Faults in the Radiocarbon Method and Problems in its Calibration
71
more strongly illuminated the light bottles are and the longer the duration of
their incubation. The latter occurs because of the more rapid response of
microheterotrophs to confinement of samples especially in darkness, where
light inhibition is absent. These features of O 2 and CO2 bottle methods
could be partly responsible for the rather high photosynthetic quotients
(02/C02 ratios) obtained on comparison of the oxygen bottle method and 14C_
uptake methods, if comparison is made between gross photosynthesis after
the Ormethod and 14C-uptake rate by phytoplankton, meaning that the
latter, when being estimated during a short-term incubation (2-3 h), is completely insensitive to the respiratory losses of CO2, because it proceeds from
the pool of unlabeled CO2 in the algae cell (Hobson et al. 1976; Dring and
Jewson 1982).
Numerous experiments comparing the O2 and CO2 methods carried out with
cultures of algae, marine and freshwater ambiental phytoplankton, as well as
with phytoplankton concentrated by gentle filtration, gave a wide range of
results and a wide range of photosynthetic quotients. Theoretically, with shortterm incubations, the 14C-method should measure intermediate values
between gross and net production, but with its limits closer to the gross. In
practice, in a wide range of experiments in which parallel sample of 14C_
method and oxygen bottle method were compared, the former produced rates
of photosynthesis which fit between gross and net photosynthesis, as measured
by the oxygen method (Ryther and Vaccaro 1955; Winberg 1960; Gieskes and
Kraay 1984; Bender 1987), or they were closer to the gross production
(Thomas 1964; Harris and Piccinin 1977; Jewson 1977; Williams et al. 1979,
1983; Andersen and Sand-Jensen 1984; Davies and Williams 1984; Irvine 1991).
A good coincidence in photosynthesis rates in coastal marine phytoplankton
was observed in parallel measurements using 14C and COr uptake methods
employing coulometric T-COz-titration (Irvine 1991). Definite agreement was
also recorded between 14C-uptake and particulated matter production rates
(Eppley and Sloan 1965; Peterson 1978).
Variations in data on phytoplankton photosynthesis obtained in parallel
experiments using different methods depend not only on the techniques
applied, but also on the kinds of phytoplankton communities (or algae cultures) dealt with, and environmental conditions such as illumination, temperature, and nutrient availability (Finenko 1978; Williams et al. 1979; Bender
1987). The same variability is experienced by the photosynthetic quotient (PO)
derived or used during comparitive experiments. These values can be obtained
in experiments measuring the gross photosynthesis by the oxygen bottle
method, and CO2 uptake by the radiocarbon method. The molar ratios of O2
produced and consumed thus received are equal to the values of P/O quotients. Normally, the range of PO values was recorded between 1.2 to 1.6
(Thomas 1964; Eppley and Sloan 1965; Harris and Piccinin 1977; Andersen and
Sand-Jensen 1984; Gieskes and Kraay 1984), but under specific conditions of
nitrogen deficiency, it can rise to 1.6-2 (Davies and Williams 1984; Irvine 1991).
71
more strongly illuminated the light bottles are and the longer the duration of
their incubation. The latter occurs because of the more rapid response of
microheterotrophs to confinement of samples especially in darkness, where
light inhibition is absent. These features of O 2 and CO2 bottle methods
could be partly responsible for the rather high photosynthetic quotients
(02/C02 ratios) obtained on comparison of the oxygen bottle method and 14C_
uptake methods, if comparison is made between gross photosynthesis after
the Ormethod and 14C-uptake rate by phytoplankton, meaning that the
latter, when being estimated during a short-term incubation (2-3 h), is completely insensitive to the respiratory losses of CO2, because it proceeds from
the pool of unlabeled CO2 in the algae cell (Hobson et al. 1976; Dring and
Jewson 1982).
Numerous experiments comparing the O2 and CO2 methods carried out with
cultures of algae, marine and freshwater ambiental phytoplankton, as well as
with phytoplankton concentrated by gentle filtration, gave a wide range of
results and a wide range of photosynthetic quotients. Theoretically, with shortterm incubations, the 14C-method should measure intermediate values
between gross and net production, but with its limits closer to the gross. In
practice, in a wide range of experiments in which parallel sample of 14C_
method and oxygen bottle method were compared, the former produced rates
of photosynthesis which fit between gross and net photosynthesis, as measured
by the oxygen method (Ryther and Vaccaro 1955; Winberg 1960; Gieskes and
Kraay 1984; Bender 1987), or they were closer to the gross production
(Thomas 1964; Harris and Piccinin 1977; Jewson 1977; Williams et al. 1979,
1983; Andersen and Sand-Jensen 1984; Davies and Williams 1984; Irvine 1991).
A good coincidence in photosynthesis rates in coastal marine phytoplankton
was observed in parallel measurements using 14C and COr uptake methods
employing coulometric T-COz-titration (Irvine 1991). Definite agreement was
also recorded between 14C-uptake and particulated matter production rates
(Eppley and Sloan 1965; Peterson 1978).
Variations in data on phytoplankton photosynthesis obtained in parallel
experiments using different methods depend not only on the techniques
applied, but also on the kinds of phytoplankton communities (or algae cultures) dealt with, and environmental conditions such as illumination, temperature, and nutrient availability (Finenko 1978; Williams et al. 1979; Bender
1987). The same variability is experienced by the photosynthetic quotient (PO)
derived or used during comparitive experiments. These values can be obtained
in experiments measuring the gross photosynthesis by the oxygen bottle
method, and CO2 uptake by the radiocarbon method. The molar ratios of O2
produced and consumed thus received are equal to the values of P/O quotients. Normally, the range of PO values was recorded between 1.2 to 1.6
(Thomas 1964; Eppley and Sloan 1965; Harris and Piccinin 1977; Andersen and
Sand-Jensen 1984; Gieskes and Kraay 1984), but under specific conditions of
nitrogen deficiency, it can rise to 1.6-2 (Davies and Williams 1984; Irvine 1991).