New-production (nitrate uptake) rates in the equatorial Pacific were estimated by parallel measurements of nitrate disappearance
from sea water using a colorimetric method and of
15N-labelled nitrate (
15NO
3
−) incorporation into particulate organic nitrogen (PON) collected on GF/F filters (net nitrate uptake, conventional
15N-tracer method) and Anopore (0.2 μm) membranes. Regression analyses of 74 sample pairs gathered during 12 and 24 h productivity
experiments revealed a significant positive relationship between decreasing nitrate level and
15NO
3
− accumulation into PON retained on GF/F filters, but the slopes of Model I and Model II regression lines were 1.18 and 1.29,
respectively, suggesting that 15 to 22% of
15NO
3
− removed from the dissolved fraction were lost to another N-pool. Two possible avenues for the missing
15NO
3
− have been examined: uptake by submicron particles passed through the GF/F filters, and loss as dissolved organic nitrogen
(DON). Nitrate uptake by small cells not recovered on GF/F filters, could be safely eliminated as a cause of loss, since
15NO
3
− uptake rates obtained from
15N entering PON collected on GF/F filters agreed well with those obtained from
15N entering PON collected on Anopore membranes (32 sample pairs). Inspection of the DON pool of 0.2 μm filtrates for excess-
15N enrichment (20 samples) revealed that in nitrate-rich waters (equatorial upwelling between 1°N and 10°S), loss of
15NO
3
− as DO
15N accounted for <5% of net nitrate uptake. In samples from subtropical oligotrophic waters (from 11°S southward), however,
15NO
3 loss as DO
15N represented up to 20% of net NO
3
− uptake. These results, as well as experimental considerations concerning the use of colorimetric and isotopic methods to
measure new production show that: (1) earlier reported high discrepancies between nitrate decreases (ΔNO
3
−) and
15NO
3
− incorporation into filterable particles (ΔNO
3
−/
15NO
3
− incorporation >2) were probably erroneous; (2) the use of GF/F filters does not result in an underestimation of new production,
although it was found to underestimate PON concentrations by up to 60%; (3) in the equatorial upwelling area (1°N to 10°S),
which has high ambient nitrate levels (>2000 nmol l
−1) but only slight changes in concentration (0 to 80 nmol l
−1 d
−1), new production is more accurately estimated by the isotopic method than by the chemical method; (4) in subtropical oligotrophic
waters (from 11°S southward) with low ambient nitrate levels (0 to 100 nmol l
−1), both procedures are appropriate as long as nitrate removal per incubation period is >3 nmol l
−1 (lower rates are only detectable with the isotopic method); (5) the traditional
15N-tracer technique does not substantially underestimate net new-production in the equatorial Pacific, and failure to account
for the loss of
15NO
3
− as DON, i.e. to estimate gross nitrate uptake (gross uptake = net uptake +
15N loss) tends to underestimate new production on an average by only 10%. Overall, the apparent low level of new production
in the nitrate-rich area of the central equatorial Pacific seems to be a fact, and may be ascribable to other nutrient (macro
and micro) deficiencies and/or to intense in situ recycling of ammonium and nitrate (regenerated production) rather than to
inaccurate nitrate uptake rates measured with the classical
15N-tracer technique.
Received: 24 November 1998 / Accepted 10 March 2000
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