Technique of Measuring Phytoplankton Primary Production
51
zone, the curve of Cpd can be derived, which reflects the values of photosynthesis per day at the depth at which the bottles were incubated. The curve is
used to calculate integrated primary production (Cpt).
I cannot recommend this method, not only in marine but even in freshwaters; although it appears a most direct and simple one, actually it is not. It
is really technically very complicated and time-consuming, and often results
in a grave underestimation of primary production. Too much effort must be
spent to obtain the Cpt value at one station, in addition to the possibility of its
underestimation. The operations of sampling, charging with isotope, and suspension of bottles need much time and many hands, as does the filtration of
numerous bottles at the end of incubation. The long-term in situ incubations
needed by this method entail many problems, especially if they are performed
from ship board. The number of depths with this method is usually restricted
to four or five to shorten the time of preparation, immersion, and filtration
of samples. A longer duration results in an increase in the losses of 14C_
assimilates due to respiration and exudation. When sampling a limited number
of depths, it is impossible to account properly for the productive popUlation
which inhabits the euphotic zone and forms the maxima and minima of its
density; and even these limited depths are selected according to underwater
illumination, ignoring phytoplankton distribution. However, as also mentioned
above, the character of the vertical distribution of phytoplankton is one of the
key factors determining the photosynthesis rate on vertical profiles and
neglect obviously results in an underestimation of primary production. The
examples of the dependence of depth-photosynthesis profiles (Ks-profiles) on
the distribution of active phytoplankton in the water column shown in Figs
2.10 and 2.11 clearly prove this.
Another approach based on Cpd estimations, the most popular among
marine hydro biologists, is the so-called simulated in situ method. This method
has a fundamental advantage: it does not need long-term overboard in situ
incubations of experimental bottles. Therefore it makes it possible to measure
integrated primary production at ordinary short-term hydrophysical stations
of 1-2-h duration. The in situ conditions are simulated with the aid of special
baths - incubators varying in respect to temperature and light conditions on
the vertical profiles. The incubators are divided into four to six sections
covered with neutral optical filters or with metallic screens, providing fractionated illumination, ranging from 100 down to 2% of penetrated photosynthetically active light (PAR). Most often, the range of fractionated light levels
in the sections of such a bath is as follows: 100,50, 10,5, and 2% PAR. The
sections are illuminated from a common source of light, either with direct
natural light on the deck of the ship, with shadowed natural light, or with
artificial light of some 10-18 x 10 3 Ix. Numerous versions of the simulated in
situ method have been used in primary production measurements, starting
with the early trials by E. Steemann Nielsen and D. Cushing, who actually
invented it (Cushing 1957; Doty and Oguri 1958; Steemann Nielsen and
Hansen 1959; Strickland 1960; TaIling 1960,1974; Jitts 1963; Mc Allister et al.
51
zone, the curve of Cpd can be derived, which reflects the values of photosynthesis per day at the depth at which the bottles were incubated. The curve is
used to calculate integrated primary production (Cpt).
I cannot recommend this method, not only in marine but even in freshwaters; although it appears a most direct and simple one, actually it is not. It
is really technically very complicated and time-consuming, and often results
in a grave underestimation of primary production. Too much effort must be
spent to obtain the Cpt value at one station, in addition to the possibility of its
underestimation. The operations of sampling, charging with isotope, and suspension of bottles need much time and many hands, as does the filtration of
numerous bottles at the end of incubation. The long-term in situ incubations
needed by this method entail many problems, especially if they are performed
from ship board. The number of depths with this method is usually restricted
to four or five to shorten the time of preparation, immersion, and filtration
of samples. A longer duration results in an increase in the losses of 14C_
assimilates due to respiration and exudation. When sampling a limited number
of depths, it is impossible to account properly for the productive popUlation
which inhabits the euphotic zone and forms the maxima and minima of its
density; and even these limited depths are selected according to underwater
illumination, ignoring phytoplankton distribution. However, as also mentioned
above, the character of the vertical distribution of phytoplankton is one of the
key factors determining the photosynthesis rate on vertical profiles and
neglect obviously results in an underestimation of primary production. The
examples of the dependence of depth-photosynthesis profiles (Ks-profiles) on
the distribution of active phytoplankton in the water column shown in Figs
2.10 and 2.11 clearly prove this.
Another approach based on Cpd estimations, the most popular among
marine hydro biologists, is the so-called simulated in situ method. This method
has a fundamental advantage: it does not need long-term overboard in situ
incubations of experimental bottles. Therefore it makes it possible to measure
integrated primary production at ordinary short-term hydrophysical stations
of 1-2-h duration. The in situ conditions are simulated with the aid of special
baths - incubators varying in respect to temperature and light conditions on
the vertical profiles. The incubators are divided into four to six sections
covered with neutral optical filters or with metallic screens, providing fractionated illumination, ranging from 100 down to 2% of penetrated photosynthetically active light (PAR). Most often, the range of fractionated light levels
in the sections of such a bath is as follows: 100,50, 10,5, and 2% PAR. The
sections are illuminated from a common source of light, either with direct
natural light on the deck of the ship, with shadowed natural light, or with
artificial light of some 10-18 x 10 3 Ix. Numerous versions of the simulated in
situ method have been used in primary production measurements, starting
with the early trials by E. Steemann Nielsen and D. Cushing, who actually
invented it (Cushing 1957; Doty and Oguri 1958; Steemann Nielsen and
Hansen 1959; Strickland 1960; TaIling 1960,1974; Jitts 1963; Mc Allister et al.