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Constraints on nitrogen cycling at the subtropical North Pacific Station ALOHA from isotopic measurements of nitrate and particulate nitrogen
Citation
Casciotti, KL and Trull, T and Glover, DM and Davies, DM, Constraints on nitrogen cycling at the subtropical North Pacific Station ALOHA from isotopic measurements of nitrate and particulate nitrogen, Deep-Sea Research Part II : Topical Studies in Oceanography, 55, (14-15) pp. 1661-1672. ISSN 0967-0645 (2008) [Refereed Article]
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DOI: doi:10.1016/j.dsr2.2008.04.017
Abstract
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 (d15NNO3 and
d18ONO3 ), as well as sinking and suspended particulate nitrogen (d15NPN). Paralleling the steep gradient
in nitrate concentration in the upper thermocline at ALOHA, we observed a steep gradient in d15NNO3 ,
decreasing from a maximum of +7.1% at 500 meters (m) to +1.5–2.4% at 150m. d18ONO3 values also
decreased from +3.0% at 300m to +0.7–0.9% at 150m. The decreases in both d15NNO3 and d18ONO3
require inputs of isotopically ‘‘light’’ nitrate to balance the upward flux of nitrate with high d15NNO3 (and
d18ONO3 ). We conclude that both nitrogen fixation and diagenetic alteration of the sinking flux
contribute to the decrease in d15NNO3 and d18ONO3 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
d15NPN between 150 and 300m, also suggests that fractionation during remineralization contributed to
the low d15NNO3 values observed in this depth range by introducing a subsurface nitrate source that is
0.5%lower in d15N 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 d15NNO3 measurements in a
time series program to allow a broader assessment of the variations in subsurface d15NNO3 values and
the links between subsurface nitrate and export flux at station ALOHA.
Item Details
Item Type: | Refereed Article |
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Keywords: | Nitrate isotopes, New production, Nitrogen fixation, Pacific Ocean |
Research Division: | Earth Sciences |
Research Group: | Oceanography |
Research Field: | Chemical oceanography |
Objective Division: | Environmental Management |
Objective Group: | Coastal and estuarine systems and management |
Objective Field: | Measurement and assessment of estuarine water quality |
UTAS Author: | Trull, T (Professor Thomas Trull) |
UTAS Author: | Davies, DM (Ms Diana Davies) |
ID Code: | 54595 |
Year Published: | 2008 |
Web of Science® Times Cited: | 112 |
Deposited By: | IASOS |
Deposited On: | 2009-02-25 |
Last Modified: | 2009-06-03 |
Downloads: | 0 |
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