[A-OS18] Chemical and Biological Oceanography

[AOS18-P04] Nutrient concentration change in deep layer in the western North Pacific Ocean

*Michio Aoyama^1,2, Daisuke Sasano³, Yuichiro Kumamoto¹, Hiroshi Uchida¹ (1.Research Institue of Global Change, The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) , 2.Center for Research in Isotopes and Environmental Dynamic, Faculty of Life and Environmental Sciences, University of Tsukuba, 3.Atmosphere and Ocean Department, Japan Meteorological Agency)

Keywords:nutrient, western North Pacific Ocean, deep layer

Data on high precise observations of the global ocean, starting with WOCE project in the 1980s, have been gradually accumulated. Analysis results using those data have also been reported. On the CCHDO site, P1, P2, P9, P10, P13 and P14 lines data are registered in the western North Pacific. In P1 line, which is the most frequently observed line, MR21-04 cruise was carried out by research vessel Mirai belong to the Japan Agency for Marine-Earth Science and Technology in 2021 in addition to four cruises conducted in 1985, 1999, 2007 and 2014. The Japan Meteorological Agency has been observing the full depth hydrography every year since 2011 on the P13 line. Water temperature is one of the best-observed items in terms of comparability and traceability of data, and the temperature increase in the bottom layer is clear from previous observations (Fukasawa et al., 2004, Masuda et al., 2010). However, studies of spatiotemporal variability of nutrients, eg. nitrate, phosphate and silicates, have not progressed compared to temperature and salinity, because of previous data do not ensure comparability and traceability.
Since the P1 observation was carried out in 2021, we can use 5 observations from 1985 to 2021 to study changes of nutrient concentrations. The data near the intersection of P1 and the 165 deg. east longitude line by the Japan Meteorological Agency since 2011 can be also used.
We decided to analyze the data up to 1200 m above the sea bottom at crossover points between P1 and 165 deg. E line. In terms of comparability and traceability of nutrient data, in-house standards were used in the 1985 and 1999 cruises on the P1 line, and reference materials, RM, were used on the 2007 cruise. In contrast, on the 2014 and 2021 cruises, certified reference material, CRM, of nutrients was used as the calibration standards, complete comparability and traceability of nutrient analysis were ensured. In addition, the precision of the analysis of nitrate, phosphate and silicate is estimated to be about 0.5% on the 1985 and 1999 cruised. On the other hand, in the 2007, 2014 and 2021 cruises, the precision of nutrients measurement was maintained at about 0.1%. In the cruises of the Japan Meteorological Agency along 165 deg. E line, RM was used in 2011 and 2012, then CRM has been used since 2013. Currently, the Japan Meteorological Agency publishes nutrient data measured based on the in-house standards. For this study, these values were corrected to CRM / RM to ensure comparability and traceability with the 2007, 2014 and 2021 P1 cruise nutrients data. Details of the obtained results will be described at the time of the presentation, but the rate of change in nitrate concentration in seawater up to 1200 m above the sea bottom is 0.14 micro mol kg-1 per 10 years, and the rate of change in dissolved oxygen concentration is -0.44 micro mol kg-1 per 10 years. The rate of phosphate concentration showed decreasing trend and it is -0.011 micro mol kg-1 per 10 years, which is the opposite of the increasing trend of nitrate. The molar ratio of the rate of change between nitrate and dissolved oxygen is -3.3, which is close to the ratio of Redfield of -6.6.