58
The Radiocarbon Method to Estimate Primary Production
fixed on to the cable with a heavy weight of 30 to 50 kg at the end, the distances between them so calculated that when the final upper set is lowered for
incubation to some 0.2-0.5 m below the surface, the other, deeper, sets of
bottles, which have also been fixed to the cable, will arrive down to the horizons programmed for them. This operation should be done rapidly to prevent
extra illumination of the bottles to be incubated in the deep layers. The number
of depths should be as many as is technically possible for the researcher
without a large extension of the operation. The position of the bottles must be
programmed so that the lesser intervals between the depths should be situated in the uppermost productive part of the euphotic zone. For example, if
the Secchi disk transparency at the station is determined to be of about 10m,
the incubation depths may be selected as follows: 0.5, 2.5, 6, 10, 15, 20, and
30m. This series can be expanded or contracted, depending on the Secchi
disk transparency (or the photometer record of depth for 2% PARS).
For immersion of bottles in lakes and coastal waters, ordinary lines may
be used attached to a raft which has an open area in the center, where the
upper end of the line is fixed with wires to avoid shading the upper bottles, as
shown in Fig. 2.27. The number of depths in this case can be abbreviated to
five. The most practical dark control is to have two dark bottles as control for
the whole series. After being charged with radioisotope solution, they are incubated in black bags in the deck-shaded incubator destined to estimate Kp
coefficients (see below).
The duration of in situ incubation should be not less than half of the light
day to record the whole scale of diurnal illumination. It may also be started
at midday and ended at dusk. In cold waters with temperatures below 15-16
°C, the duration of exposure can be prolonged to the whole light day without
significant bottle effects. To finish the incubation, the row of bottles is rapidly
raised. The sets of bottles are unhooked, separated from the cable, and
instantly stowed again in the dark box, then carried into the laboratory. Under
low illumination, the samples are fixed with a very weak Lugol iodine solution: several drops of 0.2N solution should be added until a slight yellow color
appears. Then the control dark bottles are fixed in the same way, but not
instantly. First, they should be taken out of their black covers and illuminated
on deck for approximately the period of time needed to raise the in situ
samples from the lower end of the cable. Then they are fixed too.
All sets of samples are filtered and processed as usual to estimate 14C02
assimilation by phytoplankton with correction for dark uptake. Then the K tcoefficients are calculated for each depth: K t = RaIR" if Ra is average of
radioactivity values measured in the light bottles incubated at a given depth
and corrected for 14C dark uptake; Rs is the same values measured in the
bottles incubated at the surface. Then the K t curve is derived with the K, values
on the abscissa and the depths on the ordinate (Figs 2.10,2.14). Only this curve,
but not individually measured K t coefficients, is then used to calculate the Kscoefficients needed to estimate integrated primary production in the water
column.
The Radiocarbon Method to Estimate Primary Production
fixed on to the cable with a heavy weight of 30 to 50 kg at the end, the distances between them so calculated that when the final upper set is lowered for
incubation to some 0.2-0.5 m below the surface, the other, deeper, sets of
bottles, which have also been fixed to the cable, will arrive down to the horizons programmed for them. This operation should be done rapidly to prevent
extra illumination of the bottles to be incubated in the deep layers. The number
of depths should be as many as is technically possible for the researcher
without a large extension of the operation. The position of the bottles must be
programmed so that the lesser intervals between the depths should be situated in the uppermost productive part of the euphotic zone. For example, if
the Secchi disk transparency at the station is determined to be of about 10m,
the incubation depths may be selected as follows: 0.5, 2.5, 6, 10, 15, 20, and
30m. This series can be expanded or contracted, depending on the Secchi
disk transparency (or the photometer record of depth for 2% PARS).
For immersion of bottles in lakes and coastal waters, ordinary lines may
be used attached to a raft which has an open area in the center, where the
upper end of the line is fixed with wires to avoid shading the upper bottles, as
shown in Fig. 2.27. The number of depths in this case can be abbreviated to
five. The most practical dark control is to have two dark bottles as control for
the whole series. After being charged with radioisotope solution, they are incubated in black bags in the deck-shaded incubator destined to estimate Kp
coefficients (see below).
The duration of in situ incubation should be not less than half of the light
day to record the whole scale of diurnal illumination. It may also be started
at midday and ended at dusk. In cold waters with temperatures below 15-16
°C, the duration of exposure can be prolonged to the whole light day without
significant bottle effects. To finish the incubation, the row of bottles is rapidly
raised. The sets of bottles are unhooked, separated from the cable, and
instantly stowed again in the dark box, then carried into the laboratory. Under
low illumination, the samples are fixed with a very weak Lugol iodine solution: several drops of 0.2N solution should be added until a slight yellow color
appears. Then the control dark bottles are fixed in the same way, but not
instantly. First, they should be taken out of their black covers and illuminated
on deck for approximately the period of time needed to raise the in situ
samples from the lower end of the cable. Then they are fixed too.
All sets of samples are filtered and processed as usual to estimate 14C02
assimilation by phytoplankton with correction for dark uptake. Then the K tcoefficients are calculated for each depth: K t = RaIR" if Ra is average of
radioactivity values measured in the light bottles incubated at a given depth
and corrected for 14C dark uptake; Rs is the same values measured in the
bottles incubated at the surface. Then the K t curve is derived with the K, values
on the abscissa and the depths on the ordinate (Figs 2.10,2.14). Only this curve,
but not individually measured K t coefficients, is then used to calculate the Kscoefficients needed to estimate integrated primary production in the water
column.