THE PRODUCTION OF MARINE PLANKTON
123
for calculating production in a column with a homogeneous distribution
of phytoplankton. P = photosynthesis of the population in g/C/m3/day;
R = relative photosynthetic rate, dependent on the light surface
intensity; C = chlorophyll/m3 in the column. Such a calculation gives
only an approximation since the assimilation rate assumed is a mean
value for various species. Phytoplankton is very often also markedly
stratified rather than homogeneously distributed and there is the
difficulty that the existing stock of algal cells must be estimated from
the amount of chlorophyll present.
One of the most successful and widely used methods for estimating
primary production was developed by Steemann Nielsen (1954) (cf.
Steemann Nielsen and Aabye Jensen, 1957). 1-n his technique the radioactive isotope, I4C, is added, as bicarbonate, to a bottle of sea water, the
productivity of which is to be measured. The Lotal content of CO, in the
water must be known and, assuming that the labelled carbon has been
assimilated by the algae, the total amount oS carbon photosynthesized
during the period of time may be calculated by determining the amount
of 14C present in the plankton when the experiment ends. The plankton
is removed by filtration and the amount of 14C measured by the / 3
radiation from the plankton retained on the filter. The carbon technique is undoubtedly far more sensitive and iiherefore is of great use in
measuring the primary productivity of oligotrophic waters; it avoids the
long term experiments necessary with the 0x3 gen bottle technique.
At the same time there are certain difficulties in using this method.
For example, it is necessary to assume that “C and the normal isotope
are absorbed at the same rate; Steemann Nielsen has suggested
that the error due to isotopic discrimination is not more than 5%.
Again, the precise method of filtering, the type of microfilter, and the
pressure used for filtration may also be critical in that more delicate
phytoplankton cells may be destroyed and EL loss of material encountered. The 14C method may be particularly sensitive to errors of this
sort where delicate flagellates form a relatively large part of the phytoplankton population. A difficulty found with the oxygen bottle experiment also applies to the I4C technique, namely that the enclosure of a
photosynthetic population in a static volume of sea water does not
reflect precisely the conditions operating in the natural environment.
Other errors may arise; the time of day in which the estimate is made
can be important, since diurnal variations in photosynthetic activity of
algae are now recognized, particularly the “afternoon depression”
(Doty and Oguri, 1957) which may be especially marked in plankton
from warm waters. Difficulties may also be encountered when the algal
cells have shells or inclusions of calcium carbonate (e.g. if the phytoplankton is dominated by coccolithophores). Steemann Nielsen ( 1 964a)
123
for calculating production in a column with a homogeneous distribution
of phytoplankton. P = photosynthesis of the population in g/C/m3/day;
R = relative photosynthetic rate, dependent on the light surface
intensity; C = chlorophyll/m3 in the column. Such a calculation gives
only an approximation since the assimilation rate assumed is a mean
value for various species. Phytoplankton is very often also markedly
stratified rather than homogeneously distributed and there is the
difficulty that the existing stock of algal cells must be estimated from
the amount of chlorophyll present.
One of the most successful and widely used methods for estimating
primary production was developed by Steemann Nielsen (1954) (cf.
Steemann Nielsen and Aabye Jensen, 1957). 1-n his technique the radioactive isotope, I4C, is added, as bicarbonate, to a bottle of sea water, the
productivity of which is to be measured. The Lotal content of CO, in the
water must be known and, assuming that the labelled carbon has been
assimilated by the algae, the total amount oS carbon photosynthesized
during the period of time may be calculated by determining the amount
of 14C present in the plankton when the experiment ends. The plankton
is removed by filtration and the amount of 14C measured by the / 3
radiation from the plankton retained on the filter. The carbon technique is undoubtedly far more sensitive and iiherefore is of great use in
measuring the primary productivity of oligotrophic waters; it avoids the
long term experiments necessary with the 0x3 gen bottle technique.
At the same time there are certain difficulties in using this method.
For example, it is necessary to assume that “C and the normal isotope
are absorbed at the same rate; Steemann Nielsen has suggested
that the error due to isotopic discrimination is not more than 5%.
Again, the precise method of filtering, the type of microfilter, and the
pressure used for filtration may also be critical in that more delicate
phytoplankton cells may be destroyed and EL loss of material encountered. The 14C method may be particularly sensitive to errors of this
sort where delicate flagellates form a relatively large part of the phytoplankton population. A difficulty found with the oxygen bottle experiment also applies to the I4C technique, namely that the enclosure of a
photosynthetic population in a static volume of sea water does not
reflect precisely the conditions operating in the natural environment.
Other errors may arise; the time of day in which the estimate is made
can be important, since diurnal variations in photosynthetic activity of
algae are now recognized, particularly the “afternoon depression”
(Doty and Oguri, 1957) which may be especially marked in plankton
from warm waters. Difficulties may also be encountered when the algal
cells have shells or inclusions of calcium carbonate (e.g. if the phytoplankton is dominated by coccolithophores). Steemann Nielsen ( 1 964a)