Quantification of nitracline depth in seawater

doi:10.3969/j.issn.1001-909X.2023.03.001

Abstract

Abstract:

Nitrate is the main nitrogen form available for phytoplankton life activities in the ocean, and its nitracline depth (Z_N) 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 Z_N. In this study, three different methods: difference method, gradient method and threshold method, are adopted to compute the corresponding Z_N 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 Z_N and the Z_N 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 Z_N and Z_N 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 Z_N calculated by the difference method and the observed Z_N is the smallest. According to the results of statistical error analysis, it is found that the Z_N calculated by the three methods based on BGC-Argo data show a good correlation with the observed Z_N. 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 Z_N calculated by the three methods is quite different from the observed Z_N. 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

CLC Number:

P734.44

MENG Yu, CHEN Shuangling. Quantification of nitracline depth in seawater[J]. Journal of Marine Sciences, 2023, 41(3): 1-13.

Figures/Tables 6

Fig.1 Spatial distribution of the sampling stations of in-situ data

Tab.1 Data statistics of BGC-Argo and CTD sampling sites

数据类型	时间范围	总站位数/个	预处理后站位数/个	数据利用率/%
CTD	2002-01-18—2019-08-02	3 526	383	10.86
BGC-Argo	2013-03-01—2017-02-24	427	159	37.24

Fig.2 Schematic diagrams of three nitracline depth calculation methods (cMLD represents the nitrate concentration of the MLD point of the profile, ci represents the nitrate concentration of the ith data point of the profile, ci-1 is the nitrate concentration of the (i-1)th data point, Δc1 is the difference between ci-1 and cMLD, Δc2 is the difference between ci and cMLD, d2c/d z i + 1 2 is the (i+1)th data point on the second derivative profile of nitrate concentration, σi and σi-1 represent the potential density of the ith and (i-1)th point, respective.)

Fig.3 The nitracline depth calculated by the three methods for a single profile

Fig.4 Comparison of the ZN calculated by the three methods with the observed ZN (N is the amount of data, R2 is the coefficient of determination, and RMSE is the root mean square difference.)

Tab.2 Comparison of the ZN calculated by the JOO model with the results of observed and other three methods calculated

统计参数	目视解译法	差值法	阈值法	梯度法
R²	0.82	0.78	0.71	0.68
RMSE/m	14.06	15.12	19.05	20.06

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