Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 140

You can see in Fig. 5.5 that at high light intensities the rate of carbon fixation begins
to decline away from Equation (5.4) ; this is particularly clear for the samples taken
from the thermocline. This shows the effect of the high light beginning to damage the
phytoplankton photosystem, and is called photoinhibition.
The deeper, thermocline samples in Fig. 5.5 show the greatest photoinhibition
because they were acclimated to the low light at the thermocline; the peak noon light
intensity at the thermocline (depth about 45 metres) was 165 Em 2 s 1 , compared

to a mean intensity within the surface mixed layer at noon of 487

m

Em 2 s 1 . Thus,
exposing the thermocline phytoplankton to high light in the incubation experiment
would likely overwhelm their repair mechanisms. Photoacclimation is often seen in
terms of changes in I k , with deeper phytoplankton layers having lower values of I k
compared to populations that experience higher light. This ability to alter their
physiology enables cells to find an optimum compromise between light absorption
and photosynthesis, and photodamage. Applying Equation (5.5) to the data in
Fig. 5.5 shows the surface layer phytoplankton to have I k ¼

m

87 mEm 2 s 1 and the

Em 2 s 1 .
Work in stratified shelf seas using active fluorescence has suggested that the
acclimation to low light is achieved by increasing the number of reaction centres
available to process light energy through photosynthesis (Moore et al., 2006 ) .
In other words, the PAR photon flux is low, so the cells make sure that they can
utilise the energy of as many of the photons that they intercept as possible, rather
than wasting energy through non-photochemical quenching. This increase in the
number of reaction centres in a cell will mean that there is an increase in the amount
of chlorophyll per cell. Thus photoacclimation is an important consideration when
attempting to interpret chlorophyll fluorescence in terms of phytoplankton biomass;
calibration formulae calculated by comparing analysis of water samples with
fluorometer data can be different at the sea surface compared to deeper in the
thermocline, and differ between vertically mixed and stratified regions.
There is one final point to make concerning Fig. 5.5 . You may have noticed
something a little odd about the intercept of the photosynthesis curves at zero light
intensity. We might have expected that P dark would be negative; we would not expect
photosynthesis in the absence of light. But P dark is found to be positive, suggesting
that carbon is being fixed by the plankton community in the dark. The reasons for
this are not yet entirely clear, though uptake of carbon by heterotrophic bacteria and
phytoplankton have been implicated (Li et al., 1993 ). Estimating P dark by fitting
Equation (5.4) to the incubation data allows the maximum carbon fixation rate to be
corrected for this non-photosynthetic carbon uptake.

thermocline population to have I k ¼ 39 m

5.1.5 Photosynthesis in a turbulent environment: triggering blooms

We will finish our look at the role of light in the growth of the autotrophic phyto-
plankton by considering how turbulence affects the light environment experienced by
the phytoplankton cells, and how rapid phytoplankton growth (phytoplankton
blooms) can be triggered. Let us ignore the effects of non-photosynthetic carbon