The use of ¹⁵N to measure nitrogen uptake in eutrophic oceans; experimental considerations1,2

The use of 15 N to measure the flux of nitrogen compounds has become increasingly popular as the techniques and instrumentation for stable isotope analysis have become more widely available. Questions concerning equations for calculating uptake, effect of isotope dilution (in the case of ammonium), duration of incubation, and relationship between disappearance of a nitrogen compound and the 15 N uptake measurement have arisen, especially for the research conducted in oligotrophic regions. Fewer problems seem to have occurred in eutrophic areas. However, sufficient literature now exists to allow some generally accepted experimental procedures for 15 N studies in eutrophic regions to be laid down. Incubation periods of 2–6 h appear to avoid problems related to isotope dilution and to overcome the bias introduced in some cases by initial high rate or surge uptake. During such incubation periods, assimilation is measured rather than uptake or transport into the cell. Incorporation of 15 N into the particulate fraction is usually linear with time over the periods currently used. The 15 N method provides a better estimate of incorporation into phytoplankton than 14 N disappearance, but a small fraction appears to be lost. Although most workers suggest the loss to be a result of dissolved organic nitrogen production, direct evidence is lacking. If the considerations discussed here are applied with the 15 N techniques currently available, reliable estimates of phytoplankton nitrogen flux in eutrophic areas can be obtained.