Quantification of nitracline depth in seawater

MENG Yu, CHEN Shuangling

Journal of Marine Sciences ›› 2023, Vol. 41 ›› Issue (3) : 1-13.

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Journal of Marine Sciences ›› 2023, Vol. 41 ›› Issue (3) : 1-13. DOI: 10.3969/j.issn.1001-909X.2023.03.001

Quantification of nitracline depth in seawater

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Abstract

Nitrate is the main nitrogen form available for phytoplankton life activities in the ocean, and its nitracline depth (ZN) directly affects the vertical transport of nitrate and the ocean primary productivity, and then further influences the carbon cycle. With the advancement of ocean observation technologies, the profile data of nitrate have been collected in diversified ways, such as ship-based CTD observations and BGC-Argo automatic observations. The vertical sampling resolution of these techniques varies significantly (the vertical sampling resolution of CTD is lower than that of BGC-Argo). In view of different sampling data, it is urgent to conduct systematic and quantitative comparative analysis and study on the computing methods of ZN. In this study, three different methods: difference method, gradient method and threshold method, are adopted to compute the corresponding ZN by using the historical ship-based CTD data and BGC-Argo buoy data in the Northwest Pacific Ocean. The results show that in the case of single nitrate profile, based on BGC-Argo data, the difference between observed ZN and the ZN calculated by difference method is only 0.2 m, followed by threshold method is 20.0 m and gradient method is 202.8 m at most. Based on CTD data, the difference between observed ZN and ZN calculated by difference method is 2.0 m, the threshold method is 49.0 m, and the gradient method is 155.0 m. Compared with the gradient method and threshold method, the difference between the ZN calculated by the difference method and the observed ZN is the smallest. According to the results of statistical error analysis, it is found that the ZN calculated by the three methods based on BGC-Argo data show a good correlation with the observed ZN. Among them, the error of difference method is the smallest (R2=0.77, RMSE=28.48 m). The R2 and RMSE of threshold method are 0.64 and 34.85 m, and the R2 and RMSE of gradient method are 0.52 and 53.80 m. For CTD data, due to its low vertical sampling resolution, the ZN calculated by the three methods is quite different from the observed ZN. However, compared with the gradient method and threshold method, the error of the difference method is still the smallest (R2=0.81, RMSE =16.13 m). The R2 and RMSE of threshold method are 0.47 and 27.65 m, and the R2 and RMSE of gradient method are 0.42 and 36.41 m. The applicability of each method is preliminarily explored through comparing and analyzing the characteristics and differences of them so as to provide some scientific reference for the in-depth research on the vertical distribution characteristics and upward transport process of nitrate.

Key words

nitracline depth / gradient method / difference method / threshold method / Northwest Pacific / BGC-Argo / CTD

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MENG Yu , CHEN Shuangling. Quantification of nitracline depth in seawater[J]. Journal of Marine Sciences. 2023, 41(3): 1-13 https://doi.org/10.3969/j.issn.1001-909X.2023.03.001

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感谢美国国家环境信息中心(NCEI, https://www.nodc.noaa.gov/access/index.html)和BGC-Argo数据网站中心(https://biogeochemical-argo.org/)对本文有关CTD观测数据和BGC-Argo观测数据的支持,同时特别感谢两位匿名审稿人提出的宝贵审稿意见,帮助我们完善了实验设计并提升了论文质量。

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