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Accurate and precise measurements of marine macronutrient concentrations are fundamental to our understanding of biogeochemical cycles in the ocean. Quantifying the measurement uncertainty associated with macronutrient measurements remains a challenge. Large systematic biases (up to 10%) have been identified between datasets, restricting the ability of marine biogeochemists to distinguish between the effects of environmental processes and analytical uncertainty. In this study we combine the routine analyses of certified reference materials (CRMs) with the application of a simple statistical technique to quantify the combined (random + systematic) measurement uncertainty associated with marine macronutrient measurements using gas segmented flow techniques. We demonstrate that it is realistic to achieve combined uncertainties of~1-4% for nitrate + nitrite (ΣNOx), phosphate (PO 4 3-) and silicic acid (Si(OH) 4) measurements. This approach requires only the routine analyses of CRMs (i.e. it does not require inter-comparison exercises). As CRMs for marine macronutrients are now commercially available, it is advocated that this simple approach can improve the comparability of marine macronutrient datasets and therefore should be adopted as 'best practice'. Novel autonomous Lab-on-Chip (LoC) technology is currently maturing to a point where it will soon become part of the marine chemist's standard analytical toolkit used to determine marine macronutrient concentrations. Therefore, it is critical that a complete understanding of the measurement uncertainty of data produced by LoC analysers is achieved. In this study we analysed CRMs using 7 different LoC ΣNOx analysers to estimate a combined measurement uncertainty of < 5%. This demonstrates that with high quality manufacturing and laboratory practices, LoC analysers routinely produce high quality measurements of marine macronutrient concentrations.

Microfluidic reagent-based nutrient sensors offer a promising technology to address the global undersampling of ocean chemistry but have so far not been shown to operate in the deep sea (>200 m). We report a new family of miniaturized lab-onchip (LOC) colorimetric analyzers making in situ nitrate and phosphate measurements from the surface ocean to the deep sea (>4800 m). This new technology gives users a new low-cost, highperformance tool for measuring chemistry in hyperbaric environments. Using a combination of laboratory verification and fieldbased tests, we demonstrate that the analyzers are capable of in situ measurements during profiling that are comparable to laboratorybased analyses. The sensors feature a novel and efficient inertialflow mixer that increases the mixing efficiency and reduces the back pressure and flushing time compared to a previously used serpentine mixing channel. Four separate replicate units of the nitrate and phosphate sensor were calibrated in the laboratory and showed an average limit of detection of 0.03 μM for nitrate and 0.016 μM for phosphate. Three on-chip optical absorption cell lengths provide a large linear range (to >750 μM (10.5 mg/L-N) for nitrate and >15 μM (0.47 mg/L-P) for phosphate), making the instruments suitable for typical concentrations in both ocean and freshwater aquatic environments. The LOC systems automatically collected a series of deep-sea nitrate and phosphate profiles in the northeast Atlantic while attached to a conductivity temperature depth (CTD) rosette, and the LOC nitrate sensor was attached to a PROVOR profiling float to conduct automated nitrate profiles in the Mediterranean Sea.

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