Comparison between chemical and isotopic measurements of biological nitrate utilization: further evidence of low new-production levels in the equatorial Pacific

New-production (nitrate uptake) rates in the equatorial Pacific were estima ted by parallel measurements of nitrate disappearance from sea wafer using a colorimetric method and of N-15-labelled nitrate ((NO3-)-N-15) incorporat ion into particulate organic nitrogen (PON) collected on GF/F filters (net nitrate uptake, conventional N-15-tracer method) and Anopore (0.2 mu m) mem branes. Regression analyses of 74 sample pairs gathered during 12 and 24 h productivity experiments revealed a significant positive relationship betwe en decreasing nitrate level and (NO3-)-N-15 accumulation into PON retained on CF/F filters, but the slopes of Model I and Model II regression lines we re 1.18 and 1.29, respectively, suggesting that 15 to 22% of (NO3-)-N-15; r emoved from the dissolved fraction were lost to another N-pool. Two possibl e avenues for the missing (NO3-)-N-15 have been examined: uptake by submicr on particles passed through the GF/F filters, and loss as dissolved organic nitrogen (DON). Nitrate uptake by small cells not recovered on GF/F filter s, could be safely eliminated as a cause of loss, since (NO3-)-N-15 uptake rates obtained from N-15 entering PON collected on GF/F filters agreed well with those obtained from N-15 entering PON collected on Anopore membranes (32 sample pairs). Inspection of the DON Fool of 0.2 mu m filtrates for sxc ess-N-15 enrichment (20 samples) revealed that in nitrate-rich waters (equa torial upwelling between 1 degrees N and 10 degrees S), loss of (NO3-)-N-15 as (DON)-N-15 accounted for < 5% of net nitrate uptake. In samples from su btropical oligotrophic waters (from 11 degrees S southward), however,(NO3)- N-15 loss as (DON)-N-15 represented up to 20% of net NO3- uptake. These res ults, as well as experimental considerations concerning the use of colorime tric and isotopic methods to measure new production show that: (1) earlier reported high discrepancies between nitrate decreases (Delta NO3-) and (NO3 -)-N-15 incorporation into filterable particles (Delta BNO3-/(NO3-)-N-15 in corporation, 2) were probably erroneous; (2) the use of GF/F filters does n ot result in an underestimation of new production, although it was found to underestimate PON concentrations by up to 60%; (3) in the equatorial upwel ling area 1 degrees N to 10 degrees S), which has high ambient nitrate leve ls (> 2000 nmol l(-1)) but only slight changes in concentration (0 to 80 nm ol l(-1) d(-1)), new production is more accurately estimated by the isotopi c method than by the chemical method; (4) in subtropical oligotrophic water s (from 11 degrees S southward) with low ambient nitrate levels (0 to 100 n mol l(-1)), both procedures are appropriate as long as nitrate removal per incubation period is >3 nmol 1(-1) (lower rates are only detectable with th e isotopic method); (5) the traditional N-15-tracer technique does not subs tantially underestimate net new-production in the equatorial Pacific, and f ailure to account for the loss of (NO3-)-N-15 as DON, i.e. to estimate gros s nitrate uptake (gross uptake = net uptake + N-15 loss) tends to underesti mate 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 N-15- tracer te chnique.