a number of methods that are simple in principle, more complex in practice. The basic
method is to collect a bottle of seawater from a place and depth to be studied, check
that no herbivores are present (use a coarse filter perhaps), and measure the increase
in phytoplankton biomass during some time interval. This would most appropriately
be the period of the natural illumination cycle, 24 hours, but adverse responses of the
phytoplankton to confinement often force shorter incubations. Alternately, any
photosynthetic product can be measured or the reduction of any photosynthetic
resource can be determined. In practice only a few of these are useful. Look again at
the overall reaction for photosynthesis:
(^) Oxygen production can be measured by the Winkler titration of samples before and
after incubation. That only works easily in very rich environments with large
populations of phytoplankton, because the usual change in oxygen concentration over
a reasonable interval of measurement is about the same as the variability of the
measurement. Photosynthetic O 2 evolution can also be measured with the tracer ^18 O
either directly or as the rate of ^18 O and ^16 O evolution from a mixture of H 218 O and
H 216 O (Bender et al. 1987).
(^) A long-favored method is the carbon-14 incorporation technique first introduced to
ecology by Steeman-Nielsen (1952). A bottle of seawater, to which ^14 C as sodium
bicarbonate has been added, is incubated at its depth of collection in the sea (or more
often in a simulated photic and thermal environment on deck). After an interval, the
bottle is retrieved, the phytoplankton are filtered off, and the amount of ^14 C
incorporated in them is measured by scintillation counting. The net (since respiration
goes on throughout the incubation as well as photosynthesis) primary production is
obtained by multiplying the fraction of the ^14 C taken up (e.g. counts per minute in
filtered cells/counts per minute provided) by total carbonate in the bottle. Thus, a
carbonate determination is required as well. The standard recipe for ^14 C-uptake
production measurement is given by Parsons et al. (1984). They give the following
final equation for the calculation of the result:
(Eqn. 3.1)
(^) where R is the counting rate to be expected for the entire addition of (^14) C; R
s and Rb
are counting rates for the filtered sample and a blank, respectively; W is the total
weight of carbonate carbon in the water (mg C m−3); and T is the duration of the
incubation in hours. A dark-bottle uptake experiment is usually included in the
observational design to account for carbon-isotope exchange processes other than
those of primary production per se. Note that the volume of the incubation container
is implicit in all of Rs, Rb, and R (say, ^14 C counts per liter), so it cancels.