Variations in the Ratio of Carbon and Nitrogen Stable Isotopes of Particulate Organic Matter in the Southern Sea of Japan

Abstract

Carbon and nitrogen dynamics in the Sea of Japan (SOJ) are rapidly changing. Here, we investigated the carbon and nitrogen isotope ratios of particulate organic matter (δ 13 C POM and δ 15 N POM , respectively) at a depth of [?] 100 m in the southern part of the SOJ from 2016 to 2021. Both δ 13 C POM and δ 15 N POM showed multimodal distributions and were divided into four classes (I-IV) using the Gaussian mixed model. Most of the samples were categorized into class II (n = 441), with the mean ± standard deviation (SD) of δ 13 C POM and δ 15 N POM being -23.7 ± 1.2 1.2 δ 15 N POM (-2.1 ± 0.8 class III by low δ 13 C POM (-27.1 ± 1.0 δ 13 C POM (-20.7 ± 0.8 34). Using the generalized linear model (GLM), temperature and chlorophyll-a concentration were found to positively impact δ 13 C POM in addition to sampling depth, latitude, salinity, and nitrate concentration, supporting the hypothesis that active photosynthesis and phytoplankton growth elevated δ 13 C POM . The mean δ 15 N POM value suggests that primary production mainly originates from nitrate in the SOJ. The δ 15 N POM variation, however, was explained by temperature and salinity but not by nitrate concentration in the GLM approach, which suggests that the Rayleigh distillation model was not observed in the SOJ. This suggests that primary production is supported by various nitrogenous sources from the Changjiang, Kuroshio, and atmospheric depositions in the SOJ.

Multiple nitrogen sources for primary production inferred from δ13C and δ15N in the southern Sea of Japan

“…Two recent studies have indicated that the carbon and nitrogen stable isotope ratio of POM varies in the western North Pacific marginal seas (Ho et al, 2021;Kodama et al, 2021). The δ 13 C of POM increased as the phytoplankton abundance increased in the southwestern ECS, which is consistent with isotope fractionation that occurs during photosynthesis (Ho et al, 2021).…”

“…Gao et al (2014) reported the horizontal distribution of δ 13 C and δ 15 N and that δ 13 C largely varies in this sea. Active nitrogen fixation decreases the δ 15 N of POM in offshore waters in the Kuroshio area during the summer, whereas the δ 15 N of POM remains high in the coastal waters (Kodama et al, 2021).…”

“…After the shipboard filtration, the samples were frozen (< −20 • C). Onshore laboratory analyses were performed using methods from Kodama et al (2021), whose preservation differed from that of Lorrain et al (2003), while the other protocols remained the same. The masses of 12 C, 13 C, 14 N, and 15 N were determined using the same sample.…”

“…Generalized linear models (GLMs) were applied to assess the variables that predicted the relationships between the environmental parameters and δ 13 C POM and δ 15 N POM as in Kodama et al (2021). We assumed that the error distributions of δ 13 C POM and δ15N POM were normally distributed with a linear link function in the GLMs.…”

Geographic, seasonal and ontogenetic variations of δ15N and δ13C of Japanese sardine explained by baseline variations and diverse fish movements

“…From literature that exhibited data only in its plots, data were extracted from the plots using WebPlotDigitizer (https://automeris.io/WebPlotDigitizer/). The literature data were mainly distributed in the Pacific coasts, the Kuroshio inshore, the Seto Inland Sea and the East China Sea (Minagawa et al, 2001;Takai et al, 2002;Toyokawa et al, 2003;Wu et al, 2003;Yokoyama and Ishihi, 2003;Kasai et al, 2004;Chen et al, 2006;Hoshika et al, 2006;Yamaguchi et al, 2006;Fukumori et al, 2008;Miller et al, 2010;Sano et al, 2013;Chang et al, 2014;Mino et al, 2016;Mei, 2018;Ho et al, 2021;2022b;Nakamura et al, 2022). Unpublished isotope data from POM, collected during research surveys conducted by the University of Tokyo and the Japan Fisheries Research and Education Institute in the western North Pacific and its marginal seas during 2013-2021, were added to the data set (Supplementary Table 1).…”