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In 2017, the International Ocean Carbon Coordination Project (IOCCP) and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) decided to conduct the 6th International Inter-Calibration Exercise, the “2017/18 inter-laboratory comparison exercise of Certified Reference Materials of Nutrients in Seawater, CRM”. As was the case with the previous four inter-comparison (IC) studies organized by MRI-JMA and the previous IOCCCP-JAMSTEC IC exercise in 2015, the aim of this IC exercise was to improve the level of comparability by exchanging knowledge among participating laboratories. The scale of the study was expanded. One hundred seven laboratories in 33 countries across five continents received a letter of invitation to the IC exercise, and 71 laboratories in 30 counties agreed to participate in this IC exercise. Results were returned from 69 laboratories in 30 countries. The data were statistically analyzed, and the results are described in this report. The agreement between consensus median/mean and certified values was within uncertainty for all five samples used in this IC exercise. Only small discrepancies existed among core laboratories that reported close-to-consensus values and certified values. Normalized cumulative distributions for nitrate and phosphate obtained in 2018 were better (i.e., flatter) than the normalized cumulative distributions obtained from previous IC exercises conducted in 2008, 2012, and 2015. The indication is that comparability of nitrate and phosphate analysis among the laboratories has gradually improved from 2008 to 2018. In contrast with the nitrate and phosphate results, the normalized cumulative distribution for silicate obtained in 2018 was similar to the distributions in previous years. The indication is that comparability of silicate analysis among the laboratories has not changed during these years. The results of this IC exercise also showed that nonlinearity of the calibration curves for the nutrient analyses in each laboratory is still a significant source of reduced comparability of nutrient data. Analysis of ranked scatter plots led to this conclusion. The implication is that we need to use a set of nutrient CRMs, the concentrations of which cover the whole range of nutrient concentrations in the world’s oceans to maintain comparability of results. Thirty-eight laboratories among 69 laboratories used a CRM/RM. The remaining 31 laboratories did not use a CRM/RM or did not reply to the questionnaire. It is obvious that the number of the laboratories that use CRMs has been increasing since 2008. In general, the reported results from laboratories that used CRMs were located in the central part of the ranked plots and showed good Z-scores, as expected. The results of this IC exercise imply that the majority of the participating laboratories are very capable of measuring nutrient concentrations in seawater, and using CRMs will further increase the comparability of results. The results may be SI traceable in the near future. Fil: Aoyama, Michio. Fukushima University; Japón Fil: Abad, Mara. Consejo Superior de Investigaciones Científicas. Instituto de Ciencias del Mar; España Fil: Aguilar-Islas, Ana. University of Alaska; Estados Unidos Fil: Ashraf P, Muhamed. Central Institute of Fisheries Education. Fishing Technology Division; India Fil: Azetsu-Scott, Kumiko. Bedford Institute of Oceanography; Canadá Fil: Bakir, Adil. University of Portsmouth; Reino Unido Fil: Becker, Susan. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos Fil: Benoit-Cattin-Breton, Alice. Marine Research Institute; Islandia Fil: Berdalet, Elisa. Consejo Superior de Investigaciones Científicas. Instituto de Ciencias del Mar; España Fil: Björkman, Karin. University of Hawaii at Manoa; Estados Unidos Fil: Blum, Marguerite. Monterey Bay Aquarium Research Institute; Estados Unidos Fil: de Santis Braga, Elisabete. Universidade de Sao Paulo; Brasil Fil: Caradec, Florian. French Research Institute for Exploitation of the Sea; Francia Fil: Cariou, Thierry. Université Pierre et Marie Curie; Francia. Centre National de la Recherche Scientifique; Francia Fil: Gonsalez Chiozzini, Vitor. Universidade de Sao Paulo; Brasil Fil: Collin, Karine. French Research Institute for Exploitation of the Sea; Francia Fil: Coppola, Laurent. Centre National de la Recherche Scientifique; Francia Fil: Crump, Mike. National Institute of Water and Atmospheric Research; Nueva Zelanda Fil: Dai, Minhan. Xiamen University. Laboratory of Marine Environmental Science; China Fil: Daniel, Anne. French Research Institute for Exploitation of the Sea; Francia Fil: Davis, Clare. University of Liverpool; Reino Unido Fil: Solis, Miriam Edith. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina Fil: Edelvang, Karen. Technical University of Denmark; Dinamarca Fil: Faber, David. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos Fil: Fidel, Robinson. Centro de Investigaciones Oceanográficas e Hidrográficas del Pacífico; Colombia Fil: Lunde Fonnes, Linda. Institute of Marine Research; Noruega Fil: Frank, Jerry. University of Maryland; Estados Unidos Fil: Frew, Pauline. Scottish Environment Protection Agency; Reino Unido Fil: Funkey, Carolina. University of Hawaii at Manoa; Estados Unidos Fil: Gallia, Roberto. Stazione Zoologica Anton Dohrn; Italia

The objective of this inter-laboratory calibration exercise is to evaluate and improve comparability of global nutrients data in the world ocean. IOCCP and JAMSTEC co-organized an inter-laboratory calibration exercise of nutrients in seawater using four lots of recently certified RM produced by KANSO and three CRMs provided by National Metrology Institute of Japan which are certified in Marine 2014. 71 laboratories in 28 countries had replied to the call for participants. Results were returned from 59 laboratories. Korean Institute of Ocean Science and Technology, KIOST, also offered to provide their recently developed RMs to this I/C exercise. The Royal Netherlands Institute for Sea Research, NIOZ, also offers to provide silicate stock solution to contribute to the overall assessment of results regarding with this I/C exercise. Mean, median and standard deviation were calculated, robust mean and standard deviation were also calculated. Successive t-tests at the 95% confidence level were applied to the results before estimating the consensus mean, consensus median, and consensus standard deviation. Z-scores were also calculated to evaluate the performance of laboratories as in the previous inter-comparison studies. The ranked concentration plots for a particular nutrient would be proportional and roughly parallel to each other for samples with different nutrient concentrations if each laboratory appropriately compensated for the non-linearity of the calibration curves. However there are non-proportional results from some laboratories for all of the determinants as well as observed in the previous I/C studies. These results indicate that non-linearity of the calibration curves for nutrient analysis is one of significant sources of less comparability of nutrients data. This implies that we need to use a set of CRM of which nutrients concentrations can cover whole range of measurements of nutrients concentration to keep comparability of whole range of nutrients concentration in the world ocean. It is clear that present comparability among the participants in 2015 I/C exercise is quite similar with previously obtained comparability in 2012 I/C study and previous I/C studies. Consensus standard deviations of all determinands are one order of magnitude large rather than homogeneity of the CRMs distributed and consensus standard deviations are about double of reported precision of measurements of the laboratories. Therefore these I/C results show that use of CRM will be able to greatly improve comparability of nutrient data among the laboratories of the world. There are good signal in the results that although consensus standard deviations are relatively large, consensus median/mean of each samples showed good agreement with certified values of the samples within consensus SDs. This implies that majority of the participating laboratories have good capability to measure nutrients concentration in seawater and using CRM will increase more on the comparability and could be their results to be SI traceable quickly. Fil: Aoyama Michio. Fukushima University; Japón Fil: Abad, Mara. Consejo Superior de Investigaciones Científicas. Instituto de Ciencias del Mar; España Fil: Anstey, Carol. CSIRO Marine and Atmospheric Research; Australia Fil: Ashraf, Muhamed. Central Institute of Fisheries Technology; India Fil: Bakir, Adil. University of Portsmouth; Reino Unido Fil: Solis, Miriam Edith. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina Fil: Becker, Susan. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos Fil: Bell, Steven. Bermuda Institute of Ocean Science; Bermudas Fil: Berdalet, Elisa. Consejo Superior de Investigaciones Científicas. Instituto de Ciencias del Mar; España Fil: Blum, Marguerite. Monterey Bay Aquarium Research Institute; Estados Unidos Fil: Briggs, Rebecca. SOEST Laboratory for Analytical Biogeochemistry. University of Hawaii; Estados Unidos Fil: Caradec, Florian. Ifremer; Francia Fil: Cariou, Thierry. Université Pierre et Marie Curie; Francia. Centre National de la Recherche Scientifique; Francia Fil: Church, Matt. Hawaii Ocean Time-series; Estados Unidos Fil: Coppola, Laurent. Centre National de la Recherche Scientifique; Francia Fil: Crump, Mike. National Institute of Water and Atmospheric Research; Nueva Zelanda Fil: Curless, Susan. Hawaii Ocean Time-series; Estados Unidos Fil: Dai, Minhan. Xiamen University. State Key laboratory of Marine Environmental Science; China Fil: Daniel, Anne. Ifremer; Francia Fil: Davis, Clare. University of Liverpool; Reino Unido Fil: de Santis Braga, Elisabete. Universidade de Sao Paulo; Brasil Fil: Ekern, Lindsey. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos Fil: Faber, David. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos Fil: Fraser, Tamara. Fisheries And Ocean Canada; Canadá Fil: Gundersen, Kjell. Institute of Marine Research; Noruega Fil: Jacobsen, Sólvá. Faroe Marine Research Institute; Dinamarca Fil: Knockaert, Marc. Directorate Natural Environment. Ecosystems Physico-Chemistry; Bélgica Fil: Komada, Taketoshi. University Of Tartu.; Estonia Fil: Kralj, Martina. Istituto Nazionale di Oceanografía e Geofísica Sperimentale; Italia Fil: Kramer, Rita. Bundesamt für Seeschifffahrt und Hydrographie. Laboratorium Sülldorf; Alemania