That simple "down arrow"
in the ocean on the previous diagram is really a very complex food
web that is the underlying engine for the pump. Phytoplankton, which
are microscopic photosynthetic plants in the oceans, draw CO2
out of the sea water and use it in photosynthesis in order to grow.
They're eaten by zooplankton, and small animals and fish, and as
these organisms respire they convert the phytoplankton biomass back
to CO2. Thus there are huge CO2-in/CO2-out
fluxes in the surface ocean.
But some fraction of that biomass produced by the
phytoplankton finds its way to the deep ocean, where it's regenerated
to CO2 - thus creating a concentration
gradient of CO2 from the surface
to the deep ocean, and maintaining a large reservoir of CO2
in this deep ocean ecosystem. It is well known that the fraction
of phytoplankton biomass that makes it to the deep ocean is related
to the entire structure of this complex community.
And it's well known that the rate of photosynthesis of the phytoplankton
is a function of the amount of nutrients that are available to these
cells. Most of those nutrients, especially nitrogen and phosphorus
are supplied to the sun-lit surface waters from upwelling of the
deep ocean water.
For years we thought that nitrogen and phosphorus were the key
nutrients that limited photosynthesis in the oceans, but there were
some puzzling observations. Looking at satellite img of the global
distribution of phytoplankton biomass, we can see that it varies
you see green and red and yellow, on
this map are areas of high phytoplankton biomass, and where
you see purple and blue, low phytoplankton biomass. But we have
learned that in the equatorial Pacific, the suparctic Pacific, and
the Southern ocean, there are much higher levels of N and P than
one would expect based on the relatively low phytoplankton biomass.
Why aren't these regions more productive?
In other words, if the phytoplankton were utilizing all of the
available nitrogen and phosphorus, these regions would have much
higher levels of phytoplankton. This was a real enigma to oceanographers
for years until the late John Martin developed his iron hypothesis.
He proposed that the reason the phytoplankton can't utilize the
available nutrients in these areas-- primarily the Equatorial Pacific
and the Southern Ocean -- is that there's not enough iron available