Calculations of 15N uptake rates by phytoplankton assimilating one or several nitrogen sources

A general equation is derived, which allows one to compute unbiased values of nitrogen uptake rates in the presence of a single nitrogen compound. In the case of simultaneous uptake of several nitrogen sources, current designs of 15N uptake measurements yield underestimates of specific uptake rates (in units of time-1) of the labeled compound. This is due to a characteristic of stable isotope techniques where, unlike radioactive techniques, a ratio of isotopic abundances is measured. A consequence of this is that reports of inhibition of nitrate or urea uptake by ammonium in studies using the 15N method may be calculation artefacts rather than physiological phenomena. (author)