Constraints on nitrogen cycling at the subtropical North Pacific Station ALOHA from isotopic measurements of nitrate and particulate nitrogen
Additional Document Info
Nitrogen supply to surface waters can play an important role in the productivity and ecology of subtropical ecosystems. As part of the Vertical Transport in the Global Ocean (VERTIGO) program, we examined the fluxes of nitrogen into and out of the euphotic zone at station ALOHA in the North Pacific Subtropical Gyre using natural abundance stable isotopic measurements of nitrate (ONNO, and delta(18) O(NO3), as well as sinking and suspended particulate nitrogen (delta(15)N(PN))- Paralleling the steep gradient in nitrate concentration in the upper thermocline at ALOHA, we observed a steep gradient in delta(15)N(NO3), decreasing from a maximum of +7.1 parts per thousand at 500 meters (m) to +1.5-2.4 parts per thousand at 150m. delta(18)O(NO3) values also decreased from + 3.01 parts per thousand at 300m to +0.7-0.9 parts per thousand at 150m. The decreases in both delta(15)N(NO3), and delta(18)O(NO3) require inputs of isotopically “light” nitrate to balance the upward flux of nitrate with high delta(15)N(NO3) (and 618 ONO,). We conclude that both nitrogen fixation and diagenetic alteration of the sinking flux contribute to the decrease in delta(15)N(NO3) and delta(18)O(NO3) in the upper thermocline at station ALOHA. While nitrogen fixation is required to explain the nitrogen isotope patterns, the rates of nitrogen fixation may be lower than previously estimated. By including high-resolution nitrate isotope measurements in the nitrogen isotope budget for the euphotic zone at ALOHA, we estimate that approximately 25%. rather than 50%, of export production was fueled by N(2) fixation during our study. On the other hand, this input of N(2)-derived production accumulates in the upper thermocline over time, playing a significant role in subtropical nutrient cycling through maintenance of the subsurface nitrate pool. An increase in sinking delta(15)N(PN) between 150 and 300 m, also suggests that fractionation during remineralization contributed to the low delta(15)N(NO3) values observed in this depth range by introducing a subsurface nitrate source that is 0.5 parts per thousand. lower in delta(15)N than the particle flux exported from the euphotic zone. While the time scale of these observations are currently limited, they highlight the need for inclusion of delta 15N(NO3) measurements in a time series program to allow a broader assessment of the variations in subsurface delta(15)N(NO3) values and the links between subsurface nitrate and export flux at station ALOHA. (C) 2008 Elsevier Ltd. All rights reserved.