Estimating the nitrate concentration from the dissolved oxygen concentration and seawater temperature in the Kuroshio extension, Oyashio, and mixed water regions
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For the purpose of estimating nitrate concentration using the dissolved oxygen concentration and seawater temperature recorded in an instrument such as a profiling float with an oxygen sensor, we examined the relationship between oxygen and nitrate concentrations in the Kuroshio Extension, Oyashio, and mixed water regions (34–43ºN, 140–150ºE) using vertical profile data, which includes temperature, salinity, oxygen, and nitrate, from the World Ocean Database 2005. Examining the NO3/O2 ratio along isotherms, we found a clear linear regression between the oxygen and nitrate concentrations in each potential temperature range, irrespective of water mass and seasonal variation. The slopes represented by the NO3/O2 ratio in each temperature range varied from −0.07 to −0.12. The benefit of analyzing data on constant potential temperature surfaces arises because of the strong dependence of the oxygen saturation of surface waters on temperature. This sets the end point of subducted waters and helps distinguish waters that may have similar preformed nitrate values. Our results suggest that subsequent reminearlization in the subducted water retained the Redfield ratio between N and O2. The estimate of nitrate concentration, by using the oxygen–nitrate relationship discovered in the present study, showed good accuracy with in general a mean error of less than 2.0 μmol/kg. The present method has been demonstrated to be a useful tool for estimating the nitrate concentration from temperature and oxygen.
KeywordsOxygen Nitrate Mixed water region between Kuroshio and Oyashio
The authors thank the members of the Oceanography Group at Tohoku University and at University of Hawaii at Manoa for their helpful discussions throughout this study. Thanks also to Dr. B. Qiu and M. Izumi who carefully read this paper and made suggestions. We would also like to thank anonymous reviewers whose comments have greatly improved the manuscript. This work was supported by “Studies on Prediction and Application of Fish Species Alteration (SUPRFISH)” sponsored by the Agriculture, Forestry and Fisheries Research Council (AFFRC), Japan and the Japanese Agency for Marine Earth Science and Technology. TS was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Grant-in-Aid for Scientific Research on Innovative Areas (22106007).
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