Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 134

5.1.3 Techniques for measuring primary production and respiration

There are two commonly used methods for direct measurements of rates of carbon
fixation, both based on incubating samples of seawater. After describing these two
methods and identifying some of their limitations. we shall look briefly at some
emerging new techniques.

Dissolved oxygen, primary production and respiration
We mentioned earlier that photosynthesis releases O 2 into the water. This provides
the basis for a technique to measure rates of primary production by tracking changes
in dissolved oxygen, first suggested in 1927 by Gaarder and Gran (Gaarder and
Gran, 1927 ). Three bottled samples of water are required. The first is immediately
analysed for its dissolved oxygen content. The other two are incubated, one illumin-
ated by some representative intensity of radiation and the other kept in the dark.
After some time the dissolved oxygen concentration in the 'light' and 'dark' bottles is
measured. The increase in oxygen in the light bottle over the initial concentration will
be due to both photosynthesis (adding oxygen to the water) and respiration (remov-
ing oxygen), and so provides a measure of the net community production in the bottle.
The dark bottle should show a reduction in oxygen as without light only respiration
should have been taking place, hence it provides a measure of net community
respiration. An estimate of gross community production is formed by adding the
amount of oxygen consumed in the dark bottle to that gained in the light bottle.
Note here that we have added the word 'community' to all of these rate measure-
ments; we will discuss this in more detail below.
The results can be expressed in terms of oxygen production per unit time, or they can
be converted to carbon fixation rate by assuming a ratio of oxygen production to
carbon fixation called the photosynthetic quotient with a value between 1.0 and 1.36
(Williams and Robertson, 1991 ). The basic oxygen method is simple and inexpensive,
andwidely used (e.g. Robinson andWilliams, 1999 ). Adevelopment of themethod uses
H 2 18 O, i.e. water with a stable isotope of oxygen (Bender et al., 1987 ). The isotope
18 O amounts to only

0.2%of naturally occurring oxygen, which is mainly made up of
16 O. Photosynthesis will release the 18 O into the water, while respiration consumes the
ambient 16 O. This isotopic method thus separates the effects of autotrophs and hetero-
trophs, and provides a measure of gross photosynthetic production.

∼

14

C: the standard measurement of primary production
Currently, the standard method for measuring rates of carbon fixation is to use radio-
active carbon-14 ( 14 C), first suggested by Steeman-Nielsen in 1951 (Steeman-Nielsen,
1951 ). A bottle of seawater is 'spiked' with 14 C (in the formof sodium bicarbonate), and
then incubated at the required intensity of light. After a few hours the phytoplankton in
the sample are filtered and the filter analysed in a scintillation counter. The radioactivity
of the filter indicates the amount of carbon fixed by the phytoplankton, so it provides a
measure of photosynthetic production. The advantages of this method over the oxygen
method described above are that the measure of carbon fixation is direct, i.e. not
requiring an estimate of the photosynthetic quotient, and it is more sensitive. The