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Chemical Oceanography, 4th Edition
net and gross production. The method is thus useful in separating the gross and net productivity. It has been shown to yield values for the gross production that agree with the
changes in total oxygen. The 14 C production is 65% or less of the gross production (Grande
et al., 1989).
9.1.4 uptake of CO 2 Method of Measuring Primary Productivity
The most common method of measuring primary productivity is using carbon-14. The
300-ml bottles are filled with seawater and 2 cm 3 of bicarbonate solution with labeled 14 C
(1 to 25 μCi depending on productivity expected). The bottles are suspended at various
depths or placed in a thermostatic bath and illuminated with light from fluorescent lamps
filtered to give the intensities and spectral distribution of a given depth. After 2 to 6 h, the
seawater is gently filtered through a 0.45-μm filter. The filters are washed with seawater or
HCl vapor to remove calcareous inorganic carbon. The radiocarbon flux is counted with a
scintillation counter to determine the activity. About 1 to 2% of the CO 2 may be exchanged
in the dark (10% in tropical waters). The productivity is given by
Productivity (mg C m –3 h –1 ) = 1.05 (C S – C D )W/ C N
(9.11)
where W is the total CO 2 in the sample (mg C m –3 ); N is the number of hours exposed;
1.05 compensates for slow uptake of alga cells of 14 CO 2 compared to 12 CO 2 ; C S and C D are
counting rates of light and dark bottles, respectively; and C is the normalized counting
rate of 14 C that has been added to the bottles. Strickland and Parson (1968) gave the full
details of using this technique. Some problems with the method are as follows:
1. It is important to measure net production (60 to 90% of gross production).
2. Extracellular metabolites may not be retained by the filter (30%).
3. Sample enclosure may cause changes.
4. Diurnal effects may be missed.
5. Measures only fixation of inorganic C.
Laboratory measurements with cultures show reasonable agreement between estimates
of primary productivity based on O 2 production, CO 2 assimilation, uptake of P, and the
volume of packed cells obtained after centrifugation. The O 2 method gives more meaningful results for longer than 24-h sampling and highly productive waters. 14 C is best for
oligotrophic waters—after correcting for extracellular products.
9.1.5 Determining New Production
In recent years, the new production has been determined by a number of chemical and
physical methods. Since nitrogen is thought to limit the production of phytoplankton in
the oceans, it is frequently used to define and measure new production. New production
can thus be estimated as the rate of transport of nitrate into the photic zone of the oceans
or the rate of organic nitrogen that leaves this area. This role of nitrogen- fixing organisms
still remains to be fully elucidated. The in vitro techniques that can be used to determine
new production are the assimilation of 15 NO 3
– and the evolution of 18 O 2 . The collection of
organic nitrogen in surface sediment traps can also be used to determine the new production. Other methods that measure bulk properties include the determination of the flux of
Chemical Oceanography, 4th Edition
net and gross production. The method is thus useful in separating the gross and net productivity. It has been shown to yield values for the gross production that agree with the
changes in total oxygen. The 14 C production is 65% or less of the gross production (Grande
et al., 1989).
9.1.4 uptake of CO 2 Method of Measuring Primary Productivity
The most common method of measuring primary productivity is using carbon-14. The
300-ml bottles are filled with seawater and 2 cm 3 of bicarbonate solution with labeled 14 C
(1 to 25 μCi depending on productivity expected). The bottles are suspended at various
depths or placed in a thermostatic bath and illuminated with light from fluorescent lamps
filtered to give the intensities and spectral distribution of a given depth. After 2 to 6 h, the
seawater is gently filtered through a 0.45-μm filter. The filters are washed with seawater or
HCl vapor to remove calcareous inorganic carbon. The radiocarbon flux is counted with a
scintillation counter to determine the activity. About 1 to 2% of the CO 2 may be exchanged
in the dark (10% in tropical waters). The productivity is given by
Productivity (mg C m –3 h –1 ) = 1.05 (C S – C D )W/ C N
(9.11)
where W is the total CO 2 in the sample (mg C m –3 ); N is the number of hours exposed;
1.05 compensates for slow uptake of alga cells of 14 CO 2 compared to 12 CO 2 ; C S and C D are
counting rates of light and dark bottles, respectively; and C is the normalized counting
rate of 14 C that has been added to the bottles. Strickland and Parson (1968) gave the full
details of using this technique. Some problems with the method are as follows:
1. It is important to measure net production (60 to 90% of gross production).
2. Extracellular metabolites may not be retained by the filter (30%).
3. Sample enclosure may cause changes.
4. Diurnal effects may be missed.
5. Measures only fixation of inorganic C.
Laboratory measurements with cultures show reasonable agreement between estimates
of primary productivity based on O 2 production, CO 2 assimilation, uptake of P, and the
volume of packed cells obtained after centrifugation. The O 2 method gives more meaningful results for longer than 24-h sampling and highly productive waters. 14 C is best for
oligotrophic waters—after correcting for extracellular products.
9.1.5 Determining New Production
In recent years, the new production has been determined by a number of chemical and
physical methods. Since nitrogen is thought to limit the production of phytoplankton in
the oceans, it is frequently used to define and measure new production. New production
can thus be estimated as the rate of transport of nitrate into the photic zone of the oceans
or the rate of organic nitrogen that leaves this area. This role of nitrogen- fixing organisms
still remains to be fully elucidated. The in vitro techniques that can be used to determine
new production are the assimilation of 15 NO 3
– and the evolution of 18 O 2 . The collection of
organic nitrogen in surface sediment traps can also be used to determine the new production. Other methods that measure bulk properties include the determination of the flux of