Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 132

PSI

NADPH

PSII

e -

e -

Chl a

CO 2 CH 2 O

ATP

Chl a

H 2 O O 2

Figure 5.2 Schematic of the sequence of energy transfer through photosynthetic reaction
centre. The two photosystems, PSII and PSI, both require light energy to excite an electron
in the chlorophyll molecule, which is replaced by an electron from a water molecule, so
releasing oxygen. The electron energy is used in a series of redox reactions that produce the
molecules ATP and NADPH, required to fix carbon from CO 2 and produce CH 2 O.

seawater. As we saw in Chapter 2 , light attenuation is exponential through the water
column, so we describe the vertical change in I PAR , the downward flux of PAR, as:
@ I PAR

@ z ¼

K PAR z ð 5 : 2 Þ

where K PAR is an attenuation coefficient for PAR in seawater. At this stage we
often take into account the effect on light attenuation of 'shading' due to the
phytoplankton themselves by setting K PAR to be the sum of the attenuation of
PAR due to the seawater plus an extra attenuation proportional to the amount of
phytoplankton chlorophyll in the water. For now, however, we assume K PAR is
constant through the water column, so that integrating Equation (5.2) yields:

I 0 e K PAR z

I PAR ¼ ð 5 : 3 Þ

0.1 m 1 in
the surface waters of a summer stratified shelf sea so that PAR decreases to 10%
and then to 1% of its surface value at depths of 23 and 46 metres. These changes
with depth imply strong vertical gradients in PAR which will be important when
we come to deal with photosynthesis in turbulent environments. There is an
important contrast between the light attenuation in open ocean and that in
the shelf seas. In the open ocean, variability in light attenuation is dominated

with I 0 the incident PAR on the sea surface. Typically we see K PAR ∼