Variation of nitrate concentrations and δ¹⁵N values of Seawater in the Drake Passage, Antarctic Ocean

Seawater samples were collected at discrete depths from five stations across the polar front in the Drake Passage (Antarctic Ocean) by the 20th Korea Antarctic Research Program in December, 2006. Nitrate concentrations of seawater increase with depth within the photic zone above the depth of Upper Circumpolar Deep Water (UCDW). In contrast, δ¹⁵N values of seawater nitrate decrease with depth, showing a mirror image to the nitrate variation. Such a distinct vertical variation is mainly attributed to the degree of nitrate assimilation by phytoplankton as well as organic matter degradation of shiking particles within the surface layer. The preferential ¹⁴NO₃^-assimilation by the phytoplankton causes ¹⁵NO₃^-concentration to become high in a closed-system surface-water environment during the primary production, whereas more ¹⁴NO₃^- is added to the seawater during the degradation of shiking organic particles. The water-mass mixing seems to play an important role in the alteration of δ¹⁵N values in the deep layer below the UCDW. Across the polar front, nitrate concentrations of surface seawater decrease and corresponding δ¹⁵N values increase northward, which is likely due to the degree of nitrate utilization during the primary production. Based on the Rayleigh model, the calculated ε (isotope effect of nitrate uptake) values between 4.0‰ and 5.8‰ were validated by the previously reported data, although the preformed δ¹⁵ NO₃^-_initial value of UCDW is important in the calculation of ε values.