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Revisiting N2 fixation in the North Atlantic Ocean: Significance of deviations from the Redfield Ratio, atmospheric deposition and climate variability

Revisiting N2 fixation in the North Atlantic Ocean: Significance of deviations from the Redfield Ratio, atmospheric deposition and climate variability
Revisiting N2 fixation in the North Atlantic Ocean: Significance of deviations from the Redfield Ratio, atmospheric deposition and climate variability
The average oceanic nitrate-to-phosphate molar ratio (NO3?:PO43??16:1, referred to as the Redfield Ratio) in subsurface waters, which is similar to the average ratio of particulate nitrogen (N)-to-phosphorus (P) in phytoplankton, is the cornerstone in calculating geochemical estimates of N2 fixation and denitrification rates. Any deviations from this canonical Redfield Ratio in intermediate ocean waters, expressed as N* (a measure of NO3? in excess or deficit of 16×PO43?), provides an integrated estimate of net N fluxes into and out of the ocean. In well-oxygenated ocean basins such as the North Atlantic Ocean, N* estimates are usually positive and can be used to infer that rates of N2 fixation exceed rates of denitrification. We use this approach to estimate N2 fixation over the last two decades (1988–2009) based on data collected at the Bermuda Atlantic Time-series Study (BATS) site in the North Atlantic Ocean near Bermuda. Our results indicate that interpretation of the N* tracer as an estimate of N2 fixation should be undertaken with caution, as N2 fixation is not the only process that results in a positive N* estimate. The impacts of a locally variable nitrogen-to-phosphorus ratio, relative to the fixed Redfield Ratio, in the suspended particulate matter as well as in the subsurface water nutrients and atmospheric N deposition on N* variability were examined. Furthermore, we explored the role of climate modes (i.e., North Atlantic Oscillation and Arctic Oscillation) on N* variability. We found that N* in the subsurface waters was significantly affected by these factors and hence previous estimates of N2 fixation using this technique might have been substantially overestimated. Our revised estimate of N2 fixation in the North Atlantic Ocean (0°N–50°N, 20°W–80°W) is 12.2±0.9×1011 mol N yr?1, and based on long-term BATS data provides better constraints than both earlier indirect and direct estimates N2 fixation.
Nitrogen, Phosphorus, Redfield Ratio, N*, Geochemical estimates, N2 fixation, North Atlantic Ocean
0967-0645
148-158
Singh, Arvind
227b0d30-fca6-4fe0-ab38-8c6f2e290006
Lomas, M.W.
84d523c2-4876-41d4-a160-f32c69b7c7c8
Bates, N.R.
954a83d6-8424-49e9-8acd-e606221c9c57
Singh, Arvind
227b0d30-fca6-4fe0-ab38-8c6f2e290006
Lomas, M.W.
84d523c2-4876-41d4-a160-f32c69b7c7c8
Bates, N.R.
954a83d6-8424-49e9-8acd-e606221c9c57

Singh, Arvind, Lomas, M.W. and Bates, N.R. (2013) Revisiting N2 fixation in the North Atlantic Ocean: Significance of deviations from the Redfield Ratio, atmospheric deposition and climate variability. Deep Sea Research Part II: Topical Studies in Oceanography, 93, 148-158. (doi:10.1016/j.dsr2.2013.04.008).

Record type: Article

Abstract

The average oceanic nitrate-to-phosphate molar ratio (NO3?:PO43??16:1, referred to as the Redfield Ratio) in subsurface waters, which is similar to the average ratio of particulate nitrogen (N)-to-phosphorus (P) in phytoplankton, is the cornerstone in calculating geochemical estimates of N2 fixation and denitrification rates. Any deviations from this canonical Redfield Ratio in intermediate ocean waters, expressed as N* (a measure of NO3? in excess or deficit of 16×PO43?), provides an integrated estimate of net N fluxes into and out of the ocean. In well-oxygenated ocean basins such as the North Atlantic Ocean, N* estimates are usually positive and can be used to infer that rates of N2 fixation exceed rates of denitrification. We use this approach to estimate N2 fixation over the last two decades (1988–2009) based on data collected at the Bermuda Atlantic Time-series Study (BATS) site in the North Atlantic Ocean near Bermuda. Our results indicate that interpretation of the N* tracer as an estimate of N2 fixation should be undertaken with caution, as N2 fixation is not the only process that results in a positive N* estimate. The impacts of a locally variable nitrogen-to-phosphorus ratio, relative to the fixed Redfield Ratio, in the suspended particulate matter as well as in the subsurface water nutrients and atmospheric N deposition on N* variability were examined. Furthermore, we explored the role of climate modes (i.e., North Atlantic Oscillation and Arctic Oscillation) on N* variability. We found that N* in the subsurface waters was significantly affected by these factors and hence previous estimates of N2 fixation using this technique might have been substantially overestimated. Our revised estimate of N2 fixation in the North Atlantic Ocean (0°N–50°N, 20°W–80°W) is 12.2±0.9×1011 mol N yr?1, and based on long-term BATS data provides better constraints than both earlier indirect and direct estimates N2 fixation.

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More information

Published date: September 2013
Keywords: Nitrogen, Phosphorus, Redfield Ratio, N*, Geochemical estimates, N2 fixation, North Atlantic Ocean
Organisations: Ocean Biochemistry & Ecosystems

Identifiers

Local EPrints ID: 357252
URI: http://eprints.soton.ac.uk/id/eprint/357252
ISSN: 0967-0645
PURE UUID: 11dfaa82-c60b-4635-83fc-18d29c94f58e

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Date deposited: 23 Sep 2013 15:34
Last modified: 14 Mar 2024 14:56

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Contributors

Author: Arvind Singh
Author: M.W. Lomas
Author: N.R. Bates

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