Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 139

1.6

1.2

0.8

0.4

I K (thermocline)

I K (surface)

0

0 100 200 300 400 500 600

I PAR ( µ E m -2 s -1 )

Figure 5.5 Photosynthesis-PAR curve based on 2-hour 14 C incubations at fixed light intensities.
Samples were from the Celtic Sea surface layer (black) and the thermocline (grey). The curves
are the best fits to Equation (5.4) . Data courtesy of Anna Hickman, University of Essex UK.

a different light intensity, and then analysed for the amount of carbon taken up. An
example of a photosynthesis-PAR curve generated using this technique is shown in
Fig. 5.5 . As the resource (light) increases, the photosynthesis rate gradually increases
to eventually reach a maximum value. This type of response to a resource is common
to many other ecosystem components. It is similar to a Monod growth curve, named
after after the Nobel prize-winning French molecular biologist Jacques-Louis
Monod. The curve shape is an example of a type II functional response in ecology,
where the capacity for growth is eventually limited by the rate at which the organism
is able to process the resource (in this case, the ability of the photosystem to utilise
light energy and fix carbon). We can think of the curve as summarising the end result
of the host of biochemical processes going on within the phytoplankton cells. It can
be fitted to a number of mathematical descriptions of the curve shape (Jassby and
Platt, 1976 ), with the most common being the hyperbolic tangent function. The data
in Fig. 5.5 were fitted to:

þ

h i

a q I PAR
P b max

P p ¼ P b max tanh P dark Þ 1 h 1 gC ð g Chl ð 5 : 4 Þ

with P p the phytoplankton specific photosynthesis rate (photosynthesis rate normal-
ised by the phytoplankton chlorophyll biomass), P b max is the maximum specific
photosynthesis rate and P dark is the specific photosynthesis rate in the dark. The
parameter a q (units: g C (g Chl) 1 m 2 s E 1 h 1 ) is the maximum light utilisation

coefficient, and represents the slope of the initial part of the photosynthesis-PAR
curve. The value of PAR where the photosynthesis roughly reaches its maximum
value, called the saturating light, is given by:

m

P b max
a q

I K ¼ ð 5 : 5 Þ