Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 133

by the contribution from the phytoplankton. In shelf seas we also need to include
the effects of riverine sources of organic material as well as the re-suspension of
sediments from the seabed. Particularly in the vertically mixed shallower waters
on the shelf, these additional components can lead to K PAR exceeding of 0.3 m 1 .
(ii) We tend to focus on chlorophyll a for the good reason that it is the most
important pigment in photosynthesis. However, other pigments are also involved.
Different phytoplankton groups utilise different accessory pigments as a part of
their light-gathering antennae. These pigments work alongside chlorophyll a,
extending the range of wavelengths which can be used by the photosystem.
Measurements of accessory pigments provide a useful means of detecting key
phytoplankton groups in the ocean (e.g. Barlow, et al., 1993 ; Lohrenz et al., 2003 ;
Qian et al., 2003 )). The different pigments have response peaks at slightly different
wavelengths of light, which allows scope for competition between phytoplankton
groups. For instance, as the light spectrum changes with depth through the water
column, contrasts in pigment composition can result in vertical separation of
phytoplankton groups; you will see an example of this later in Chapter 7 .
(iii) The light-dependent stage of photosynthesis involves photons exciting electrons
within chlorophyll. The capacity for photosynthesis can become saturated under
high light conditions if light energy is intercepted by the chlorophyll at a rate
greater than the rate at which electron energy can be transferred and used within
the photosystems. The continuing supply of light will keep exciting electrons
within the chlorophyll and has the potential to damage the photosystem, so the
chlorophyll has two mechanisms for shedding the excess light energy. A small
amount ( 1%) of absorbed energy can be re-emitted as light at a longer

wavelength. This is the basis for the use of chlorophyll fluorescence as an
indicator of phytoplankton biomass. Most of the excess energy that needs
to be shed by the chlorophyll is transformed to heat in a process called
non-photochemical quenching. High light levels can lead to a shift from fluorescence
to non-photochemical quenching, which is important to remember when using
fluorometers to assess phytoplankton biomass. For instance, at the sea surface,
phytoplankton chlorophyll could fluoresce more during the night than during
daylight hours, so that a time series of fluorescence could by misinterpreted
as showing a diurnal signal in phytoplankton concentration.

∼

5.1.2 Cell respiration: net and gross primary production

Some of the energy fixed chemically during photosynthesis is used in cellular respir-
ation, maintaining the internal structure and operating capability of the cell. In the
photo-autotrophs respiration is aerobic, meaning that it consumes oxygen. Respir-
ation uses up some of the carbohydrates, lipids and amino acids generated by
photosynthesis. We can describe carbon fixation as either gross or net primary
production. Gross primary production is the total amount of carbon fixed by the
phytoplankton, while net production is the gross production minus the amount of
carbon consumed in cell respiration. Thus net primary production reflects the energy
available for phytoplankton growth.