A model for the vertical flux of nitrogen in the upper ocean: Simulating the alteration of isotopic ratios

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Abstract
An idealized, one-dimensional, constant diffusivity mathematical model for the study of the vertical flux of nitrogen in the upper-ocean is presented. We attempt to simulate observed patterns in vertical profiles for the natural abundance of 15N in particulate organic nitrogen (PON) and the concentrations of PON and NO3-. The concentration of phytoplankton nitrogen (II) increased as a result of either increasing the upward flux of NO3- (N) or by increasing the residence time of II. A minimum in the .delta.15N of phytoplankton nitrogen (.delta.2) appeared near a maximum in II at the inflection point of the N profile. Increasing the residence time or the vertical eddy diffusivity, reduced the amplitude of the .delta.2 profile. The model was able to produce reasonably good simulations of observed profiles from two warm-core rings, Rings 82-E and 82-H, using the most appropriate values for the light extinction coefficient and the residence time of PON. These results lend general support to current views regarding the nature and significance of the vertical fluxes of nitrogen in the upper-ocean and hypotheses presented previously concerning the factors which affect the .delta.15N of PON.