84
The Radiocarbon Method to Estimate Primary Production
radioactivity Ri of the isotopic working solution in the nonquenched scintillation vial.
When using this simplified approach to Ri and Cr evaluations, the
researcher must make sure that the Ri values in the experimental vessels do
not change during the time of incubation. There can be two probable causes
of its rapid (detectable) change: lusses of !4C02 during gas cxchange of watcr
with the atmosphere, and losses ofT 14 C02 due to incomplete isolation of water
in the in situ enclosures worked into the bottom sediments or in those attached
to the stems of hydrophytes.
The second cause is much more probable. In any case, both these possible ways of 14C02 losses should be controlled experimentally by directly measuring T CO2 radioactivity in water due to the exposure of experimental
vessels (without plants) with open and closed surfaces not containing an air
phase, and also in open and closed vessels worked into the sediments. The
samples of water taken from the vessels at the beginning and end of incubations are transferred into closed test tubes containing 0.1 mIl N KOH. In the
laboratory, 1-2 ml subsamples of this water are radioassayed using any available water-miscible scintillation cocktail. If during 4 h of incubation the
changes in 14C radioactivity thus measured do not exceed 5%, they can be
ignored. If they are more, the corresponding correction coefficients can be
derived and used to correct radioisotope values when performing later routine
incubations.
When working with benthic hydrophytes in the field, a rather small
portion (0.5-1.5ml) of isotopic working solution should be injected into the
large volume of water contained in the incubation vessels. To facilitate rapid
mixing, the injection of working solution into the volume of a vessel is most
practical is in two steps. First, the standard portions of working solution programmed to be injected into the definite volumes of experimental vessels are
distributed into a series of closed test tubes containing 10 ml of the alkaline
carrier solution (20mg of Na2CO} + 20mg of NaOH per 11 of either distilled
water or 3.5% NaCl solution). When working in the field, the contents of each
test tube is injected completely into a chosen experimental vessel by hypodermic syringe.
The contents of inorganic carbon (T CO2) in water Ci may be measured
with any of the above-mentioned methods (see Sect. 2.3.2.2). The simplest and
most practical procedure is titration. The samples, preserved in closed bottles
in a cold place, will retain constant T CO2 contents for a long time, so they can
be collected as work proceeds, and then analyzed in one set for this parameter. Because of the above-mentioned stability of carbonate system, the Ci
values are usually very conservative, and only need to be controlled from time
to time. If the experiments are conducted during the same time interval of the
light day (for example, between 7.00 and 11.00h) samples for Ci measurements
may be taken ca. once a week in the middle of this period, at ca. 9.00h.
Having estimated Ri and Ci values, the reverse specific radioactivity of
inorganic carbon in the experimental vessels (Ci) can be calculated: Ci = C;lRi
The Radiocarbon Method to Estimate Primary Production
radioactivity Ri of the isotopic working solution in the nonquenched scintillation vial.
When using this simplified approach to Ri and Cr evaluations, the
researcher must make sure that the Ri values in the experimental vessels do
not change during the time of incubation. There can be two probable causes
of its rapid (detectable) change: lusses of !4C02 during gas cxchange of watcr
with the atmosphere, and losses ofT 14 C02 due to incomplete isolation of water
in the in situ enclosures worked into the bottom sediments or in those attached
to the stems of hydrophytes.
The second cause is much more probable. In any case, both these possible ways of 14C02 losses should be controlled experimentally by directly measuring T CO2 radioactivity in water due to the exposure of experimental
vessels (without plants) with open and closed surfaces not containing an air
phase, and also in open and closed vessels worked into the sediments. The
samples of water taken from the vessels at the beginning and end of incubations are transferred into closed test tubes containing 0.1 mIl N KOH. In the
laboratory, 1-2 ml subsamples of this water are radioassayed using any available water-miscible scintillation cocktail. If during 4 h of incubation the
changes in 14C radioactivity thus measured do not exceed 5%, they can be
ignored. If they are more, the corresponding correction coefficients can be
derived and used to correct radioisotope values when performing later routine
incubations.
When working with benthic hydrophytes in the field, a rather small
portion (0.5-1.5ml) of isotopic working solution should be injected into the
large volume of water contained in the incubation vessels. To facilitate rapid
mixing, the injection of working solution into the volume of a vessel is most
practical is in two steps. First, the standard portions of working solution programmed to be injected into the definite volumes of experimental vessels are
distributed into a series of closed test tubes containing 10 ml of the alkaline
carrier solution (20mg of Na2CO} + 20mg of NaOH per 11 of either distilled
water or 3.5% NaCl solution). When working in the field, the contents of each
test tube is injected completely into a chosen experimental vessel by hypodermic syringe.
The contents of inorganic carbon (T CO2) in water Ci may be measured
with any of the above-mentioned methods (see Sect. 2.3.2.2). The simplest and
most practical procedure is titration. The samples, preserved in closed bottles
in a cold place, will retain constant T CO2 contents for a long time, so they can
be collected as work proceeds, and then analyzed in one set for this parameter. Because of the above-mentioned stability of carbonate system, the Ci
values are usually very conservative, and only need to be controlled from time
to time. If the experiments are conducted during the same time interval of the
light day (for example, between 7.00 and 11.00h) samples for Ci measurements
may be taken ca. once a week in the middle of this period, at ca. 9.00h.
Having estimated Ri and Ci values, the reverse specific radioactivity of
inorganic carbon in the experimental vessels (Ci) can be calculated: Ci = C;lRi