Use of the Radiocarbon Method to Determine Primary Production
97
taken together with the hard substrate they inhabit. Their photosynthesis rates
are preferably calculated per area of substrate, not per wet or dry weight.
The samples, arriving in the laboratory from the field in the thermoisolated box with dry ice, are placed into the refrigerator, and removed one by
one, for treatment. The sample is placed into the plastic cap. With the aid of a
scalpel, tht: periphytonic turf is scraped out together with the upper millimeter of substrate. The substrate itself is washed, dried, and then used to measure
its surface area. The scraped material is collected into the weighed 30-ml
beaker, and first treated with 10% HCI solution drop by drop until the evolution of CO2 stops, to avoid calcareous material. Then the contents of the
beaker is dried at -70°C and it is weighed again to know the amount of dry
material in the scraped samples. It is necessary to know the weight of the material in order to equalize its approximate contents in the scintillation vials and
thus to simplify the quench correction (see Sect. 2.5.3). After the beaker is
weighed, 5% solution of NaOH is added into it in the proportion 5ml per 19
of dry weight of the scraped material. The beaker is then heated to 100°C until
all the liquid evaporates. The subsequent procedure is as described before for
radioassay of plant tissue alkaline hydrolyzates.
Treatment of microphytobenthos cores after 14C-incubation is as follows.
The cores arrive at the laboratory still contained in the corer tubes. In the laboratory, the lower stopper is taken out from the tube and replaced by a piston,
with the aid of which the core is moved up until its surface is 2mm over the
edge of the corer tube in the case of silty sediment (or 4-5 mm if it is sandy
sediment). This layer up the edge of the tube is then cut off and placed into a
beaker. This same operations can be accomplished even more easily with the
cores previously frozen. The subsequent procedure is the same as described
above for the treatment of periphyton.
2.5.6 Calculation of Results; Comparison with Alternative Methods
The most convenient basic parameters for the normalization of results of
primary production determinations carried out by the 14C technique are the
hourly photosynthesis rates calculated per g-l dry weight of plant material for
the thalli of macrophytes and for the leaves of sea grasses, or per dm- 2 substrate area for periphyton or microphytobenthos. In this case, the hourly rate
of photosynthesis (Cp ) can be calculated as follows: Cp = Re Cr 1.15ltmgCg- 1
h- 1 (or mg C dm- 2 h- 1 ), if Re is the radioactivity either of plant material (cpm
g-l) or of the substance of microphytobenthic-periphytonic associations
corrected for dark control (cpm dm- 2 ), Cr the inverse specific radioactivity of
inorganic (T CO2) carbon in experimental vessels, mgCcpm- 1 ; 1.30 the joint
correction coefficient accounting for the isotopic effect (see Sect. 2.3.1) and
for the assumed 10% losses of 14C-Iabeled assimilates as the exudated organic
matter (mainly glycollate) and as the respiratory CO2 (Arnold and Littler
97
taken together with the hard substrate they inhabit. Their photosynthesis rates
are preferably calculated per area of substrate, not per wet or dry weight.
The samples, arriving in the laboratory from the field in the thermoisolated box with dry ice, are placed into the refrigerator, and removed one by
one, for treatment. The sample is placed into the plastic cap. With the aid of a
scalpel, tht: periphytonic turf is scraped out together with the upper millimeter of substrate. The substrate itself is washed, dried, and then used to measure
its surface area. The scraped material is collected into the weighed 30-ml
beaker, and first treated with 10% HCI solution drop by drop until the evolution of CO2 stops, to avoid calcareous material. Then the contents of the
beaker is dried at -70°C and it is weighed again to know the amount of dry
material in the scraped samples. It is necessary to know the weight of the material in order to equalize its approximate contents in the scintillation vials and
thus to simplify the quench correction (see Sect. 2.5.3). After the beaker is
weighed, 5% solution of NaOH is added into it in the proportion 5ml per 19
of dry weight of the scraped material. The beaker is then heated to 100°C until
all the liquid evaporates. The subsequent procedure is as described before for
radioassay of plant tissue alkaline hydrolyzates.
Treatment of microphytobenthos cores after 14C-incubation is as follows.
The cores arrive at the laboratory still contained in the corer tubes. In the laboratory, the lower stopper is taken out from the tube and replaced by a piston,
with the aid of which the core is moved up until its surface is 2mm over the
edge of the corer tube in the case of silty sediment (or 4-5 mm if it is sandy
sediment). This layer up the edge of the tube is then cut off and placed into a
beaker. This same operations can be accomplished even more easily with the
cores previously frozen. The subsequent procedure is the same as described
above for the treatment of periphyton.
2.5.6 Calculation of Results; Comparison with Alternative Methods
The most convenient basic parameters for the normalization of results of
primary production determinations carried out by the 14C technique are the
hourly photosynthesis rates calculated per g-l dry weight of plant material for
the thalli of macrophytes and for the leaves of sea grasses, or per dm- 2 substrate area for periphyton or microphytobenthos. In this case, the hourly rate
of photosynthesis (Cp ) can be calculated as follows: Cp = Re Cr 1.15ltmgCg- 1
h- 1 (or mg C dm- 2 h- 1 ), if Re is the radioactivity either of plant material (cpm
g-l) or of the substance of microphytobenthic-periphytonic associations
corrected for dark control (cpm dm- 2 ), Cr the inverse specific radioactivity of
inorganic (T CO2) carbon in experimental vessels, mgCcpm- 1 ; 1.30 the joint
correction coefficient accounting for the isotopic effect (see Sect. 2.3.1) and
for the assumed 10% losses of 14C-Iabeled assimilates as the exudated organic
matter (mainly glycollate) and as the respiratory CO2 (Arnold and Littler