Natural abundance of 15N in particulate nitrogen and zooplankton in the Gulf Stream region and warm-core ring 86A

Abstract

The natural abundance of 15N (δ 15N) was measured in samples of suspended particles (PN) and macrozooplankton collected in the Slope Water and warm-core ring 86A in late May 1986. The vertical distribution of 15N in PN (δ 15PN) was characterized by a maximum below the mixed layer, and frequently by a minimum above the thermocline. The range of δ 15N values in PN, as well as the mean δ 15N for PN in the upper 100 m of the water column were comparable to literature values of these quantities. The δ 15N and concentration of PN were inversely correlated, a pattern that could be generated by isotopie fractionation during the remineralization of sinking PN. The fractionation factor estimated for this process (1.003–1.006) was similar to fractionation factors for the bacterially mediated decomposition of phytoplankton in laboratory studies. The δ 15N of the zooplankton sampled showed a general increase with trophic level: herbivorous species ( Salpa aspera and copepods) were characterized by δ 15NN values of ca 4‰, omnivorous species spanned a range of δ 15N values between 5.5. and 6.5‰, and carnivorous species had a δ 15N of 8–9‰. Despite this trophic level effect on δ 15N among the zooplankton, the range of δ 15N values measured for the animals was not distinctly different from the range of values of δ 15PN. The absence of a difference in δ 15N between the PN and the zooplankton in this ecosystem may be a consequence of the normal variation in δ 15N through the seasonal production cycle and the different turnover times for nitrogen in PN and the higher trophic levels of the food web. Representatives of five zooplankton species were maintained in filtered sea water for up to 74 h. Three of the species ( Euphausia krohnii, Paralhemisto gaudichaudii and Vibilia sp.) showed no significant changes in the δ 15N of body nitrogen with time. The data for the other two species ( Meganyctiphanes norvegica and Euphausia mulica) suggest that the δ 15N of body nitrogen increased with time of starvation. The fecal pellets released by zooplankton used in the starvation experiment were typically similar to or higher in δ 15N than the animals proper. In contrast, the (δ 15N of the nitrogen in molted exoskeletons was much lower than the whole-body δ 15N of the animals which produced them. The possible implications of these data are discussed in the context of the distribution of 15N in plankton in the upper ocean.