48
The Radiocarbon Method to Estimate Primary Production
Concluding this section, it should be mentioned that most authors who
estimated the degree of losses of labeled assimilates due to filtration measured
the radioactivity of 14C-Iabeled organic matter directly in filtrates after avoiding 14C-carbonate radioactivity by bubbling them with air or with COrgas, thus
escaping the above rather complex wet combustion procedures. For this, some
2- to 5-ml aliquots of filtrate were mixed with a special scintillation cocklail,
such as unisolve, Mallincrodt Hand Fluor Bray, or Aquasol (see Sect. 1.2.2.2)
which can take several (up to 8) ml of water per counting sample of 15-20ml
without significant quenching effects; but this easy approach has been
justifiably criticized (Thomas 1971; Williams et al. 1976; Sharp 1977). Its sensitivity is, in fact, insufficient, especially when dealing with marine water, which
is significantly less miscible with the scintillation cocktails, and is not suited for
concentration by evaporation because of the high salt load. This direct method
is acceptable only for measurements of 14C losses in samples from productive
freshwater basins. In this case, the filtrates should in any case be concentrated
by freeze-drying in the scintillation vial, into which the cocktail can then be
added. The problems of measuring exudation and losses of 14C-assimilates will
be discussed in more detail below (see Sect. 2.4).
2.3.3 Estimating Integrated Pelagic Primary Production in
the Water Column (Cpt)
As was mentioned above (see Sect. 2.3), the final goal of primary production
estimations is its integrated value in the water column (Cpt, g C m- 2 day-I). Only
the integrated photosynthetic production of new organic matter corresponds
to the primary energy input into the ecosystem. To estimate this, it is sufficient
to measure either the absolute rates of primary production at different depths
on the vertical profile (Cpd, mgCm- 3 ,day-l), or to measure the absolute value
of primary production in the upper layer of water (Cps, mg Cm- 3 ,day-l) and its
relative rates at different depths on the vertical profile (Ks). Both Cpd and Ks
should be measured per half of the light day or per whole light day to account
for the diurnal course of illumination. For Cpt calculation, the appropriate Cpd
and Ks curves are usually constructed. The practical approach to the construction of these curves is very different. The curves Cpd are derived from data
obtained during in situ (or simulated in situ light) incubations, when the
absolute values of half or whole light day primary production are measured
on the vertical profile, and the integrated production Cpt is then calculated ..
The Ks curves are constructed by calculating the appropriate Ks coefficients at
different depths within the euphotic zone with the use of data on the relative
vertical distribution of the two main factors deciding the character of the phytoplankton photosynthesis rate on the vertical profile; one of these is light
attenuation within the water column, and the second the relative vertical distribution of active phytoplankton on the vertical profile.
The Radiocarbon Method to Estimate Primary Production
Concluding this section, it should be mentioned that most authors who
estimated the degree of losses of labeled assimilates due to filtration measured
the radioactivity of 14C-Iabeled organic matter directly in filtrates after avoiding 14C-carbonate radioactivity by bubbling them with air or with COrgas, thus
escaping the above rather complex wet combustion procedures. For this, some
2- to 5-ml aliquots of filtrate were mixed with a special scintillation cocklail,
such as unisolve, Mallincrodt Hand Fluor Bray, or Aquasol (see Sect. 1.2.2.2)
which can take several (up to 8) ml of water per counting sample of 15-20ml
without significant quenching effects; but this easy approach has been
justifiably criticized (Thomas 1971; Williams et al. 1976; Sharp 1977). Its sensitivity is, in fact, insufficient, especially when dealing with marine water, which
is significantly less miscible with the scintillation cocktails, and is not suited for
concentration by evaporation because of the high salt load. This direct method
is acceptable only for measurements of 14C losses in samples from productive
freshwater basins. In this case, the filtrates should in any case be concentrated
by freeze-drying in the scintillation vial, into which the cocktail can then be
added. The problems of measuring exudation and losses of 14C-assimilates will
be discussed in more detail below (see Sect. 2.4).
2.3.3 Estimating Integrated Pelagic Primary Production in
the Water Column (Cpt)
As was mentioned above (see Sect. 2.3), the final goal of primary production
estimations is its integrated value in the water column (Cpt, g C m- 2 day-I). Only
the integrated photosynthetic production of new organic matter corresponds
to the primary energy input into the ecosystem. To estimate this, it is sufficient
to measure either the absolute rates of primary production at different depths
on the vertical profile (Cpd, mgCm- 3 ,day-l), or to measure the absolute value
of primary production in the upper layer of water (Cps, mg Cm- 3 ,day-l) and its
relative rates at different depths on the vertical profile (Ks). Both Cpd and Ks
should be measured per half of the light day or per whole light day to account
for the diurnal course of illumination. For Cpt calculation, the appropriate Cpd
and Ks curves are usually constructed. The practical approach to the construction of these curves is very different. The curves Cpd are derived from data
obtained during in situ (or simulated in situ light) incubations, when the
absolute values of half or whole light day primary production are measured
on the vertical profile, and the integrated production Cpt is then calculated ..
The Ks curves are constructed by calculating the appropriate Ks coefficients at
different depths within the euphotic zone with the use of data on the relative
vertical distribution of the two main factors deciding the character of the phytoplankton photosynthesis rate on the vertical profile; one of these is light
attenuation within the water column, and the second the relative vertical distribution of active phytoplankton on the vertical profile.