{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,15]],"date-time":"2026-03-15T14:51:59Z","timestamp":1773586319533,"version":"3.50.1"},"reference-count":84,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2022,5,3]],"date-time":"2022-05-03T00:00:00Z","timestamp":1651536000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"GEOMAR, Helmholtz for Ocean Research Centre"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Accurate, on-site determinations of macronutrients (phosphate (PO43\u2212), nitrate (NO3\u2212), and silicic acid (H4SiO4)) in seawater in real time are essential to obtain information on their distribution, flux, and role in marine biogeochemical cycles. The development of robust sensors for long-term on-site analysis of macronutrients in seawater is a great challenge. Here, we present improvements of a commercial automated sensor for nutrients (including PO43\u2212, H4SiO4, and NO2\u2212 plus NO3\u2212), suitable for a variety of aquatic environments. The sensor uses the phosphomolybdate blue method for PO43\u2212, the silicomolybdate blue method for H4SiO4 and the Griess reagent method for NO2\u2212, modified with vanadium chloride as reducing agent for the determination of NO3\u2212. Here, we report the optimization of analytical conditions, including reaction time for PO43\u2212 analysis, complexation time for H4SiO4 analysis, and analyte to reagent ratio for NO3\u2212 analysis. The instrument showed wide linear ranges, from 0.2 to 100 \u03bcM PO43\u2212, between 0.2 and 100 \u03bcM H4SiO4, from 0.5 to 100 \u03bcM NO3\u2212, and between 0.4 and 100 \u03bcM NO2\u2212, with detection limits of 0.18 \u03bcM, 0.15 \u03bcM, 0.45 \u03bcM, and 0.35 \u03bcM for PO43\u2212, H4SiO4, NO3\u2212, and NO2\u2212, respectively. The analyzer showed good precision with a relative standard deviation of 8.9% for PO43\u2212, 4.8% for H4SiO4, and 7.4% for NO2\u2212 plus NO3\u2212 during routine analysis of certified reference materials (KANSO, Japan). The analyzer performed well in the field during a 46-day deployment on a pontoon in the Kiel Fjord (located in the southwestern Baltic Sea), with a water supply from a depth of 1 m. The system successfully collected 443, 440, and 409 on-site data points for PO43\u2212, \u03a3(NO3\u2212 + NO2\u2212), and H4SiO4, respectively. Time series data agreed well with data obtained from the analysis of discretely collected samples using standard reference laboratory procedures and showed clear correlations with key hydrographic parameters throughout the deployment period.<\/jats:p>","DOI":"10.3390\/s22093479","type":"journal-article","created":{"date-parts":[[2022,5,3]],"date-time":"2022-05-03T08:26:35Z","timestamp":1651566395000},"page":"3479","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":22,"title":["Improvement of On-Site Sensor for Simultaneous Determination of Phosphate, Silicic Acid, Nitrate plus Nitrite in Seawater"],"prefix":"10.3390","volume":"22","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-8447-2907","authenticated-orcid":false,"given":"Mahmoud Fatehy","family":"Altahan","sequence":"first","affiliation":[{"name":"GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany"},{"name":"Central Laboratory for Environmental Quality Monitoring, National Water Research Center, El-Qanater El-Khairia 13621, Egypt"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2575-7814","authenticated-orcid":false,"given":"Mario","family":"Esposito","sequence":"additional","affiliation":[{"name":"GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3061-2767","authenticated-orcid":false,"given":"Eric P.","family":"Achterberg","sequence":"additional","affiliation":[{"name":"GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany"}]}],"member":"1968","published-online":{"date-parts":[[2022,5,3]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"20160318","DOI":"10.1098\/rsta.2016.0318","article-title":"Ocean deoxygenation, the global phosphorus cycle and the possibility of human-caused large-scale ocean anoxia","volume":"375","author":"Watson","year":"2017","journal-title":"Philos. 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