Nitrate isotope dynamics in the lower euphotic-upper mesopelagic zones of the western South China Sea
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Abstract: The dual isotopes (N and O) of nitrate were measured using a denitrifier bacterial method in the western South China Sea (WSCS) during September 2015 to elucidate key information during N transformation in the lower euphotic zone (LEZ)-upper mesopelagic zone (UMZ, down to 500 m in this study) continuum, which is a vital sub-environment for marine N cycle and sequestration of atmospheric CO2 as well. The N isotopic composition (δ15N) of nitrate generally decreased from 500 m toward the base of the euphotic zone (~100 m), reaching a value of ~4.6‰ (vs. air N2) at the base of the LEZ, suggesting the imprint of remineralization (nitrification) of isotopically light N from atmospheric source. The δ15N and δ18O of nitrate only generally conform to a 1:1 line at 50 m and 75 m, suggesting that nitrate assimilation is a dominant process to shape nitrate isotope signature in this light-limited and relatively N-replete lower part of the euphotic zone. The fractionation factors of N and O isotopes during nitrate fractionation (15εASSIM, 18εASSIM) using a steady-state model were estimated to be 4.0‰±0.3‰ and 5.4‰±0.3‰, respectively. The occurrence of nitrification at the base of the LEZ and most of the UMZ is corroborated by the decoupling of δ15N and the oxygen isotopic composition (δ18O) of nitrate. Our results will provide insights for better understanding N cycle in the South China Sea from a perspective of present and past.
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Key words:
- N and O isotopes /
- nitrate assimilation /
- nitrification /
- western South China Sea
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Figure 1. Sampling locations in the western South China Sea. Arrows indicate mean sea surface geostrophic current during the period of the cruise (data source: Archiving, Validation and Interpretation of Satellite Oceanographic Data).
Figure 2. Temperature-salinity diagram (a); depth profiles of temperature (b), practical salinity (c), and density anomaly (d) in the upper 500 m of the western South China Sea (during sampling period). Major water masses include the near-shore surface water (SW), the surface water of the SCS (SCSSW), the subsurface water (SSW), and the intermediate water (IW), respectively. The grey line in a is isopycnal. σ0: potential density. The colorful lines and shapes of the dots in the figure represent different sampling stations.
Figure 3. Depth profiles of nitrate+nitrite concentration (a, b), δ15N (c, d) and δ18O (e, f) in the upper 500 m and the upper 150 m, specifically. UEZ: upper oligotrophic zone; LEZ: lower euphotic zone. The colorful lines and shapes of the dots in the figure represent different sampling stations.
Figure 4. Depth profile of Δ(15-18) in the upper 500 m. For those samples at 50 m and 75 m, values of δ15N and δ18O had been corrected for nitrite. See Section 4.1 for details. UEZ: upper oligotrophic zone; LEZ: lower euphotic zone; UMZ: upper portion of mesopelagic zone. The colorful lines and shapes of the dots in the figure represent different sampling stations.
Figure 5. Relationship of δ15N and δ18O of nitrate+nitrite and nitrate only in the LEZ. Nitrite correction is applied for samples at 50–75 m water column. The correlation for nitrate-only isotope ratios at 50 m and 75 m yields a linear relationship (δ18O=1.0×δ15N–1.4, R2=0.95, p<0.05). N2 in the air (x-axis) and the Vienna Standard Mean Ocean Water (y-axis) are chosen as the standards for N and O isotope analysis, respectively.
Figure 6. The N and O isotope fractionation during nitrate assimilation in Rayleigh (a) or steady-state (b) models. Data points of 50 m and 75 m are used for regression. The blue triangles represent the mean endmember value at 100 m. The standard deviation is 0.10‰ and 0.20‰ for δ15N and δ18O at 100 m, respectively. The dotted lines represent the confidence interval of 95%. The f means the ratio of the remaining reactant and initial reactant.
Figure 7. A brief compilation of 18εASSIM and 15εASSIM during nitrate assimilation by phytoplankton. Data are cited from the subtropical North Atlantic (Fawcett et al., 2015), the equatorial Pacific (Rafter and Sigman, 2016) and the Southern Ocean (Fripiat et al., 2019), and in-lab culture (Granger et al., 2004, 2010).
Figure 8. Schematic of N cycle as inferred from nitrate isotopes in the upper 500 m of the western South China Sea. Depth profiles of nitrate+nitrite concentration, δ15N, δ18O and Δ(15-18) are based on the mean values from this study. The vertical patterns of NH4+ uptake rate and nitrification rate based on isotope tracer assay are considered (Wan et al., 2018). N2 in the air and the Vienna Standard Mean Ocean Water are chosen as the standards for N and O isotope analysis, respectively. UEZ: upper oligotrophic zone; LEZ: lower euphotic zone; UMZ: upper portion of mesopelagic zone.
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Bai Peng, Yang Jingling, Zhang Shuwen, et al. 2019. Upwelling off the west coast of Hainan Island: sensitivity to wave-mixing. Acta Oceanologica Sinica, 38(11): 11–19. doi: 10.1007/s13131-019-1494-3 Braman R S, Hendrix S A. 1989. Nanogram nitrite and nitrate determination in environmental and biological materials by vanadium (III) reduction with chemi luminescence detection. Analytical Chemistry, 61(24): 2715–2718. doi: 10.1021/ac00199a007 Buchwald C, Casciotti K L. 2013. Isotopic ratios of nitrite as tracers of the sources and age of oceanic nitrite. Nature Geoscience, 6(4): 308–313. doi: 10.1038/ngeo1745 Cai Pinghe, Chen Weifang, Dai Minhan, et al. 2008. A high-resolution study of particle export in the southern South China Sea based on 234Th: 238U disequilibrium. Journal of Geophysical Research: Oceans, 113: C04019 Casciotti K L. 2016a. Nitrite isotopes as tracers of marine N cycle processes. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 374(2081): 20150295 Casciotti K L. 2016b. Nitrogen and oxygen isotopic studies of the marine nitrogen cycle. Annual Review of Marine Science, 8: 379–407. doi: 10.1146/annurev-marine-010213-135052 Casciotti K L, Buchwald C, Santoro A E, et al. 2011. Assessment of nitrogen and oxygen isotopic fractionation during nitrification and its expression in the marine environment. Methods in Enzymology, 486: 253–280 Casciotti K L, Sigman D M, Hastings M G, et al. 2002. Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry, 74(19): 4905–4912. doi: 10.1021/ac020113w Chen Yangjun, Bardhan P, Zhao Xiufeng, et al. 2021. Nitrite cycle indicated by dual isotopes in the northern South China Sea. Journal of Geophysical Research: Biogeosciences, 126(7): e2020JG006129 Chen Yuh-ling Lee, Chen Houng-Yung, Tuo Sing-how, et al. 2008. Seasonal dynamics of new production from Trichodesmium N2 fixation and nitrate uptake in the upstream Kuroshio and South China Sea basin. Limnology and Oceanography, 53(5): 1705–1721. doi: 10.4319/lo.2008.53.5.1705 Chen Fajin, Lao Qibin, Zhang Shuwen, et al. 2020. Nitrate sources and biogeochemical processes identified using nitrogen and oxygen isotopes on the eastern coast of Hainan Island. Continental Shelf Research, 207: 104209. doi: 10.1016/j.csr.2020.104209 Coale K H, Bruland K W. 1987. Oceanic stratified euphotic zone as elucidated by 234Th: 238U disequilibria. Limnology and Oceanography, 32(1): 189–200. doi: 10.4319/lo.1987.32.1.0189 Craig H, Gordon L. 1964. Deuterium and Oxygen 18 Variations in the Ocean and Marine Atmosphere. Pisa: Laboratorio di Geologia Nucleare, 277–374 Deman F, Fonseca-Batista D, Roukaerts A, et al. 2021. Nitrate supply routes and impact of internal cycling in the North Atlantic Ocean inferred from nitrate isotopic composition. Global Biogeochemical Cycles, 35(4): e2020GB006887 Dore J E, Karl D M. 1996. Nitrification in the euphotic zone as a source for nitrite, nitrate, and nitrous oxide at Station ALOHA. Limnology and Oceanography, 41(8): 1619–1628. doi: 10.4319/lo.1996.41.8.1619 Dore J E, Letelier R M, Church M J, et al. 2008. Summer phytoplankton blooms in the oligotrophic North Pacific Subtropical Gyre: Historical perspective and recent observations. Progress in Oceanography, 76(1): 2–38. doi: 10.1016/j.pocean.2007.10.002 Du Chuanjun, Liu Zhiyu, Kao Shuh-Ji, et al. 2017. Diapycnal fluxes of nutrients in an oligotrophic oceanic regime: The South China Sea. Geophysical Research Letters, 44(22): 11510–11518. doi: 10.1002/2017GL074921 Eppley R W, Peterson B J. 1979. Particulate organic matter flux and planktonic new production in the deep ocean. Nature, 282(5740): 677–680. doi: 10.1038/282677a0 Fawcett S E, Ward B B, Lomas M W, et al. 2015. Vertical decoupling of nitrate assimilation and nitrification in the Sargasso Sea. Deep-Sea Research Part I: Oceanographic Research Papers, 103: 64–72. doi: 10.1016/j.dsr.2015.05.004 Fripiat F, Martínez-García A, Fawcett S E, et al. 2019. The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications. Geochimica et Cosmochimica Acta, 247: 261–279. doi: 10.1016/j.gca.2018.12.003 Granger J, Sigman D M, Needoba J A, et al. 2004. Coupled nitrogen and oxygen isotope fractionation of nitrate during assimilation by cultures of marine phytoplankton. Limnology and Oceanography, 49(5): 1763–1773. doi: 10.4319/lo.2004.49.5.1763 Granger J, Sigman D M, Rohde M M, et al. 2010. N and O isotope effects during nitrate assimilation by unicellular prokaryotic and eukaryotic plankton cultures. Geochimica et Cosmochimica Acta, 74(3): 1030–1040. doi: 10.1016/j.gca.2009.10.044 Kao Shuh-Ji, Yang Jin-Yu Terence, Liu Kon-Kee, et al. 2012. Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea. Global Biogeochemical Cycles, 26(2): GB2033 Karl D M, Letelier R M, Bidigare R R, et al. 2021. Seasonal-to-decadal scale variability in primary production and particulate matter export at Station ALOHA. Progress in Oceanography, 195: 102563. doi: 10.1016/j.pocean.2021.102563 Karl D, Letelier R, Tupas L, et al. 1997. The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean. Nature, 388(6642): 533–538. doi: 10.1038/41474 Karsh K L, Granger J, Kritee K, et al. 2012. Eukaryotic assimilatory nitrate reductase fractionates N and O isotopes with a ratio near unity. Environmental Science & Technology, 46(11): 5727–5735 Karsh K L, Trull T W, Lourey M J, et al. 2003. Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron Release Experiment (SOIREE). Limnology and Oceanography, 48(3): 1058–1068. doi: 10.4319/lo.2003.48.3.1058 Kemeny P C, Weigand M A, Zhang R, et al. 2016. Enzyme-level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean. Global Biogeochemical Cycles, 30(7): 1069–1085. doi: 10.1002/2015GB005350 Lao Qibin, Chen Fajin, Liu Guoqiang, et al. 2019. Isotopic evidence for the shift of nitrate sources and active biological transformation on the western coast of Guangdong Province, South China. Marine Pollution Bulletin, 142: 603–612. doi: 10.1016/j.marpolbul.2019.04.026 Li Denghui, Zhou Meng, Zhang Zhaoru, et al. 2018. Intrusions of Kuroshio and shelf waters on northern slope of South China Sea in summer 2015. Journal of Ocean University of China, 17(3): 477–486. doi: 10.1007/s11802-018-3384-2 Liang Wenzhao, Tang Danling, Luo Xin. 2018. Phytoplankton size structure in the western South China Sea under the influence of a ‘jet-eddy system’. Journal of Marine Systems, 187: 82–95. doi: 10.1016/j.jmarsys.2018.07.001 Liu Kon-Kee, Atkinson L, Quiñones P R, et al. 2010. Carbon and Nutrient Fluxes in Continental Margins: A Global Synthesis. Berlin, Heidelberg: Springer, 423–493 Loick N, Dippner J, Doan H N, et al. 2007. Pelagic nitrogen dynamics in the Vietnamese upwelling area according to stable nitrogen and carbon isotope data. Deep-Sea Research Part I: Oceanographic Research Papers, 54(4): 596–607. doi: 10.1016/j.dsr.2006.12.009 Mariotti A, Germon J C, Hubert P, et al. 1981. Experimental determination of nitrogen kinetic isotope fractionation: Some principles; illustration for the denitrification and nitrification processes. Plant and Soil, 62(3): 413–430. doi: 10.1007/BF02374138 Montoya J P, Carpenter E J, Capone D G. 2002. Nitrogen fixation and nitrogen isotope abundances in zooplankton of the oligotrophic North Atlantic. Limnology and Oceanography, 47: 1617–1628 Moore C M, Mills M M, Arrigo K R, et al. 2013. Processes and patterns of oceanic nutrient limitation. Nature Geoscience, 6(9): 701–710. doi: 10.1038/ngeo1765 Needoba J A, Harrison P J. 2004. Influence of low light and a light: Dark cycle on ${{\rm {NO}}_3^-} $ uptake, intracellular${{\rm {NO}}_3^-} $ , and nitrogen isotope fractionation by marine phytoplankton. Journal of Phycology, 40: 505–516Peters B D, Lam P J, Casciotti K L. 2018. Nitrogen and oxygen isotope measurements of nitrate along the US GEOTRACES Eastern Pacific Zonal Transect (GP16) yield insights into nitrate supply, remineralization, and water mass transport. Marine Chemistry, 201: 137–150. doi: 10.1016/j.marchem.2017.09.009 Rafter P A, DiFiore P J, Sigman D M. 2013. Coupled nitrate nitrogen and oxygen isotopes and organic matter remineralization in the Southern and Pacific Oceans. Journal of Geophysical Research: Oceans, 118(10): 4781–4794. doi: 10.1002/jgrc.20316 Rafter P A, Sigman D M. 2016. Spatial distribution and temporal variation of nitrate nitrogen and oxygen isotopes in the upper equatorial Pacific Ocean. Limnology and Oceanography, 61(1): 14–31. doi: 10.1002/lno.10152 Ren Haojia, Chen Yichi, Wang Xingchen T, et al. 2017. 21st-century rise in anthropogenic nitrogen deposition on a remote coral reef. Science, 356(6339): 749–752. doi: 10.1126/science.aal3869 Shang Shaoling, Lee Z, Wei Guomei. 2011. Characterization of MODIS-derived euphotic zone depth: Results for the China Sea. Remote Sensing of Environment, 115(1): 180–186. doi: 10.1016/j.rse.2010.08.016 Sigman D M, Altabet M A, McCorkle D C, et al. 1999. The δ15N of nitrate in the Southern Ocean: Consumption of nitrate in surface waters. Global Biogeochemical Cycles, 13(4): 1149–1166. doi: 10.1029/1999GB900038 Sigman D M, DiFiore P J, Hain M P, et al. 2009. The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen. Deep-Sea Research Part I: Oceanographic Research Papers, 56(9): 1419–1439. doi: 10.1016/j.dsr.2009.04.007 Sigman D M, Fripiat F. 2019. Nitrogen isotopes in the ocean. In: Cochran J K, Bokuniewicz H J, Yager P L, eds. Encyclopedia of Ocean Sciences. 3rd ed. Oxford: Academic Press, 263–278 Sigman D M, Granger J, DiFiore P J, et al. 2005. Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin. Global Biogeochemical Cycles, 19(4): GB4022 Sigman D M, Hain M P, Haug G H. 2010. The polar ocean and glacial cycles in atmospheric CO2 concentration. Nature, 466(7302): 47–55. doi: 10.1038/nature09149 Stephens B M, Wankel S D, Beman J M, et al. 2020. Euphotic zone nitrification in the California Current Ecosystem. Limnology and Oceanography, 65(4): 790–806. doi: 10.1002/lno.11348 Voss M, Bombar D, Loick N, et al. 2006. Riverine influence on nitrogen fixation in the upwelling region off Vietnam, South China Sea. Geophysical Research Letters, 33(7): L07604 Wan Xianhui Sean, Sheng Huaxia, Dai Minhan, et al. 2018. Ambient nitrate switches the ammonium consumption pathway in the euphotic ocean. Nature Communications, 9: 915. doi: 10.1038/s41467-018-03363-0 Ward B B. 2005. Temporal variability in nitrification rates and related biogeochemical factors in Monterey Bay, California, USA. Marine Ecology Progress Series, 292: 97–109. doi: 10.3354/meps292097 Wong G T F, Tseng Chun-Mao, Wen Liang-Saw, et al. 2007. Nutrient dynamics and N-anomaly at the SEATS station. Deep-Sea Research Part II: Topical Studies in Oceanography, 54(14–15): 1528–1545 Wu Jingfeng, Chung Shi-Wei, Wen Liang-Saw, et al. 2003. Dissolved inorganic phosphorus, dissolved iron, and Trichodesmium in the oligotrophic South China Sea. Global Biogeochemical Cycles, 17(1): 1008 Wu Jinhui, Lao Qibin, Chen Fajin, et al. 2021. Water mass processes between the South China Sea and the western Pacific through the Luzon Strait: insights from hydrogen and oxygen Isotopes. Journal of Geophysical Research: Oceans, 126(8): e2021JC017484 Xiao Hongwei, Xiao Huayun, Luo Li, et al. 2018. Stable carbon and nitrogen isotope compositions of bulk aerosol samples over the South China Sea. Atmospheric Environment, 193: 1–10. doi: 10.1016/j.atmosenv.2018.09.006 Xu Chao. 2021. Nitrogen and oxygen isotopic compositions of nitrate in the South China Sea and the western North Pacific (in Chinese)[dissertation]. Xiamen: Xiamen University Yang Zhi, Chen Jianfang, Chen Min, et al. 2018. Sources and transformations of nitrogen in the South China Sea: insights from nitrogen isotopes. Journal of Oceanography, 74(1): 101–113. doi: 10.1007/s10872-017-0443-z Yang Jin-Yu Terence, Kao Shuh-Ji, Dai Minhan, et al. 2017. Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases. Journal of Geophysical Research: Biogeosciences, 122(8): 2118–2136. doi: 10.1002/2016JG003618 Yool A, Martin A P, Fernández C, et al. 2007. The significance of nitrification for oceanic new production. Nature, 447(7147): 999–1002. doi: 10.1038/nature05885 Zakem E J, Al-Haj A, Church M J, et al. 2018. Ecological control of nitrite in the upper ocean. Nature Communications, 9: 1206. doi: 10.1038/s41467-018-03553-w Zhang Run, Chen Min, Yang Qing, et al. 2015. Physical-biological coupling of N2 fixation in the northwestern South China Sea coastal upwelling during summer. Limnology and Oceanography, 60(4): 1411–1425. doi: 10.1002/lno.10111 Zhang Run, Wang Xingchen T, Ren Haojia, et al. 2020. Dissolved organic nitrogen cycling in the South China Sea from an isotopic perspective. Global Biogeochemical Cycles, 34(12): e2020GB006551 Zhu Yifan, Liu Jing, Mulholland M R. 2021. Dynamics of ammonium biogeochemistry in an oligotrophic regime in the South China Sea. Marine Chemistry, 237: 104040 -
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