Shining a light on the ocean's twilight zone



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).