72
The Radiocarbon Method to Estimate Primary Production
For practical purposes in the 14C-method intercalibration after the Ormethod
(see below), we recommend the value of 1.4 as most realistic. This value is
somewhat exaggerated, assuming the above mentioned possible overestimation of real gross photosynthetic oxygen production, when using the oxygen
light bottle method.
The procedure for 14C-methud intercalibration using an alternative
method, either the technically more simple oxygen-Winkler or the more
complex COrcoulometric should, in my opinion, be a necessary element in
practical use of the radiocarbon method. It creates for the researcher a solid
background of reliable data. Of these two methods, the oxygen method is,
practically speaking, more suitable.
The dark-light bottle oxygen method was originally invented by Gaarder
and Gran (1927). Later, it was significantly modernized and tested for reliability and possible faults by Winberg (1960) and Vollenveider (1974). Its
sensitivity on using three replicates and special electronic accessories for
precise Winkler titration (Williams and Jenkinson 1982) can be evaluated
within 10-20 j..lg 0 2 1- 1 , or some 2-4j..lg 0 2 1- 1 h- 1 by incubation of bottles for 6 h,
or for about half a light day. So the sensitivity of the routine oxygen method
for gross primary production estimation can be evaluated as between 5
and 20mg Cm- 3 day-I depending on the duration of incubation (full or half
light day). To determine gross photosynthesis by the oxygen-bottle method,
necessary for the inter calibration of the radiocarbon method, the appropriate experiments can be conducted only in productive water with the
lower limit of primary production at an optimal light intensity of about
100mgCm- 3 day-l, better if over 200mgCm- 3 day-l, for estimations of gross
photosynthesis during half light-day incubations, which are much more preferable from the point of the view of bottle effects (see above). Therefore, even
when working in oligotrophic basins, like oligotrophic lakes or the tropical
ocean for calibration experiments of the 14C-method as against an alternative
one (either the oxygen or the CO2), the researcher should choose sites with
enough productive waters, such as coastal areas or embayments. The wet
phytoplankton biomass should be over 500-700mgm- 3 and the chlorophyll a
content over 2-3mgm- 3 •
When using the oxygen bottle method, it its helpful to know the possible
sources of errors, the main being gas bubble formation on the inner walls, and
the consumption of iodine by reduced substances present in the water samples.
The bubbles are most often formed in the experimental bottles when the
experiment is started before midday, when the water in productive basins may
be oversaturated with dissolved oxygen. In such water samples, the bubbles
form inevitably even on slight (2-3°C) changes in temperature. A greater
change in temperature during incubation could induce bubble formation even
in samples undersaturated with oxygen. The experimental bottles in which
bubbles have appeared due to incubation should be avoided. Iodine consumption occurs in samples prepared for Winkler titration; being an oxidizer,
it reacts with the labile organic matter when the samples are kept for over
The Radiocarbon Method to Estimate Primary Production
For practical purposes in the 14C-method intercalibration after the Ormethod
(see below), we recommend the value of 1.4 as most realistic. This value is
somewhat exaggerated, assuming the above mentioned possible overestimation of real gross photosynthetic oxygen production, when using the oxygen
light bottle method.
The procedure for 14C-methud intercalibration using an alternative
method, either the technically more simple oxygen-Winkler or the more
complex COrcoulometric should, in my opinion, be a necessary element in
practical use of the radiocarbon method. It creates for the researcher a solid
background of reliable data. Of these two methods, the oxygen method is,
practically speaking, more suitable.
The dark-light bottle oxygen method was originally invented by Gaarder
and Gran (1927). Later, it was significantly modernized and tested for reliability and possible faults by Winberg (1960) and Vollenveider (1974). Its
sensitivity on using three replicates and special electronic accessories for
precise Winkler titration (Williams and Jenkinson 1982) can be evaluated
within 10-20 j..lg 0 2 1- 1 , or some 2-4j..lg 0 2 1- 1 h- 1 by incubation of bottles for 6 h,
or for about half a light day. So the sensitivity of the routine oxygen method
for gross primary production estimation can be evaluated as between 5
and 20mg Cm- 3 day-I depending on the duration of incubation (full or half
light day). To determine gross photosynthesis by the oxygen-bottle method,
necessary for the inter calibration of the radiocarbon method, the appropriate experiments can be conducted only in productive water with the
lower limit of primary production at an optimal light intensity of about
100mgCm- 3 day-l, better if over 200mgCm- 3 day-l, for estimations of gross
photosynthesis during half light-day incubations, which are much more preferable from the point of the view of bottle effects (see above). Therefore, even
when working in oligotrophic basins, like oligotrophic lakes or the tropical
ocean for calibration experiments of the 14C-method as against an alternative
one (either the oxygen or the CO2), the researcher should choose sites with
enough productive waters, such as coastal areas or embayments. The wet
phytoplankton biomass should be over 500-700mgm- 3 and the chlorophyll a
content over 2-3mgm- 3 •
When using the oxygen bottle method, it its helpful to know the possible
sources of errors, the main being gas bubble formation on the inner walls, and
the consumption of iodine by reduced substances present in the water samples.
The bubbles are most often formed in the experimental bottles when the
experiment is started before midday, when the water in productive basins may
be oversaturated with dissolved oxygen. In such water samples, the bubbles
form inevitably even on slight (2-3°C) changes in temperature. A greater
change in temperature during incubation could induce bubble formation even
in samples undersaturated with oxygen. The experimental bottles in which
bubbles have appeared due to incubation should be avoided. Iodine consumption occurs in samples prepared for Winkler titration; being an oxidizer,
it reacts with the labile organic matter when the samples are kept for over