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.
Published Version
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