The natural abundance of 15N in PON from the upper 200 m of 4 warm-core rings and the Sargasso Sea was measured. Minima in the δ15N of PON often occurred near the depth at which NO3 was first detectable. Frequently, maxima in PON concentration and minima in C/N ratio also co-occurred in this region. The average value for the δ15N of PON below the top of the nitracline was almost always greater than that above the top of the nitracline. The observed vertical pattern for the δ15N of PON is most likely the result of isotopic fractionation in the processes of NO3 uptake by phytoplankton at the base of the euphotic zone and the degradation of PON below the euphotic zone. Variations in the strength and coherence of this vertical pattern appear to occur in response to both rapid physical modification of the water column and the trophic status of the euphotic zone.