Historical Proceedings
25
contact with solid surfaces. Some scientists even used the metallic Nansen
water bottles to sample for primary production experiments. They used long
rubber tubes for subsampling from water bottles and small-capacity bottles of
60-100ml for exposure with 14C. Following the tradition of the oxygen bottle
method, they exposed the bottles also to 14C-bicarbonate for 24 h, which was
far too long, and resulted in underestimation, especially when the water temperature was over 20°C. Long exposure causes the death of the phytoplankton and accelerates the dark 14C-uptake by bacterioplankton, which is usually
greater in dark than in light bottles because of the light inhibition of bacterioplankton (Sorokin 1971). No wonder that I have often heard from inexperienced colleagues that they found negative primary production in tropical
waters, when 14C assimilation by microplankton was larger in the dark bottles
than in the light ones, even in the surface layer.
In addition, however, to such obvious artifacts, the radiocarbon method as
invented and used in the 1950s and 1960s had also some intrinsic deficiencies
which also resulted in underestimation. One of these was the loss of a
significant part of the assimilated 14C during the filtration procedure and,
as respired labeled CO2, produced due to the use of newly formed labeled assimilates by the phytoplankton cells as energy material. At that time the
researchers also did not realize that a significant part - up to 50% - of the
total phytoplankton biomass in marine and often also in freshwaters could be
composed of cells of nanophytofiagellates and picocyanobacteria of less than
3 f..lm in size. Therefore for primary production estimations, often membrane
filters with a pore size of 2-3 f..lm were used, filtration was carried out under a
strong vacuum of over 100mmHg to accelerate the filtration of numerous
samples in the field, and no attention was paid to the fact that up to 50% of
the assimilated 14C could be lost during this procedure. Significant respiratory
losses of 14C assimilated by algae were entailed by unnecessarily long incubations of 24h. It is quite obvious that the share of newly assimilated 14C-labeled
organic carbon in the pool of organic matter accessible for use as the substrate
for respiration rises during exposure, as well as the share of labeled CO2 in
the totally respired. It is negligible during the first 2-3 h of exposure, but at the
end of a 24-h exposure, it can be quite significant, up to 20-30% (Winberg
1960; Harris 1980). In conjunction with obvious respiratory losses of assimilated 14C during incubation (Steemann Nielsen 1958), the problem was long
discussed, what kind of primary production is measured with the aid of 14C:
the gross or the net one? The oxygen method clearly measures gross production (light bottle O 2 minus dark bottle O2) and also net production (light bottIe
O 2 minus initial bottle O 2). In its old version, with 24-h exposures and drastic
losses of assimilated 14C, the radiocarbon method measured values below or
equal to the net production, while in its modern modification, with 2-3h exposure and corrections for losses of assimilated 14C, it measures values close to
the gross primary production (Williams et al. 1983; Sorokin 1987). Numerous
experimental works done in the 1970s and 1980s on the adequacy of radiocarbon methods, had elucidated its inadequacies, and ways to correct them,
25
contact with solid surfaces. Some scientists even used the metallic Nansen
water bottles to sample for primary production experiments. They used long
rubber tubes for subsampling from water bottles and small-capacity bottles of
60-100ml for exposure with 14C. Following the tradition of the oxygen bottle
method, they exposed the bottles also to 14C-bicarbonate for 24 h, which was
far too long, and resulted in underestimation, especially when the water temperature was over 20°C. Long exposure causes the death of the phytoplankton and accelerates the dark 14C-uptake by bacterioplankton, which is usually
greater in dark than in light bottles because of the light inhibition of bacterioplankton (Sorokin 1971). No wonder that I have often heard from inexperienced colleagues that they found negative primary production in tropical
waters, when 14C assimilation by microplankton was larger in the dark bottles
than in the light ones, even in the surface layer.
In addition, however, to such obvious artifacts, the radiocarbon method as
invented and used in the 1950s and 1960s had also some intrinsic deficiencies
which also resulted in underestimation. One of these was the loss of a
significant part of the assimilated 14C during the filtration procedure and,
as respired labeled CO2, produced due to the use of newly formed labeled assimilates by the phytoplankton cells as energy material. At that time the
researchers also did not realize that a significant part - up to 50% - of the
total phytoplankton biomass in marine and often also in freshwaters could be
composed of cells of nanophytofiagellates and picocyanobacteria of less than
3 f..lm in size. Therefore for primary production estimations, often membrane
filters with a pore size of 2-3 f..lm were used, filtration was carried out under a
strong vacuum of over 100mmHg to accelerate the filtration of numerous
samples in the field, and no attention was paid to the fact that up to 50% of
the assimilated 14C could be lost during this procedure. Significant respiratory
losses of 14C assimilated by algae were entailed by unnecessarily long incubations of 24h. It is quite obvious that the share of newly assimilated 14C-labeled
organic carbon in the pool of organic matter accessible for use as the substrate
for respiration rises during exposure, as well as the share of labeled CO2 in
the totally respired. It is negligible during the first 2-3 h of exposure, but at the
end of a 24-h exposure, it can be quite significant, up to 20-30% (Winberg
1960; Harris 1980). In conjunction with obvious respiratory losses of assimilated 14C during incubation (Steemann Nielsen 1958), the problem was long
discussed, what kind of primary production is measured with the aid of 14C:
the gross or the net one? The oxygen method clearly measures gross production (light bottle O 2 minus dark bottle O2) and also net production (light bottIe
O 2 minus initial bottle O 2). In its old version, with 24-h exposures and drastic
losses of assimilated 14C, the radiocarbon method measured values below or
equal to the net production, while in its modern modification, with 2-3h exposure and corrections for losses of assimilated 14C, it measures values close to
the gross primary production (Williams et al. 1983; Sorokin 1987). Numerous
experimental works done in the 1970s and 1980s on the adequacy of radiocarbon methods, had elucidated its inadequacies, and ways to correct them,