Faults in the Radiocarbon Method and Problems in its Calibration
73
several tenths of minutes after dissolution of the Mn-hydroxide sediment in
the bottles. To escape these faults, calibration experiments should be started
in the morning, when the oxygen contents in the water is still within saturation limits. On in situ incubation, the bottles should be immersed to the depth
corresponding to some 0.5-0.7 of the Secchi disk transparency depth, if the
experiment is conducted on a bnght sunny day.
When using the deck incubator for such an intercalibration on a sunny
day, this should be kept in the shade or in full sun, covered with a screen or a
white tissue blocking some 50-60% of the external light. The temperature in
this incubator should in all cases be kept constant during the whole experiment. To avoid iodine losses from titration of samples, it must be performed
within a set time interval between the dissolution of the Mn-hydroxide sediment and the titration procedure itself.
The technique of the calibration experiment should be as follows. The
water sample, taken from a site with sufficiently high phytoplankton density,
should be collected into a 5-1 glass jar. Then with the aid of syphon made of
glass tubing not less than 6 mm internal diameter, the water is distributed into
four dark and four light BOD bottles of the same capacity as used for routine
14C experiments. The end of the syphon is thrust down to the bottom of the
bottle. After the bottle is full, one extra volume is passed through it, as for
analysis of dissolved oxygen. The full bottles are capped with ground glass
stoppers, which are then covered with plastic film held by a rubber band. This
series of oxygen bottles is placed for exposure in light in situ or in the deck
incubator. Then the rest of the water in the glass jar is distributed into three
light and two dark bottles of the same kind as for 14COruptake measurement
(see Cps estimation technique, Sect. 2.3.2.5). The bottles are charged with 14C_
bicarbonate working solution and incubated in the same place as the series of
oxygen bottles. The duration of their incubation should be the same as usually
used during Cps estimations, e.g., within 2 to 4h. The oxygen bottles are incubated for either half the light day or the full right day, depending on the water
temperature. Half light-day incubations are preferable if the water temperature is over 20°C.
After exposure, the value of Cps (primary production per day, mgCI- 1 ) is
estimated in the 14C-bottles, as described above (Sect. 2.3.2.5). In the oxygen
bottles the oxygen contents is measured by the Winkler procedure and the
average values are calculated for the groups of light bottles and dark bottles,
respectively. Thus, the average of the difference between the light and the dark
bottles is estimated, and the gross photosynthesis production of oxygen by the
phytoplankton (Pg, mg 0 2 1- 1 day-1) is calculated, accounting for the duration
of incubation (half light day or whole light day). Then the Cps value is calculated as measured by the oxygen bottle method using the PO quotient 1.4: Cps
= Pg x 0.27mgCI- 1 day-l.
Significant efforts have been undertaken to adapt the information on the
diurnal fluctuations of oxygen contents in water also within the euphotic zone
to evaluate primary photosynthetic production (Seivell 1935; Miyake and
73
several tenths of minutes after dissolution of the Mn-hydroxide sediment in
the bottles. To escape these faults, calibration experiments should be started
in the morning, when the oxygen contents in the water is still within saturation limits. On in situ incubation, the bottles should be immersed to the depth
corresponding to some 0.5-0.7 of the Secchi disk transparency depth, if the
experiment is conducted on a bnght sunny day.
When using the deck incubator for such an intercalibration on a sunny
day, this should be kept in the shade or in full sun, covered with a screen or a
white tissue blocking some 50-60% of the external light. The temperature in
this incubator should in all cases be kept constant during the whole experiment. To avoid iodine losses from titration of samples, it must be performed
within a set time interval between the dissolution of the Mn-hydroxide sediment and the titration procedure itself.
The technique of the calibration experiment should be as follows. The
water sample, taken from a site with sufficiently high phytoplankton density,
should be collected into a 5-1 glass jar. Then with the aid of syphon made of
glass tubing not less than 6 mm internal diameter, the water is distributed into
four dark and four light BOD bottles of the same capacity as used for routine
14C experiments. The end of the syphon is thrust down to the bottom of the
bottle. After the bottle is full, one extra volume is passed through it, as for
analysis of dissolved oxygen. The full bottles are capped with ground glass
stoppers, which are then covered with plastic film held by a rubber band. This
series of oxygen bottles is placed for exposure in light in situ or in the deck
incubator. Then the rest of the water in the glass jar is distributed into three
light and two dark bottles of the same kind as for 14COruptake measurement
(see Cps estimation technique, Sect. 2.3.2.5). The bottles are charged with 14C_
bicarbonate working solution and incubated in the same place as the series of
oxygen bottles. The duration of their incubation should be the same as usually
used during Cps estimations, e.g., within 2 to 4h. The oxygen bottles are incubated for either half the light day or the full right day, depending on the water
temperature. Half light-day incubations are preferable if the water temperature is over 20°C.
After exposure, the value of Cps (primary production per day, mgCI- 1 ) is
estimated in the 14C-bottles, as described above (Sect. 2.3.2.5). In the oxygen
bottles the oxygen contents is measured by the Winkler procedure and the
average values are calculated for the groups of light bottles and dark bottles,
respectively. Thus, the average of the difference between the light and the dark
bottles is estimated, and the gross photosynthesis production of oxygen by the
phytoplankton (Pg, mg 0 2 1- 1 day-1) is calculated, accounting for the duration
of incubation (half light day or whole light day). Then the Cps value is calculated as measured by the oxygen bottle method using the PO quotient 1.4: Cps
= Pg x 0.27mgCI- 1 day-l.
Significant efforts have been undertaken to adapt the information on the
diurnal fluctuations of oxygen contents in water also within the euphotic zone
to evaluate primary photosynthetic production (Seivell 1935; Miyake and