12.10: Organic Carbon

Most organic matter in the oceans is nonliving and exists as both dissolved compounds and particulate matter (detritus). The reason most organic matter is nonliving is that organic matter created by phytoplankton and other marine algae undergoes a series of transfers between organisms, and conversions among physical and chemical forms, before finally being converted back to dissolved inorganic constituents by decomposers.

Under normal conditions, about 10% of the organic carbon that phytoplankton create by photosynthesis is excreted to seawater as dissolved substances. When phytoplankton are stressed by low nutrient or low light levels, nonoptimal temperatures, or other unfavorable factors, they may excrete 50% or more of the organic matter they create by photosynthesis. Although most phytoplankton are eaten before they die, those that are not eaten excrete substantial amounts of dissolved and particulate organic material as they reach the end of the life cycle. In addition, dissolved and particulate organic matter are released when phytoplankton cells are ruptured by viruses and during inefficient feeding by herbivores.

The distinction between dissolved and particulate matter is arbitrary. Oceanographers usually consider compounds to be dissolved if they pass through a filter with a pore size of 0.5 μm (human hair is about 100 μm in diameter), although filters used to separate dissolved and organic matter often have pore sizes somewhat larger or smaller than this value. Material that collects on such a filter is called “particulate.” Much of the “dissolved” material that passes through a 0.5-μm filter is not truly dissolved, because it consists of colloidal-sized (CC4) inorganic or organic particles in addition to living organisms (such as microbacteria and archaea) and viruses. Nevertheless, the distinction between dissolved and particulate is useful. “Particles” less than 0.5 μm in diameter are believed to be too small to be eaten by most species in food chains that lead to higher animals, such as fishes.

Both particulate and dissolved organic matter are produced continuously wherever organisms are present in the oceans. Most organic matter is released in the photic zone, where marine organisms are concentrated. Many organic particles, such as fecal pellets and phytoplankton or zooplankton fragments, are relatively large (ranging from about 1 μm to more than 1 mm in diameter) and sink relatively fast (Chap. 6, CC4).

Marine bacteria, archaea, and fungi utilize organic particles and dissolved organic matter that is present throughout the ocean depths. These decomposers rapidly break down the more easily oxidized organic compounds in solution and in detritus. As particles are decomposed, smaller particles and more dissolved organic matter are released. Decomposition continues until all organic matter is converted to carbon dioxide and water, which may take many years or even centuries. There are three reasons for the long-term persistence of some dissolved and particulate organic matter. First, many organic compounds are extremely resistant to oxidation. Second, the rate of bacterial decomposition is very slow in the cold temperatures and high pressures of the deep oceans. Finally, dissolved organic molecules and microbial decomposers are so small and well dispersed in seawater that encounters between particles and decomposers are rare events.

The mass of dissolved and particulate matter in the oceans far exceeds the mass of living organisms because dissolved and particulate organic matter is produced continuously but much of it is decomposed only very slowly. In the photic zone, dissolved and particulate nonliving organic matter usually makes up 95% or more of the total organic carbon. An unknown proportion of the “dissolved” organic matter is, in fact, living microbes (including microbacteria and archaea) and viruses. Although the number of these microorganisms is extremely large, they are very small and their total biomass is thought to be only a small fraction of the nonliving organic matter. The declines in total mass of carbon from phytoplankton to zooplankton and from zooplankton to fishes reflect the low efficiency of food transfer between trophic levels (CC15).

Relatively little is known about the composition of dissolved or particulate organic matter. These materials are extremely difficult to study because they consist of hundreds of thousands or millions of chemical compounds, each present in extremely small concentrations. 

The concentration of particulate organic carbon is generally higher in the upper layers of the oceans than in deep waters. Within the surface layer, the concentration is lowest in areas where phytoplankton productivity is high. This seemingly paradoxical situation is created by the abundant populations of zooplankton and other filter-feeding organisms that thrive in productive waters because of the continuously abundant food supply. Particulate organic matter concentrations are high in deep waters below such productive areas because large numbers of fecal particles fall from the abundant filter-feeding organisms.

Organic particles generally account for about one-quarter of the suspended particles in ocean waters. In most areas, the majority of suspended particles are fragments of algae and animal hard parts with lesser amounts of lithogenous particles (Chap. 6).