Estimating the nitrate concentration from the dissolved oxygen concentration and seawater temperature in the Kuroshio extension, Oyashio, and mixed water regions

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.