Plants take up CO2 – they essentially breathe it in and use it to build their physical structures – and phytoplankton are basically teeny tiny microscopic plants that live in the surface of the ocean, so essentially you have in the surface of the ocean these little tiny microscopic plants are taking carbon from the water and using it to build parts of their bodies.
Now as the phytoplankton mature, there are a couple of different things that can happen. One thing that can happen is they can be recycled back into the biological processes of the surface ocean – so maybe zooplankton eats them, maybe they die and they are returned to their component minerals by natural processes and that carbon is taken up by some other phytoplankton. But the other thing that can happen is that, when they die, they can precipitate down into the deep ocean and when that happens because ocean circulation is so slow that carbon can be stored or sequestered in the ocean for a very long period of time.
Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)
Phytoplankton and zooplankton images courtesy of NIWA
Satellite image of phytoplankton bloom off East coast of NZ, by Jeff Schmaltz, courtesy of NASA and MODIS Rapid Response Team