Abstract
Colorimetric analyses of nitrate disappearance from seawater have been compared with isotopic analyses of 15N-labelled nitrate incorporation into particulate matter. The slope (1.41) of a regression line calculated from 19 sample pairs gathered during 6 time-series experiments and 2 single end-point incubations showed that nitrate incorporation is positively related to changes in nitrate concentration but that it accounts only for 71% of nitrate disappearance. 15N-isotope dilution as a consequence of nitrification, if any, would not fully explain discrepancies between the two analytical procedures. A further possible mechanism responsible for the imbalance between nitrate-incorporation and -disappearance rates suggests losses of 15N label from plankton biomass to an unanalyzed pool (dissolved organic nitrogen?) which increase (up to 65%) with incubation time. The lack of 15N-mass balance calls for the need to consider additional nitrogen pools in 15N budgets of isotope experiments and not only substrate and biomass pools as has been done so far.
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