Improvement of On-Site Sensor for Simultaneous Determination of Phosphate, Silicic Acid, Nitrate plus Nitrite in Seawater

Accurate, on-site determinations of macronutrients (phosphate (PO(4)(3−)), nitrate (NO(3)(−)), and silicic acid (H(4)SiO(4))) 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 PO(4)(3−), H(4)SiO(4), and NO(2)(−) plus NO(3)(−)), suitable for a variety of aquatic environments. The sensor uses the phosphomolybdate blue method for PO(4)(3−), the silicomolybdate blue method for H(4)SiO(4) and the Griess reagent method for NO(2)(−), modified with vanadium chloride as reducing agent for the determination of NO(3)(−). Here, we report the optimization of analytical conditions, including reaction time for PO(4)(3−) analysis, complexation time for H(4)SiO(4) analysis, and analyte to reagent ratio for NO(3)(−) analysis. The instrument showed wide linear ranges, from 0.2 to 100 μM PO(4)(3−), between 0.2 and 100 μM H(4)SiO(4), from 0.5 to 100 μM NO(3)(−), and between 0.4 and 100 μM NO(2)(−), with detection limits of 0.18 μM, 0.15 μM, 0.45 μM, and 0.35 μM for PO(4)(3−), H(4)SiO(4), NO(3)(−), and NO(2)(−), respectively. The analyzer showed good precision with a relative standard deviation of 8.9% for PO(4)(3−), 4.8% for H(4)SiO(4), and 7.4% for NO(2)(−) plus NO(3)(−) 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 PO(4)(3−), Σ(NO(3)(−) + NO(2)(−)), and H(4)SiO(4), 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.