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Low rates of nitrogen fixation in eastern tropical South Pacific surface waters.

Title: Low rates of nitrogen fixation in eastern tropical South Pacific surface waters.
Name(s): Knapp, Angela N, author
Casciotti, Karen L, author
Berelson, William M, author
Prokopenko, Maria G, author
Capone, Douglas G, author
Type of Resource: text
Genre: Journal Article
Date Issued: 2016-04-19
Physical Form: computer
online resource
Extent: 1 online resource
Language(s): English
Abstract/Description: An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N2fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N2fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N2fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 µmol N m(-2)d(-1)based on sediment trap fluxes. Whereas the fractional importance of N2fixation did not change, the N2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N2fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N2-fixation rates were higher, up to 100 µmol N m(-2)d(-1), and detected N2fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N2fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N2fixation in the ETSP.
Identifier: FSU_pmch_26976587 (IID), 10.1073/pnas.1515641113 (DOI), PMC4843426 (PMCID), 26976587 (RID), 26976587 (EID), 1515641113 (PII)
Keywords: Eastern tropical south Pacific, Nitrate, Nitrogen budgets, Nitrogen fixation, Nitrogen isotopes
Publication Note: This NIH-funded author manuscript originally appeared in PubMed Central at
Subject(s): Nitrogen Fixation
Pacific Ocean
Tropical Climate
Persistent Link to This Record:
Host Institution: FSU
Is Part Of: Proceedings of the National Academy of Sciences of the United States of America.
Issue: iss. 16, vol. 113

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Knapp, A. N., Casciotti, K. L., Berelson, W. M., Prokopenko, M. G., & Capone, D. G. (2016). Low rates of nitrogen fixation in eastern tropical South Pacific surface waters. Proceedings Of The National Academy Of Sciences Of The United States Of America. Retrieved from