Estimation of N or C uptake rates by phytoplankton using 15N or 13C: revisiting the usual computation formulae

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Abstract
The uptake of N by phytoplankton is generally estimated using the 15N technique and, under some circumstances, the uptake of C is estimated using 13C. Rigorous examination of formulae for computing net transport rates leads to several interesting and even unexpected conclusions. These are that the 15N or 13C technique formula for computing net transport rates (ρ) is identical to that of the 14C technique, in spite of apparent dissimilarities which reflect differences in equipment used for determining non-radioactive and radioactive isotopes; the so-called specific uptake rates (V) should not be used with natural samples, except as a step in the calculation of transport rates (ρ); estimation of ρ is unaffected by the presence/absence of non-phytoplanktonic paniculate organic matter (POM) in the incubated sample; the practice of adding the concentration of tracer to the denominator of expression representing the concentration of tracer in the dissolved phase at the beginning of incubation should be discontinued; and the concentration of POM should be determined from the inoculated sample at the end of incubation (or, alternatively, from a sample incubated in parallel) and not from a water sample taken at the beginning of the incubation.

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