Abstract
To identify water with an excess nitrate concentration to phosphate ratio and its potential source, the nutrient concentrations in the Tsushima Strait (TSS) were investigated over ten cruises in August and September 2007–2014, excluding 2010. On the basis of the Redfield ratio, water with an excess nitrate concentration of >1 μM (positive ExNOx water) was identified below the surface mixed layer during four cruises in 2011–2013. Positive ExNOx water was present mainly in less-saline (<34) waters with a density of 22–25 σ θ , and 25–75 m depth. However, in August 2012, positive ExNOx was detected in dense (25–25.5 σ θ ) and deep (50–110 m depth) waters near the salinity maximum, although the salinity during this period was significantly lower than that in other years. The horizontal length of positive ExNOx water was >100 km across the TSS during two cruises in August 2012 and September 2013, respectively. According to multi-regression analysis conducted on the silicate concentration, temperature, and salinity, the silicate concentration was increased in the less-saline subsurface water. The required amount of original freshwater was 108–9 m3 day−1 based on the excess nitrate concentration. This evidence indicates that the positive ExNOx water originated from large river waters such as the Changjiang. Thus, discharged water from the rivers of the East Asia is contributing to the increased N:P ratio in the Tsushima Warm Current, southern Japan Sea.
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Acknowledgements
We thank the captain, officers, crew members, and scientists aboard T/V Tenyo maru of the National Fisheries University and R/V Yoko maru of the Fisheries Research Agency for their cooperation at sea. Rainfall data were obtained from the JMA. We would like to thank two anonymous reviewers and associate editor for their suggestions and comments. Potential density calculations used the Microsoft Excel VBA programmed by Shigeho Kakehi of Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency. H. Kodama helped in the construction of the shipboard observation database. We would like to thank Editage (http://www.editage.jp) for English language editing. This study was financially supported by general research funds to the Fisheries Research Agency from the Fisheries Agency and by JSPS Grant-in-Aid for Scientific Research (B) to AM (KAKENHI; Grant Number 22310009).
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Kodama, T., Morimoto, A., Takikawa, T. et al. Presence of high nitrate to phosphate ratio subsurface water in the Tsushima Strait during summer. J Oceanogr 73, 759–769 (2017). https://doi.org/10.1007/s10872-017-0430-4
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DOI: https://doi.org/10.1007/s10872-017-0430-4