Variation of Nitrate Concentrations and δ 15N Values of Seawater in the Drake Passage, Antarctic Ocean

Abstract

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, δ 15N 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 sinking particles within the surface layer. The preferential 14NO3- assimilation by the phytoplankton causes 15NO3 concentration to become high in a closedsystem surface-water environment during the primary production, whereas more 14NO3 is added to the seawater during the degradation of sinking organic particles. The water-mass mixing seems to play an importantrole in the alteration of δ 15N values in the deep layer below the UCDW. Across the polar front, nitrate concentrations of surface seawater decrease and corresponding δ 15N values increase northward, which is likelydue 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 δ 15NO3 initial value of UCDW is important in the calculation of ε values.