Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 156

flux of particulate organic carbon (POC) to the seafloor (Eppley and Peterson, 1979 ).
If the phytoplankton using this source of nitrate are able to leave the surface layer to
the deeper water or to the sediments (e.g. by sinking), then the POC has been
exported. Assuming a steady state over the region and time in question, new produc-
tion is also treated as export production, and the f-ratio equated with the export ratio
(Laws et al., 2000 ).

The role of cell size
The sinking speed of a phytoplankton cell can be estimated by looking at the balance
between the frictional force between the cell and the water, the buoyancy of the cell
arising from the contrast in density between the cell and the water and the gravita-
tional force pulling the cell downward. The physicist George Stokes calculated the
frictional force for the case of very small Reynolds numbers (see Chapter 4 ), which is
applicable to phytoplankton cells in water. The resulting settling velocity of the cell is
given by:

2

9 ð p Þ

ga p w cell ¼ ð 5 : 14 Þ

10 3
Nm 2 s is the dynamic viscosity of seawater, and remembering that a p is the radius of
the phytoplankton cell. Note that there is a strong dependence on cell size; larger cells
sink faster than small cells. In the phytoplankton community, this typically means
that dinoflagellates, coccolithophores, and particularly diatoms can export carbon,
while small flagellates and the cyanobacteria do so much less efficiently. A cell needs
to be bigger than about 5

with r p the density of the cell surrounded by water of density r, m ¼ 1.1

m before it is viewed as playing a significant role in the
vertical transport of carbon to depth (Legendre and Rivkin, 2002 ).

m

The Redfield ratio and achieving carbon export
We need to be careful in determining this potential for carbon export arising from
new primary production. The water that is mixed upward with a quantity of
nitrate will also contain DIC. If everything was in a strict Redfield balance then
that source of DIC would be exactly what is required by the phytoplankton when
they utilise the nitrate, and additional carbon from the atmosphere would not be
required. Carbon export requires the Redfield balance to be contravened at some
stage. For instance, source waters could have an excess of nitrate compared to the
Redfield C:N. In the open ocean deep water can contain 'pre-formed' nutrients
arising from before the water sank away from the surface, plus nutrients added
later due to the recycling of sinking detritus (Redfield et al., 1963 ). Alternatively,
the C:N ratio of the particulate organic material that sinks to the seabed could
have an excess in C due to different rates of recycling of the C and N components
of the particles (Kiriakoulakis, Stutt, et al., 2001 ; Schneider, Schlitzer, et al.,
2003 ), leading to a relative excess of C potentially buried and an excess of
inorganic nitrogen left behind in solution after the material's journey downward