42
The Radiocarbon Method to Estimate Primary Production
radioactivity; but loss of small phytoplankton during filtration is equally undesirable. In this case, it is better to select a neutral position accounting for
and minimizing the above negative effects. From this point of view, the most
practical are the membrane filters with pore sizes between 0.6 and 0.8 f..lm, or
ultrafine GF glass fiber filters, which retain almost completely even picocyanobacteria of the same size as the largest bactenal cells -0.6-3 f..lm, while
most of really planktonic bacteria with an average size of 0.5 1.5 f..lm pass
through this filter. Glass fiber filters are preferable for use in turbid or
phytoplankton-rich (blooming) waters. The membrane filters should be boiled
in distilled water for some minutes, dried, and stored. Before use they are
moistened with a weak detergent solution.
The counting procedure was also briefly discussed above (see Sect.
1.2.2.2). For counting 14C-radioactivity on filters, either large 20-ml vials or
small ones (10-ml) may be used. For the former the filter is placed at the
bottom of the glass or plastic scintillation vial, or on its side if small vials are
used. In this case, the lower edge of the filter should touch the bottom of the
vial. As scintillation liquid any kind of cocktail can be used, starting from
toluene-based, under one condition: the cpm radioactivity of the isotope
working solution (R;) should be estimated using the same cocktail as that of
the phytoplankton (R/) and in the same volume. The minimal volume of cocktail in vials for counting filters can be about 6ml, for both large and small ones.
The optimal time of counting depends on the radioactivity of the filters; but
to spend a long time for precise counting of minor activities is unreasonable.
The radiocarbon method is so sensitive that even a small (0.01-0.05 f..lg)
amount of assimilated carbon corresponds to significant radioactivity (some
50 to 200cpm), which can be estimated precisely enough during 2-3min of
counting. Because this instrument counts radioactivity automatically, an
optimal counting method for primary production measurement can be the
cpm counts 2 x 2min. Another method is the use of a program for the accumulation of a definite number of counts (some 2000cpm is optimum) and
recording the time.
2.3.2.7 Estimating Correction Coefficients (K/, Kc)
Krcoefficients are used for calculation of primary production per day (Cps)
from rates of photosynthesis per hour (Cp ), measured during short-term (1.52h chain) incubations (see Sect. 2.3.2.5; Fig. 2.6). Thus, the Krcoefficients are
equal to the ratios CpJCp estimated for 2-h intervals through the whole light
day. Examples of these coefficients are given in Table 2.1, estimated empirically for a given water body and a given season with the aid of curves of the
diurnal course of phytoplankton photosynthesis in the upper water layer.
These curves have to be established separately for bright and dull days. To
derive them, the relative rates of photosynthesis should be estimated during
2-h intervals from sunrise to sunset. To make these estimations, a 3-41 glass
The Radiocarbon Method to Estimate Primary Production
radioactivity; but loss of small phytoplankton during filtration is equally undesirable. In this case, it is better to select a neutral position accounting for
and minimizing the above negative effects. From this point of view, the most
practical are the membrane filters with pore sizes between 0.6 and 0.8 f..lm, or
ultrafine GF glass fiber filters, which retain almost completely even picocyanobacteria of the same size as the largest bactenal cells -0.6-3 f..lm, while
most of really planktonic bacteria with an average size of 0.5 1.5 f..lm pass
through this filter. Glass fiber filters are preferable for use in turbid or
phytoplankton-rich (blooming) waters. The membrane filters should be boiled
in distilled water for some minutes, dried, and stored. Before use they are
moistened with a weak detergent solution.
The counting procedure was also briefly discussed above (see Sect.
1.2.2.2). For counting 14C-radioactivity on filters, either large 20-ml vials or
small ones (10-ml) may be used. For the former the filter is placed at the
bottom of the glass or plastic scintillation vial, or on its side if small vials are
used. In this case, the lower edge of the filter should touch the bottom of the
vial. As scintillation liquid any kind of cocktail can be used, starting from
toluene-based, under one condition: the cpm radioactivity of the isotope
working solution (R;) should be estimated using the same cocktail as that of
the phytoplankton (R/) and in the same volume. The minimal volume of cocktail in vials for counting filters can be about 6ml, for both large and small ones.
The optimal time of counting depends on the radioactivity of the filters; but
to spend a long time for precise counting of minor activities is unreasonable.
The radiocarbon method is so sensitive that even a small (0.01-0.05 f..lg)
amount of assimilated carbon corresponds to significant radioactivity (some
50 to 200cpm), which can be estimated precisely enough during 2-3min of
counting. Because this instrument counts radioactivity automatically, an
optimal counting method for primary production measurement can be the
cpm counts 2 x 2min. Another method is the use of a program for the accumulation of a definite number of counts (some 2000cpm is optimum) and
recording the time.
2.3.2.7 Estimating Correction Coefficients (K/, Kc)
Krcoefficients are used for calculation of primary production per day (Cps)
from rates of photosynthesis per hour (Cp ), measured during short-term (1.52h chain) incubations (see Sect. 2.3.2.5; Fig. 2.6). Thus, the Krcoefficients are
equal to the ratios CpJCp estimated for 2-h intervals through the whole light
day. Examples of these coefficients are given in Table 2.1, estimated empirically for a given water body and a given season with the aid of curves of the
diurnal course of phytoplankton photosynthesis in the upper water layer.
These curves have to be established separately for bright and dull days. To
derive them, the relative rates of photosynthesis should be estimated during
2-h intervals from sunrise to sunset. To make these estimations, a 3-41 glass