The use of 15 N to measure nitrogen uptake in eutrophic oceans ; experimental considerations 1 t 2

The use of 15N 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 15N 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 15N 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 15N into the particulate fraction is usually linear with time over the periods currently used. The 15N method provides a better estimate of incorporation into phytoplankton than 14N 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 lSN techniques currently available, reliable estimates of phytoplankton nitrogen flux in eutrophic areas can be obtained. Although the 15N technique was introduced into studies of the nitrogen cycle in the sea 25 years ago (Dugdale et al. 196 l), its use has been confined to a relatively small number of laboratories, primarily to make estimates of phytoplankton uptake of inorganic nitrogen. Usually mass spectrometers have been used to obtain 15N : 14N isotope ratios. Improved instrumentation l This study was supported by National Science Foundation grants OCE 77-27006 for the CUEA studies and OCE 82-l 5221 and OCE 85-05400 for the OPUS program. 2 Contribution 4 14 of the Allan Hancock Foundation. (Harrison 1983a; Lipschultz et al. 1985), along with a surge of interest in the oligotrophic ocean and the need to understand the relative proportions of new and regenerated production (Dugdale and Goering 1967; Eppley and Peterson 1979) have resulted in increasing use of the 15N technique in oceanographic studies. With the proliferation of laboratories using 15N in marine investigations, some disagreement has arisen over both 15N measurement techniques and the appropriate equations for calculating uptake of dissolved inorganic nitrogen (DIN). The major problems that have arisen concern the disappearance of l 5N during incubations (Laws