Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 142

Figure 5.6 Comparing the
compensation and critical depths.
The thin curve is a light profile with
k PAR ¼

% Surface light

0 20 40 60 80 100

0

0.15 m 1 . The bold line is
layer-averaged light: at a point on the
curve, depth ¼ z, the light is averaged
between z and the sea surface. If a cell
requires light of 10% of the surface
value in order to balance respiration
and grazing losses, it would either have
to be held at the compensation depth
or alternatively mixed continuously
between the critical depth and the
sea surface.

-30

Compensation depth

Critical depth

-60

-90

-120

-150

a phytoplankton cell at the depth at which we found this light level, referred to as the
compensation depth, photosynthesis would just balance the requirements of cellular
respiration and heterotroph grazing.
Note that both r p and g p are not constant. Respiration rate will be temperature
dependent, with cellular energy demands typically higher in warmer water.
The grazing rate will have a seasonal signal, lower in winter and highest in summer
(e.g. Lee et al., 2002 ). Grazers will also respond to an increase in the biomass of their
food supply, which is a topic we will return to later in this chapter.
For much of the time, especially in the near surface layers, the ocean is turbulent, so
a phytoplankton cell is more likely to be travelling a random path in response to
vertical turbulent mixing. Sometimes it will be near the sea surface, and receiving more
than enough light to satisfy Equation (5.7) , while at other times it could be much
deeper and need to use stored energy to fuel its respiration. It is possible for this
turbulent trajectory to take the cell below the compensation depth, but the respiration
could later be exceeded by growth when nearer the surface and on average the cell may
be able to grow. The depth at which the average light received is just able to balance
respiration is the critical depth, a concept first introduced by Harald Sverdrup in 1953
(Sverdrup, 1953 ). In Fig. 5.6 you can compare these concepts of critical and compen-
sation depths. Notice that they are substantially different: the critical depth is typically
about a factor of 4 greater than the compensation depth.
Since Sverdrup's seminal paper, the concept of critical depth has been pivotal in
our understanding of how rapid growth of phytoplankton (blooms) are triggered in
the ocean. We will look at phytoplankton blooms in more detail in Chapter 6 . For