24
The Radiocarbon Method to Estimate Primary Production
Another reason for replacing the oxygen method by the radiocarbon one
was that, in addition to its high sensitivity, the latter appeared to be much more
simple and less time-consuming, which was very important for field research.
All that is necessary with the radiocarbon technique is to fill the bottles with
samples, inject radioisotope solution, expose them in situ or in the board
incubator for some hours, and filter the samples. This is much easier than
the precise oxygen titration of endless rows of bottles of the oxygen method,
whose low sensitivity requires high precision and many parallels: at least three
dark and three light bottles for one estimate. Moreover, on using the oxygen
method, researchers were confronted with many often inexplicable phenomena arising from the quality of the natural water, which could contain substances interfering with iodine and behaving differently in light and in dark
bottles, such as dissolved labile organic matter, peroxides, and reduced sulfur
and manganese compounds. Therefore, cases of negative gross photosynthesis
results were no rare phenomenon with the oxygen method, when the oxygen
contents in the dark bottles at the end of the exposure was larger than in the
light ones (Winberg 1960). Oxygen oversaturation, which often happens in
summer, especially in shallow basins with rich bottom vegetation and phytoplankton blooms, severely distorts the results of primary production measurements by the oxygen method because part of the oxygen produced escapes
as a bubbles. The phenomenon of bubble formation in the experimental bottles
during shifts of temperature makes deck incubation difficult with the oxygen
method. Finally, one of the basic disadvantages of this method connected with
its low sensitivity is that it needs long exposure times, not less than 24 h or
often more to obtain any results. This increases the errors due to the bottle
effects, as after 8-10 h or exposure in the tropics, and some 12-24 h in temperate waters, the microplankton communities begin to changes. First, rapid
growth of heterotrophic bacteria starts, which influences the rate of oxygen
uptake, and evidently the results of gross primary production estimates
calculated by the difference in oxygen content of light and dark bottles.
All these problems were avoided with the introduction of the radiocarbon method. However, it soon became clear that, although extremely simple
technically, the interpretation of the results thus obtained was rather complicated. In the original version of Steeman Nielsen, it could result in an underestimation of gross primary production by as much as two to three times
(Sorokin 1971; Eppley 1981). Gieskes et al. (1979) insisted upon an even
greater underestimation - up to five to ten times, but this was an apparent
exaggeration not verified later (Sorokin 1987). A tendency to underestimate
pelagic primary production by the 14C-method was common, especially during
the 1960s at the initial stage of its triumphal march through the hydrobiological laboratories. Its use became a kind of fashion at that time. It was often
applied by people with no knowledge of plankton biology, attracted by the
technical simplicity of the method itself. They took samples in the same way
as they took them for hydrochemical analyses, not taking into account that
planktonic algae are fragile and very sensitive to mechanical disturbance and
The Radiocarbon Method to Estimate Primary Production
Another reason for replacing the oxygen method by the radiocarbon one
was that, in addition to its high sensitivity, the latter appeared to be much more
simple and less time-consuming, which was very important for field research.
All that is necessary with the radiocarbon technique is to fill the bottles with
samples, inject radioisotope solution, expose them in situ or in the board
incubator for some hours, and filter the samples. This is much easier than
the precise oxygen titration of endless rows of bottles of the oxygen method,
whose low sensitivity requires high precision and many parallels: at least three
dark and three light bottles for one estimate. Moreover, on using the oxygen
method, researchers were confronted with many often inexplicable phenomena arising from the quality of the natural water, which could contain substances interfering with iodine and behaving differently in light and in dark
bottles, such as dissolved labile organic matter, peroxides, and reduced sulfur
and manganese compounds. Therefore, cases of negative gross photosynthesis
results were no rare phenomenon with the oxygen method, when the oxygen
contents in the dark bottles at the end of the exposure was larger than in the
light ones (Winberg 1960). Oxygen oversaturation, which often happens in
summer, especially in shallow basins with rich bottom vegetation and phytoplankton blooms, severely distorts the results of primary production measurements by the oxygen method because part of the oxygen produced escapes
as a bubbles. The phenomenon of bubble formation in the experimental bottles
during shifts of temperature makes deck incubation difficult with the oxygen
method. Finally, one of the basic disadvantages of this method connected with
its low sensitivity is that it needs long exposure times, not less than 24 h or
often more to obtain any results. This increases the errors due to the bottle
effects, as after 8-10 h or exposure in the tropics, and some 12-24 h in temperate waters, the microplankton communities begin to changes. First, rapid
growth of heterotrophic bacteria starts, which influences the rate of oxygen
uptake, and evidently the results of gross primary production estimates
calculated by the difference in oxygen content of light and dark bottles.
All these problems were avoided with the introduction of the radiocarbon method. However, it soon became clear that, although extremely simple
technically, the interpretation of the results thus obtained was rather complicated. In the original version of Steeman Nielsen, it could result in an underestimation of gross primary production by as much as two to three times
(Sorokin 1971; Eppley 1981). Gieskes et al. (1979) insisted upon an even
greater underestimation - up to five to ten times, but this was an apparent
exaggeration not verified later (Sorokin 1987). A tendency to underestimate
pelagic primary production by the 14C-method was common, especially during
the 1960s at the initial stage of its triumphal march through the hydrobiological laboratories. Its use became a kind of fashion at that time. It was often
applied by people with no knowledge of plankton biology, attracted by the
technical simplicity of the method itself. They took samples in the same way
as they took them for hydrochemical analyses, not taking into account that
planktonic algae are fragile and very sensitive to mechanical disturbance and