More Accurate Nutrients Measurements Along Reoccupied P06 Section Using Reference Material of Nutrients in Seawater

Michio AOYAMA* (Meteorological Res. Inst./Japan Meteorological Agency)Junko HAMANAKA, Asako KUBO, Yuki OTSUBO, Kenichiro SATO, Ai YASUDA, Shinichiro YOKOGAWA (Department of Marine Science, Marine Works Japan Ltd.)Contact: M. AOYAMA E-mail:maoyama@mri-jma.go.jp

Objectives1) To describe the present status of nutrients concentration in 2003-2004 in good traceability throughout the cruises. The target nutrients are nitrate, nitrite, phosphate and silicic acid .2) To study the temporal and spatial variation of nutrients concentration.3) To study the temporal and spatial variation of nitrate:phosphate ratio.4) To obtain more accurate estimation of inventory of nitrate, phosphate and silicate in the interested area.5) To provide more accurate nutrients data for physical oceanographers to use nutrients as tracers of water mass movement.

OS21E-09 2004 OSM 26-30 Jan. 2004 Portland

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Scientific goalBetter understanding on temporal and spatial variations of nutrients in seawater for studies on climate research and biogeochemical cycling in the ocean.

1, Cruise information: Re-occupied PO6 which was completed in 1992, 11 years before this cruise.Ship: R/V Mirai Leg1: EXPOCODE 49MR0304/1 3 Aug. 2003 to 5 Spet. 2003 Brisbane to Tahiti 121 stations Leg 2:EXPOCODE: 49MR0304/2 9 Sept. 2003 to 16 Oct. 2003 Tahiti to Valparaiso 114 stations

Fig. 1 Station locations during 49MR0304/1 and /2

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2, Nutrients analyses and RMNSAnalytical detail using TRAACS 800 systems (BRAN+LUEBBE)The phosphate analysis is a modification of the procedure of Murphy and Riley (1962). Molybdic acid is added to the seawater sample to form phosphomolybdic acid which is in turn reduced to phosphomolybdous acid using L-ascorbic acid as the reductant. Nitrate + nitrite and nitrite are analyzed according to the modification method of Grasshoff (1970).The sample nitrate is reduced to nitrite in a cadmium tube inside of which is coated with metallic copper. The sample stream with its equivalent nitrite is treated with an acidic, sulfanilamide reagent and the nitrite forms nitrous acid which reacts with the sulfanilamide to produce a diazoniumion. N1-Naphthylethylene-diamine added to the sample stream then couples with the diazonium ion to produce a red, azo dye. With reduction of the nitrate to nitrite, both nitrate and nitrite react and are measured; without reduction, only nitrite reacts. Thus, for the nitrite analysis, no reduction is performed and the alkaline buffer is not necessary. Nitrate is computed by difference.The silicic acid method is analogous to that described for phosphate. The method used is essentially that of Grasshoff et al. (1983), wherein silicomolybdic acid is first formed from the silicic acid in the sample and added molybdic acid; then the silicomolybdic acid is reduced to silicomolybdous acid, or "molybdenum blue," using ascorbic acid as the reductant.Precisions of nutrients analyses Precision of nutrients analyses during the cruise was evaluated based on the 13 measurements, which are measured every 10-15 samples, during a run at the concentration of C-5. Analytical precisions previously evaluated were 0.39% for phosphate, 0.36% for nitrate and 0.13% for silicic acid, respectively. During leg 1,2 and leg4, the analytical precisions for phosphate, nitrate and silicate were 0.18%, 0.17% and 0.16% in terms of median of all analyses, respectively. The analytical precisions for nitrate were shown in figure 3.Consensus values of nutrients concentration of lot AHConsensus values of nitrate, phosphate and silicic acid of lot AH were derived by the successive application of a t-test at the 95% confidence level to remove outliters. Those were 35.32+-0.11 μmol kg-1, 2.11+-0.01 μmol kg-1 and 133.91+-0.31 μmol kg-1, respectively. The nitrate concentrations of lot AH based on in-house working standard solution were shown in figure 4.

Reference Material of Nutrients in Seawater (hereafter RMNS)To get the more accurate and high quality nutrients data to achieve the objectives of this cruise, huge number of the bottles of the RMNS are prepared. In the previous world wide expeditions, such as WOCE cruises, the higher reproducibility and precision of nutrients measurements were required (Joyce and Corry, 1994). Since no standards were available for the measurement of nutrients in seawater at that time, the requirements were described in term of reproducibility. The required reproducibility was 1%, 1-2%, 1-3% for nitrate, phosphate and silicate, respectively. Although nutrient data from the WOCE one-time survey was of unprecedented quality and coverage due to much care in sampling and measurements, the differences of nutrients concentration at crossover points are still found among the expeditions (Aoyama and Joyce, 1996, Mordy et al., 2000, Gouretski and Jancke, 2001). For instance, the mean offset of nitrate concentration at deep waters was 0.5 µmol kg-1 for 345 crossovers at world oceans, though the maximum was 1.7 µmol kg-1 (Gouretski and Jancke, 2001). At the 31 crossover points in the Pacific WHP one-time lines, the WOCE standard of reproducibility for nitrate of 1% was fulfilled at about half of the crossover points and the maximum difference was 7% at deeper layers below 1.6 deg. C in potential temperature (Aoyama and Joyce, 1996, Aoyama et al., 2002). Methos of RMNS preparation The study on RMNS of the seawater base has been carried out to establish traceability on

nutrient analyses in seawater since 1992 in Japan. Autoclaving to produce RMNS has been studied (Aminot and Kerouel, 1991, 1995) and autoclaving was used to stabilize the samples for the 5th intercompariosn exercise in 1992/1993 (Aminot and Kirkwood, 1995). Aminot and Kerouel (1995) concluded that nitrate and nitrite were extremely stable throughout their 27 months storage experiment with overall standard deviations lower than 0.3% (range 5-50 µmol l-l) and 0.8% (range 0.5-5 µmol l-1), respectively. Previous study showed that autoclaving to inactivate the biological activity is acceptable for RMNS preparation. The seawater for RMNS production was sampled in the North Pacific Ocean at the depths of

surface where the nutrients are almost depleted and 1500-2000 meters depth where the nutrients concentrations are the maximum. The seawater was gravity-filtered through a membrane filter with a pore size of 0.45 µm (Millipore HA). The latest procedure of autoclaving for RMNS preparation is that the seawater in a stainless steel container of 40 liters was autoclaved at 120 deg. C, 2 hours, 2 times during two days. The filling procedure of autoclaved seawater was basically same throughout our study. Following cooling at room temperature in two days, polypropylene bottle of 100 ml capacity were filled by the autoclaved seawater of 90 ml through a membrane filter with a pore size of 0.2 µm (Millipore HA) at a clean bench in a clean room. The polypropylene caps were immediately tightly screwed on and a label containing lot number and serial number of the bottle was attached on all of the bottles. Then the bottles were vacuum-sealed to avid potential contamination from the environment.

180 RMNS packages and 500 bottles of lot AH, total 1400 bottles, were prepared for this cruiseRMNS lots T, AN, AK, AM and O are prepared to cover the nutrients concentrations in the interested sea area. About 180 sets of 5 RMNS lots are prepared. These packages will be used daily when in-house standard solutions renewed daily. 500 bottles of RMNS lot AH are prepared to use every analysis at every hydrographic stations planed about 500 during the cruise. These RMNS assignment were completely done based on random number. RMNS correctionAlthough nutrients data obtained during this cruise showed higher quality, we correct small station-to-station variability, which might be caused by the various analytical conditions, based on the results of RMNS analyses. In general RMNS corrections were applied to the results of the analysis in which RMNS concentrations were out of 95% confidence level of consensus concentrations of RMNS.Corrected results show that corrections by using RMNS results led smaller variability in deep water concentrations comparing previously reported nutrients data in the interested area.

Phosphate Nitrate Silicate

Pre-cruise N=30 x 2 ＊ 0.80% 0.44% 0.15%

Analytical Precision CV% 0.18% 0.17% 0.16%

Leg1,2,4 N=537＊＊ 1.0% 0.42% 0.34%

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*: Homogeneity of lot AH obtained based on a measurement onboard Mirai in May 2003.Analytical precisions stated here were medians of those obtained from each analysis during leg 1,2 and 4**: One sigma of standard deviation derived from ca. 350 analyses during leg 1,2 and 4.

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Fig. 2 Analytical precision of nitrate during leg1,2 and 4 Fig. 3 Nitrate concentration of lot-AH based on inhouse standard

3, Results of observation and long term trend of nutirients3-1, Nitrate, phosphate and silicic acid section along Pacific sector

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3-2, Comparison between 2003 section and 1992 section and long term trend east of New Zealand and the Tonga-Kermadec Ridge

SummaryOne of the highest quality nutrients concentration dataset was obtained

throughout R/V Mirai around the world cruise along 30 deg. S.Traceability throughout this cruise, in which 236 hydrographic stations were completed and ca. 250 stations were completed in the Atlantic and Indian sector, was established using RMNS (Reference Material ofNutrients in Seawater). Preliminaly results show that nutrients concentrations in deeper water

east of Tonga-Kermadec Ridge, where Circumpolar Deep Water flows north, decreased at least after the WOCE P06 observation in 1992. Nitrate concentration seems to be decreasing since 1928.

Future work to establish traceabilityof nutrients concentrations in seawaterWe need International Nutrients Scale System together with the RMNS.

Fig. 5a Sampling locations and layers Fig.5b Nitrate section Fig.5c Phosphate section Fig.5d Silicic acid section (micro mol kg-1) (micro mol kg-1) (micro mol kg-1)

Fig. 6 Comparison results. Fig. 7 Long term trend deeper than 400dbMedians in a grid of 2 deg. x 500db were compared. between 182 deg. E and 190deg. E(a) Phosphate (b) Nitrate (c) Silicic acid (a) Phosphate (b) Nitrate (c) Silicic acid

Nutrient concentrations deeper than 400 db east of Tonga-Kermadec Ridge , marked in red in fig. 5a, observed in 2003 decreased from those observed in 1992. The differences tend to small at the eastern part of Pacific sector as shown in fig. 6(a). Historical nutrients data at same region, deeper than 4000db between 182-190

deg. E, were extracted from OSD01 and plotted in figures 7(a), 7(b) and 7( c). It is clear that nutrients concentration decreased after WOCE observation in 1992. Nitrate concentration may have been decreasing in the 1920s.

Fig. 8 Phosphate profiles in 1992 and 2003 at 182-190 deg. E (a) and 260-270 deg. E (b)

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