Protocol for Net and Gross O Production Measurements

Michael L. Bender

General

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 H

O 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 Pyrex mates.

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 Measurements

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.

Notes

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.