Faults in the Radiocarbon Method and Problems in its Calibration
69
bottles in situ on sunny days. So I advise those researchers, who have some
reserves about the reality of photoinhibition in the upper layer to control it
experimentally by comparing the photosynthesis rate in bottles incubated, for
example, for 1 h during a sunny midday at the surface, and at the depth of half
Secchi disk transparency (-30% of PAR) at optimal illumination. Then the
researcher l:all l:unviul:t: him:sdf a:s tu the rt:ality uf phutoinhibition and its
range in concrete conditions of the water body under investigation.
There is another way to avoid preoccupation with photoinhibition, which
in any case may strongly influence the results of primary production measurements during midsummer in the temperate zone or tropics. This is to use,
for estimation of the absolute rate of photosynthesis, a sample from the layer
of photosynthesis illumination optimum, e.g., from a depth of 25-30% of
PARS during bright sunny days. These samples should be incubated at the
same depth in situ or in the desk incubator, under a neutral filter which retains
-70% of light. The Cpd values thus obtained, expressed in mgCm- 3 day-\ can
then be used to calculate integrated primary production (Cpt) in the water
column, like the Cps values (see Sect. 2.3.3.2.).
Estimations of integrated primary production in the water column may
also be influenced by the photoadaptation of planktonic algal, which inhabit
the lower, poorly illuminated, part of the euphotic zone. The adaptation of phytoplankton associations inhabiting layers of deep chlorophyll maxima near the
lower boundary of the euphotic zone to a low light intensity is a matter of fact
(Sorokin 1964a; Falkovski 1980,1981; Lewis et al. 1984; see Fig. 2.12). The deep
chlorophyll maxima are located in the upper part of the thermocline layer
where illumination is between 5 and 10% PARS (Eppley 1981). The cells of
algae which inhabit this layer have a larger lower limit of light saturation (Fig.
2.19) and a larger chlorophyll content per cell volume. They exhibit a higher
rate of photosynthesis per biomass at lower light limits for compensation point
in comparison with phytoplankton which inhabits the upper, well illuminated
mixed layer. These features of deep-living phytoplankton can lead to the
expectation of a probable high photosynthetic production in this poorly illuminated zone (Venrick et al. 1973). Again, such expectation can be tested
experimentally by in situ 14C-incubations in this layer and in the layer of
optimal illumination. Also the calculative method of measuring integral
primary production accounts well for possible ranges of photosynthesis rates
in layers of deep chlorophyll maxima (see Sect. 2.3.3.2, Fig. 2.11).
8. Evaluating the validity of the 14C method, and comparison with other
methods. The validity of the radiocarbon method to estimate phytoplankton
primary production has been tested and discussed in numerous publications.
For control and calibration of the 14C method, two alternative methods are
being used most often in practice: the traditional dark-light oxygen bottle
method and the recent unlabeled CO2 uptake dark-light bottle method. The
oxygen method is based mainly on Winkler titration fortified with electronic
accessories to increase its precision (Williams and Jenkinson 1982; Irvin 1991).
The COr uptake method is based on high-precision coulometric total CO2
69
bottles in situ on sunny days. So I advise those researchers, who have some
reserves about the reality of photoinhibition in the upper layer to control it
experimentally by comparing the photosynthesis rate in bottles incubated, for
example, for 1 h during a sunny midday at the surface, and at the depth of half
Secchi disk transparency (-30% of PAR) at optimal illumination. Then the
researcher l:all l:unviul:t: him:sdf a:s tu the rt:ality uf phutoinhibition and its
range in concrete conditions of the water body under investigation.
There is another way to avoid preoccupation with photoinhibition, which
in any case may strongly influence the results of primary production measurements during midsummer in the temperate zone or tropics. This is to use,
for estimation of the absolute rate of photosynthesis, a sample from the layer
of photosynthesis illumination optimum, e.g., from a depth of 25-30% of
PARS during bright sunny days. These samples should be incubated at the
same depth in situ or in the desk incubator, under a neutral filter which retains
-70% of light. The Cpd values thus obtained, expressed in mgCm- 3 day-\ can
then be used to calculate integrated primary production (Cpt) in the water
column, like the Cps values (see Sect. 2.3.3.2.).
Estimations of integrated primary production in the water column may
also be influenced by the photoadaptation of planktonic algal, which inhabit
the lower, poorly illuminated, part of the euphotic zone. The adaptation of phytoplankton associations inhabiting layers of deep chlorophyll maxima near the
lower boundary of the euphotic zone to a low light intensity is a matter of fact
(Sorokin 1964a; Falkovski 1980,1981; Lewis et al. 1984; see Fig. 2.12). The deep
chlorophyll maxima are located in the upper part of the thermocline layer
where illumination is between 5 and 10% PARS (Eppley 1981). The cells of
algae which inhabit this layer have a larger lower limit of light saturation (Fig.
2.19) and a larger chlorophyll content per cell volume. They exhibit a higher
rate of photosynthesis per biomass at lower light limits for compensation point
in comparison with phytoplankton which inhabits the upper, well illuminated
mixed layer. These features of deep-living phytoplankton can lead to the
expectation of a probable high photosynthetic production in this poorly illuminated zone (Venrick et al. 1973). Again, such expectation can be tested
experimentally by in situ 14C-incubations in this layer and in the layer of
optimal illumination. Also the calculative method of measuring integral
primary production accounts well for possible ranges of photosynthesis rates
in layers of deep chlorophyll maxima (see Sect. 2.3.3.2, Fig. 2.11).
8. Evaluating the validity of the 14C method, and comparison with other
methods. The validity of the radiocarbon method to estimate phytoplankton
primary production has been tested and discussed in numerous publications.
For control and calibration of the 14C method, two alternative methods are
being used most often in practice: the traditional dark-light oxygen bottle
method and the recent unlabeled CO2 uptake dark-light bottle method. The
oxygen method is based mainly on Winkler titration fortified with electronic
accessories to increase its precision (Williams and Jenkinson 1982; Irvin 1991).
The COr uptake method is based on high-precision coulometric total CO2