Protocol for Net and Gross O
Michael L. Bender
Samples are incubated in 100 ml quartz bottles at ambient
temperature and irradiance for 24 hours. In situ incubations are
preferable in that they do a better job of reproducing irradiance,
spectral quality, and possibly temperature. Net O production is
determined from the change in the O concentration after 24 hours.
Gross O production is determined by measuring the rate at which
O-labeled O in HO is transformed into O -labeled O.
Quartz bottles are carefully precleaned by long soaking in 1 NHCl before
the cruise, and by extensive rinsing with 18 Meg deionized water between
incubations. Quartz bottles have ground glass female mouths; stoppers are
In practice, two analytical methods are used. In the first, the change in
the O concentration is determined by measuring [O] in
incubated and unincubated samples; only net production is measured. In
the second, incubation samples are labeled with H O. The
dissolved gases are extracted from incubated and unincubated samples. In
the laboratory, the change in the O of O and the change in the O /N ratio are measured mass spectrometrically.
Net production is equal to the fractional change in the O /N
ratio times the O concentration. Gross production is equal to the
change in the O times the O concentration divided by the
difference between the O of the spiked water and the
O of the ambient O.
Procedure I : Net O Production Measurements Only
Eight bottles are filled sequentially from a Go-Flo bottle.
Bottles 1, 3, 5 and 7 are ``fixed'' for O analysis and stored in the
dark at room temperature. Bottles 2, 4, 6 and 8 are incubated for 24
hours and ``fixed'' for O analysis upon retrieval at the end of the
incubation. 4--12 hours after the incubation, samples are titrated in the
same order that bottles were filled. Titrations are done by the Winkler
method using a Radiometer automated titrator (TM-90). Analytical
precision is about ±0.1 %, giving a pooled standard error for net
production of about ±0.07 % of the ambient O concentration. In
practice, we have found analytical precision to be poorer because of
problems in drawing O samples from Go-Flo bottles.
Procedure II : Net and Gross O Production
Four 100 ml quartz bottles are filled sequentially from a
single Go-Flo bottle. To bottles 2 and 4, an amount of spike is added
that will cause the O of the O to rise by
at least 3/ by the end of the incubation. Bottles 2 and 4 are then
incubated at ambient temperature and irradiance. Dissolved gas is sampled
from bottles 1 and 3 immediately after collection using the method of
Emerson and Quay (personal communication). In this method, 50 ml of water
is admitted to a pre-evacuated bottle and equilibrated with the
~100 ml head space of this bottle. The water is then drained off the
gas sample stored for analysis. At the end of the incubation, dissolved
gas is sampled from bottles 2 and 4 in the identical manner.
In the laboratory, noncondensibles are frozen out using liquid nitrogen,
and the O-N-AR mixture is admitted to the mass spectrometer.
The O of O is measured, N of N
is measured, and the O /N /Ar ratios are measured with respect to
appropriate standards. Gross production is calculated from the change in
O of O, as described above. Net production is
calculated from the change in O /N (or O /Ar, which may
give a more precise measure, since these gases have more similar
solubility characteristics). Data on the N of N
serve as a test for the possiblity that gases are fractionated during
extraction; this variable is nearly conservative in seawater.
- Net productivity determined during a 24 hour period and gross
productivity determined during the photoperiod (this latter term is the
same as gross productivity after 24 hours) will in general be the most
informative measures. In principle, however, net and gross productivity
can be measured for any interval of time using the above method.
- In general, one will want to compare net and gross rates of O
production with other rate measurements (C, new and regenerated N
production,...). In order for such a comparison to be valid, one must
determine whether there are differential bottle effects. This is done by
doing C incubations, N incubations,... in the 100 ml
quartz bottles used for net and gross O production measurements, and
comparing rates determined in this way with rates measured in the bottles
normally used for those other measures of production.