The marine biological pump depends on a number of key pools, components and processes that influence its functioning. There are four main pools of carbon in the ocean.
* Dissolved inorganic carbon (DIC) is the largest pool. It constitutes around 38,000 Pg C and includes dissolved carbon dioxide (CO), bicarbonate (), carbonate (), and carbonic acid (). The equilibrium between carbonic acid and carbonate determines the pH of the seawater. Carbon dioxide dissolves easily in water and its solubility is inversely related to temperature. Dissolved CO is taken up in the process of photosynthesis, and can reduce the partial pressure of CO in the seawater, favouring drawdown from the atmosphere. The reverse process respiration, releases CO back into the water, can increase partial pressure of CO in the seawater, favouring release back to the atmosphere. The formation of calcium carbonate by organisms such as coccolithophores has the effect of releasing CO into the water.
* Dissolved organic carbon (DOC) is the next largest pool at around 662 Pg C. DOC can be classified according to its reactivity as refractory, semi-labile or labile. The labile pool constitutes around 0.2 Pg C, is bioavailable, and has a high production rate (~ 15−25 Pg C y). The refractory component is the biggest pool (~642 Pg C ± 32; but has a very low turnover rate (0.043 Pg C y). The turnover time for refractory DOC is thought to be greater than 1000 years.
* Particulate organic carbon (POC) constitutes around 2.3 Pg C, and is relatively small compared with DIC and DOC. Though small in size, this pool is highly dynamic, having the highest turnover rate of any organic carbon pool on the planet. Driven by primary production, it produces around 50 Pg C y globally. It can be separated into living (e.g. phytoplankton, zooplankton, bacteria) and non-living (e.g. detritus) material. Of these, the phytoplankton carbon is particularly important, because of its role in marine primary production, and also because it serves as the food resource for all the larger organisms in the pelagic ecosystem.
* Particulate inorganic carbon (PIC) is the smallest of the pools at around 0.03 Pg C. It is present in the form of calcium carbonate (CaCO) in particulate form, and impacts the carbonate system and pH of the seawater. Estimates for PIC production are in the region of 0.8–1.4 Pg C y, with at least 65% of it being dissolved in the upper water column, the rest contributing to deep sediments. Coccolithophores and foraminifera are estimated to be the dominant sources of PIC in the open ocean. The PIC pool is of particular importance due to its role in the ocean carbonate system, and in facilitating the export of carbon to the deep ocean through the carbonate pump, whereby PIC is exported out of the photic zone and deposited in the bottom sediments.