270
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
samples. The mixture is shaken before use. The precipitates 1 h later are filtered
through GF filters in a funnel 2 cm in diameter. The wet filters are placed
into the scintillation vials, treated with 0.3 ml 1: 1 acetic acid and dried at
80°C to evaporate the liquid. Then 0.1 ml of 10% hyamine solution in toluene
+3ml methoxyethanol +1O-15ml of the scintillation cocktail are added to
them. After mixing, the counting samples thus assembled are radioassayed.
The content of P04-P in the samples is measured by the standard method.
Killed controls are not necessary in this case because the chemical absorption of P04-P by benthic objects is quite negligible (Sorokin 1992).
The rate of P04-P uptake by benthic objects (Ac) is calculated as follows:
(R - R.) x (P + P.) x V
.
A = 0
•
0
•
Ilg P kg- 1 h- 1 , where Ro and Po are, respectIvely,
c
W x t x (Rm + R;)
radioactivity (cpml-l) and the absolute phosphorus content (llgPI-1) calculated as means in the initial subsamples and in those taken from the control
aquarium at the end of incubation, R; and P; are the same as taken from
the experimental aquarium at the end of incubation, V the mean volume
of the experimental vessel corrected for subsampling, 1; W the wet weight
of the benthic object, kg, and t the time of incubation, h. The rate of the
release of P04-P by elements of benthic communities Ae is calculated
as the sum of Ac and the rate of change of P04-P absolute concentration in the experimental aquarium (APt): Ae = Ac + APt Ilg P kg- 1 h- 1 • The
value of P t is calculated acording to the results of the same experiment:
(P -P.)x V
AP = 0
•
Ilg P kg- 1 h- 1 •
t
txW
To measure the same values of Ao Ae and P t in soft sediments in situ, the
enclosure technique can be employed. Another possibility in this case is the
use of a sample taken with the aid of a bottom sampler which brings up an
undisturbed core of sediment with overlying water. For the enclosure technique, Plexiglas transparent and black boxes measuring 30 x 30 x 30cm are
convenient to use. The boxes must be fitted with propellers to mix the water
and with a tube or hole closed with a serum plug to inject the isotope solution
and to take subsamples (Fig. 2.20). The boxes are firmly seated on the bottom
sediment and aliquots of labeled phosphate are injected into them, as
described above (see Sect. 2.5.4). After the water inside the box has been thoroughly mixed with the propeller, subsamples are taken from it with a large
syringe to estimate Ro and Po. When the exposure time of 3-5 h ends, a final
set of subsamples is extracted to estimate R; and Pi.
To estimate a possible exchange of water between the enclosures and the
surrounding water, thiosulfate solution is added to one of the parallel boxes
instead 0 f the labeled phosphate, and its concentration in the box is estimated
iodometrically in the beginning (ko) and at the end (k;) of the incubation
of all the boxes. A correction coefficient, K, is calculated for a possible washout of P04-P from the boxes during the experiment: K = k;lko• Usually, its
values range between 0.8 and 0.9. The chemical absorption of P04-P from
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
samples. The mixture is shaken before use. The precipitates 1 h later are filtered
through GF filters in a funnel 2 cm in diameter. The wet filters are placed
into the scintillation vials, treated with 0.3 ml 1: 1 acetic acid and dried at
80°C to evaporate the liquid. Then 0.1 ml of 10% hyamine solution in toluene
+3ml methoxyethanol +1O-15ml of the scintillation cocktail are added to
them. After mixing, the counting samples thus assembled are radioassayed.
The content of P04-P in the samples is measured by the standard method.
Killed controls are not necessary in this case because the chemical absorption of P04-P by benthic objects is quite negligible (Sorokin 1992).
The rate of P04-P uptake by benthic objects (Ac) is calculated as follows:
(R - R.) x (P + P.) x V
.
A = 0
•
0
•
Ilg P kg- 1 h- 1 , where Ro and Po are, respectIvely,
c
W x t x (Rm + R;)
radioactivity (cpml-l) and the absolute phosphorus content (llgPI-1) calculated as means in the initial subsamples and in those taken from the control
aquarium at the end of incubation, R; and P; are the same as taken from
the experimental aquarium at the end of incubation, V the mean volume
of the experimental vessel corrected for subsampling, 1; W the wet weight
of the benthic object, kg, and t the time of incubation, h. The rate of the
release of P04-P by elements of benthic communities Ae is calculated
as the sum of Ac and the rate of change of P04-P absolute concentration in the experimental aquarium (APt): Ae = Ac + APt Ilg P kg- 1 h- 1 • The
value of P t is calculated acording to the results of the same experiment:
(P -P.)x V
AP = 0
•
Ilg P kg- 1 h- 1 •
t
txW
To measure the same values of Ao Ae and P t in soft sediments in situ, the
enclosure technique can be employed. Another possibility in this case is the
use of a sample taken with the aid of a bottom sampler which brings up an
undisturbed core of sediment with overlying water. For the enclosure technique, Plexiglas transparent and black boxes measuring 30 x 30 x 30cm are
convenient to use. The boxes must be fitted with propellers to mix the water
and with a tube or hole closed with a serum plug to inject the isotope solution
and to take subsamples (Fig. 2.20). The boxes are firmly seated on the bottom
sediment and aliquots of labeled phosphate are injected into them, as
described above (see Sect. 2.5.4). After the water inside the box has been thoroughly mixed with the propeller, subsamples are taken from it with a large
syringe to estimate Ro and Po. When the exposure time of 3-5 h ends, a final
set of subsamples is extracted to estimate R; and Pi.
To estimate a possible exchange of water between the enclosures and the
surrounding water, thiosulfate solution is added to one of the parallel boxes
instead 0 f the labeled phosphate, and its concentration in the box is estimated
iodometrically in the beginning (ko) and at the end (k;) of the incubation
of all the boxes. A correction coefficient, K, is calculated for a possible washout of P04-P from the boxes during the experiment: K = k;lko• Usually, its
values range between 0.8 and 0.9. The chemical absorption of P04-P from