Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 150

with

N
max the maximum growth rate. The quotas Q min and Q max are the minimum
(sometimes referred to as 'subsistence') and maximum nutrient quotas within the cell.
Remember that cell respiration and grazing would have to be applied to both
calculations of growth rate. Now imagine a phytoplankton cell in the nutrient
environment of Fig. 5.8 . In the bottom layer, the cell may have a high nutrient quota
and a correspondingly high growth rate from Equation (5.11) . However, there is not
a lot of light in the bottom layer, so the growth rate predicted by Equation (5.10)
would be very low. Conversely, a cell in the surface layer has access to plenty of light,
but could be starved of sufficient nitrate, in which case Equation (5.11) would predict
a lower growth rate. One method of incorporating the possibility of either light or
nutrients being limiting to growth is to calculate growth rates based on Equations
(5.10) and (5.11) and use whichever is the lower (e.g. Tett et al., 1986 ). This is an
application of 'Liebig's Law of the Minimum', where growth is controlled by the
scarcest resource. An alternative approach is to use the cell nutrient status to
moderate the maximum photosynthetic rate, i.e. in Equation (5.10) we replace P b max
(e.g. Geider et al., 1998 ):

Q min
Q max

Q N

P b max ! P b max

: ð : Þ

5 12

Q min

Redfield ratios
In 1934 the American oceanographer Alfred Redfield reported a remarkable consist-
ency in the relative amounts of carbon, nitrogen and phosphorus in marine organic
matter (Redfield, 1934 ). Observations show that, on average, the numbers of carbon,
nitrogen and phosphorus atoms in organic material are found in the ratio C:N:P

¼
106:16:1. This basic piece of information about marine microbial life, called the Red-
field ratio, has become fundamental to our understanding of ocean biogeochemistry.
Observations can sometimes show departures from this Redfield ratio (Arrigo, 2005 ),
often as a result of the phytoplankton optimising resource allocation during growth or
because many phytoplankton are able to store nutrients internally. Also, notice in
Fig. 5.8 that the N:P ratio in the bottom water is about 11, rather than the Redfield
value of 16. This will be the result of denitification by bacteria. However, on average,
the observed C:N:P ratio is frequently close to the Redfield value. Why the ratio should
have the particular values that it does is not entirely understood, though it must be
telling us something fundamental about the interactions within microbial communities
and between the microbes and the physical and chemical marine environment.

5.1.8 Phytoplankton species

There are possibly about 5000 species of phytoplankton in the ocean (Sournia et al.,
1991 ) ranging from

< >

m in size. We will see below that their role in
the ecosystem is largely determined by cell size. Small cells do not sink and are food
for similarly small heterotrophs, while large cells can sink rapidly, exporting carbon
to depth. The larger phytoplankton also provide a food source for larger

0.5 m mto 100 m