Carbon and nitrogen dynamics in the coastal Japan Sea inferred from 15 years of measurements of stable isotope ratios of Calanus sinicus

Abstract

Abstract. Human activities have caused sometimes dramatic changes to the marine environment globally and locally during the last half century. We hypothesized that the carbon and nitrogen stable isotope ratios (δ13C and δ15N) of the copepod Calanus sinicus, one of the dominant secondary producers of North Pacific coastal waters, would record anthropogenic impacts on the coastal environment of the Japan Sea. We monitored these isotope ratios during the spring at four stations in the Japan Sea from 2006 to 2020. The δ13C values ranged from −24.7 ‰ to −15.0 ‰ and decreased from the spring bloom (February/March) to the post-bloom (June/July). This monthly variation was attributed to changes in both the physiology of C. sinicus and phytoplankton δ13C. The negative correlation between the δ13C values of C. sinicus and their carbon:nitrogen ratios reflected lipid accumulation by the copepods; high δ13C values were associated with high sea surface chlorophyll a concentrations. The δ15N values ranged from 2.8 ‰ to 8.8 ‰. The tendency of the δ15N values to increase from the bloom to post-bloom was attributable to an increase of the δ15N of the phytoplankton associated with nitrate depletion and Rayleigh fractionation. These monthly changes were synchronized among the four stations, but δ13C and δ15N differed significantly between stations. Interannual variations were statistically significant, but there were no significant monotonic trends. Interannual variations differed between δ13C and δ15N as well as among stations. These results suggest that local conditions rather than global-scale trends were the primary determinants of elemental cycles in this coastal ecosystem.

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).…”