60
The Radiocarbon Method to Estimate Primary Production
Then the water column is rapidly sampled at six to eight depths down to the
lower margins of the euphotic zone. The sampling depths are selected depending on the position of chlorophyll minima and maxima. If chlorophyll profiles
are not available, the depths of sampling are selected in conjunction with temperature. The first sample is taken in the surface layer, then one or two samples
are taken within the upper mixed layer, another two to four samples within
the thermocline layer, and one or two samples below the thermocline, if the
euphotic zone still extends to this depth. The samples are taken in pairs in light
bottles. For dark controls are taken: one bottle from the surface, one from the
layer of chlorophyll maximum, and one below it, at a lower boundary of the
euphotic zone. All the bottles thus prepared are charged with equal portions
of 14C-carbonate working solution, and incubated in the deck aquarium at a
temperature close to that in situ and at a temperate illumination of some 510000lx in the shade or under a corresponding neutral optical filter, made, for
example, of white cloth. The length of incubation in the tropics and temperate waters in summer should be 1.5-2h, and in cold waters up to 4h. At the
end of incubation, the samples are instantly filtered. The radioactivity of 14C
in phytoplankton is measured at the filters as has been described above (Sect.
2.3.2.6) with correction of its dark uptake. Then the Kp-coefficients are calculated: Kp = RdIR" if Rd is the average of radioactivity of phytoplankton in taken
from various corresponding depths, and Rs the same in samples taken from
the surface layer.
From the first approach it may appear that the Kp curves may be replaced
by profiles of chlorophyll fluorescence, which also definitely reflect the vertical distribution of phytoplankton; but comparison of these two kinds of curves
shows that the maxima of chlorophyll fluorescence largely reflect accumulations of photosynthetically low active chlorophyll, and therefore do not suit
this purpose.
Having estimated the K, curve and the Kp-coefficients, the Ks-coefficients
could then be calculated as follows (Table 2.3). First, with the aid of the appropriate K t curve, the Kt-coefficients are estimated corresponding to the depths
at which the Kp-coefficients were measured. Then the Ks-coefficients are calculated for those depths only as the products of K t and Kp-coefficients attributed to the same depths. These coefficients are then used to derive the Ks curve
for a given station. Finally, the integral primary production in the water column
(Cpt) is calculated as described above (Fig. 2.15).
Calculations of Cpt values during cruises with many stations in areas with
rather similar parameters of water column stratification can be made with a
regression curve between the surface photosynthesis rate (Cps) and the ratio
Cp,lCps. An example of such a curve, calculated for the early summer period
in the Bering Sea, is given in Fig. 2.16. In this case, the values of photosynthesis are measured only in the surface samples (Cps). Then the corresponding
values of Cp,lCps ratios are found with the use of this curve, and the Cpt values
are calculated. Such curves, derived once for a given season, could then be
used for Cpt estimations in samples taken from a moving ship (Sorokin 1975c).
The Radiocarbon Method to Estimate Primary Production
Then the water column is rapidly sampled at six to eight depths down to the
lower margins of the euphotic zone. The sampling depths are selected depending on the position of chlorophyll minima and maxima. If chlorophyll profiles
are not available, the depths of sampling are selected in conjunction with temperature. The first sample is taken in the surface layer, then one or two samples
are taken within the upper mixed layer, another two to four samples within
the thermocline layer, and one or two samples below the thermocline, if the
euphotic zone still extends to this depth. The samples are taken in pairs in light
bottles. For dark controls are taken: one bottle from the surface, one from the
layer of chlorophyll maximum, and one below it, at a lower boundary of the
euphotic zone. All the bottles thus prepared are charged with equal portions
of 14C-carbonate working solution, and incubated in the deck aquarium at a
temperature close to that in situ and at a temperate illumination of some 510000lx in the shade or under a corresponding neutral optical filter, made, for
example, of white cloth. The length of incubation in the tropics and temperate waters in summer should be 1.5-2h, and in cold waters up to 4h. At the
end of incubation, the samples are instantly filtered. The radioactivity of 14C
in phytoplankton is measured at the filters as has been described above (Sect.
2.3.2.6) with correction of its dark uptake. Then the Kp-coefficients are calculated: Kp = RdIR" if Rd is the average of radioactivity of phytoplankton in taken
from various corresponding depths, and Rs the same in samples taken from
the surface layer.
From the first approach it may appear that the Kp curves may be replaced
by profiles of chlorophyll fluorescence, which also definitely reflect the vertical distribution of phytoplankton; but comparison of these two kinds of curves
shows that the maxima of chlorophyll fluorescence largely reflect accumulations of photosynthetically low active chlorophyll, and therefore do not suit
this purpose.
Having estimated the K, curve and the Kp-coefficients, the Ks-coefficients
could then be calculated as follows (Table 2.3). First, with the aid of the appropriate K t curve, the Kt-coefficients are estimated corresponding to the depths
at which the Kp-coefficients were measured. Then the Ks-coefficients are calculated for those depths only as the products of K t and Kp-coefficients attributed to the same depths. These coefficients are then used to derive the Ks curve
for a given station. Finally, the integral primary production in the water column
(Cpt) is calculated as described above (Fig. 2.15).
Calculations of Cpt values during cruises with many stations in areas with
rather similar parameters of water column stratification can be made with a
regression curve between the surface photosynthesis rate (Cps) and the ratio
Cp,lCps. An example of such a curve, calculated for the early summer period
in the Bering Sea, is given in Fig. 2.16. In this case, the values of photosynthesis are measured only in the surface samples (Cps). Then the corresponding
values of Cp,lCps ratios are found with the use of this curve, and the Cpt values
are calculated. Such curves, derived once for a given season, could then be
used for Cpt estimations in samples taken from a moving ship (Sorokin 1975c).