Constraints on nitrogen cycling at the subtropical North Pacific Station ALOHA from isotopic measurements of nitrate and particulate nitrogen

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 (δ15NNO3δ15NNO3 and δ18ONO3δ18ONO3), as well as sinking and suspended particulate nitrogen (δ15NPN). Paralleling the steep gradient in nitrate concentration in the upper thermocline at ALOHA, we observed a steep gradient in δ15NNO3δ15NNO3, decreasing from a maximum of +7.1‰ at 500 meters (m) to +1.5–2.4‰ at 150 m. δ18ONO3δ18ONO3 values also decreased from +3.0‰ at 300 m to +0.7–0.9‰ at 150 m. The decreases in both δ15NNO3δ15NNO3 and δ18ONO3δ18ONO3 require inputs of isotopically “light” nitrate to balance the upward flux of nitrate with high δ15NNO3δ15NNO3 (and δ18ONO3δ18ONO3). We conclude that both nitrogen fixation and diagenetic alteration of the sinking flux contribute to the decrease in δ15NNO3δ15NNO3 and δ18ONO3δ18ONO3 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 N2 fixation during our study. On the other hand, this input of N2-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 δ15NPN between 150 and 300 m, also suggests that fractionation during remineralization contributed to the low δ15NNO3δ15NNO3 values observed in this depth range by introducing a subsurface nitrate source that is 0.5‰ lower in δ15N 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 δ15NNO3δ15NNO3 measurements in a time series program to allow a broader assessment of the variations in subsurface δ15NNO3δ15NNO3 values and the links between subsurface nitrate and export flux at station ALOHA.