Carbon in the environment continually cycles between the land, the atmosphere, and the oceans. The oceanic carbon pool is one of the largest of these, with a capacity under equilibrium conditions to contain 50 times more carbon than the atmosphere.
Phytoplankton in the surface ocean incorporate this inorganic carbon into organic carbon through photosynthesis (primary production). Between 2% and 50% of this organic material settles out of the surface ocean as particulate material (POC), consisting of heterogeneous aggregates (marine snow), zooplankton fecal material, and phytoplankton aggregates. This POC provides a food source for both zooplankton and bacteria, which can recycle some of the material back into inorganic forms (e.g., through respiration) or repackage it for further settling (Figure 1).