For Scientists

The Biological Pump

 

The ocean has the capacity to absorb far more carbon dioxide than the atmosphere.  However, much less of the carbon dioxide produced by humans has gone into the oceans than you might expect, because the oceans are stratified.  This means that the deep layers of the ocean are much denser than the shallow layers and hence these two layers do not mix.  For this reason, plenty of anthropogenic carbon dioxide can go into the surface ocean (which is at equilibrium with the atmosphere), but very little of it makes it into the deep ocean.  Since less than 2% of the volume of the oceans is in the shallow layer that "communicates" with the atmosphere most of the ocean is not currently storing anthropogenic carbon dioxide.

 

There are two main processes that transport carbon dioxide from the surface to the deep ocean: the biological pump and the solubility pump.  The solubility pump refers to the fact that carbon dioxide is more soluble in cold water and that cold water is also denser and more likely to sink into the deep ocean.  The solubility pump is thus driven by a combination of ocean physics and chemistry.  The biological pump, as the name implies, is driven by marine biota.  Phytoplankton in the sunlit surface layers of the ocean take up carbon dioxide during photosynthesis.  This decreases the carbon dioxide concentration in the surface ocean, and draws carbon dioxide from the atmosphere into the surface ocean.  However, unlike trees on land, phytoplankton do not live very long.  Most have a life span of only days to a week.  Thus the biomass that they form from carbon dioxide is rapidly converted back into carbon dioxide due to the respiration of bacteria or zooplankton that feed on them.  This carbon dioxide can then be re-emitted into the atmosphere.  The only way that this carbon dioxide will be sequestered from the atmosphere for periods of interest to humans is if some of the organic matter formed by the phytoplankton is transferred into the deep ocean before it is converted back to carbon dioxide. The processes that transport this organic matter into the deep ocean are referred to as the biological pump.

 

The biological pump is driven by a suite of processes including: 1) Sinking particles (including dead phyto- or zooplankton and fecal pellets) and aggregates, 2) active transport by zooplankton and fish that feed in surface layers at night and but reside (and respire) at deeper depths during the day, and 3) vertical mixing and subduction of water masses that lead to passive flux of particles and dissolved organic molecules to depth.  Understanding how these processes vary in time and space is one of the primary goals of our research.  Sinking particles can be produced when zooplankton (e.g. krill) feed and defecate.  The fecal pellets produced by large zooplankton typically sink rapidly, although some other taxa may preferentially feed on such fecal pellets.  Furthermore, rapidly swimming zooplankton often fragment fecal pellets before they can sink out of the surface layers.  As these fecal pellets sink to deeper and deeper layers in the ocean, they are often colonized by microbial communities, that respire the carbon in these particles as they sink.  Other important classes of sinking particles are produced when zooplankton discard mucous feeding webs or when dead phytoplankton produce sticky molecules that cause them to aggregate into large "marine snow" particles that sink out of the surface ocean.  Understanding the biological pump thus requires study of ecological interactions amongst many different types of organisms living at different layers in the ocean.