Abstract
Improved comparability of nutrient concentrations in seawater is required to enhance the quality and utility of measurements reported to global databases. Significant progress has been made over recent decades in improving the analysis and data quality for traditional laboratory measurements of nutrients. Similar efforts are required to establish high-quality data outputs from in situ nutrient sensors, which are rapidly becoming integral components of ocean observing systems. This paper suggests using the good practices routine established for laboratory reference methods to propose a harmonized set of deployment protocols and of quality control procedures for nutrient measurements obtained from in situ sensors. These procedures are intended to establish a framework to standardize the technical and analytical controls carried out on the three main types of in situ nutrient sensors currently available (wet chemical analyzers, ultraviolet optical sensors, electrochemical sensors) for their deployments on all kinds of platform. The routine reference controls that can be applied to the sensors are listed for each step of sensor use: initial qualification under controlled conditions in the laboratory, preparation of the sensor before deployment, field deployment and finally the sensor recovery. The fundamental principles applied to the laboratory reference method are then reviewed in terms of the calibration protocol, instrumental interferences, environmental interferences, external controls, and method performance assessment. Data corrections (linearity, sensitivity, drifts, interferences and outliers) are finally identified along with the concepts and calculations for qualification for both real time and time delayed data. This paper emphasizes the necessity of future collaborations between research groups, reference-accredited laboratories, and technology developers, to maintain comparability of the concentrations reported for the various nutrient parameters measured by in situ sensors.
Highlights
Nutrients are identified by the Global Ocean Observing System (GOOS) expert panels as one of the EOVs1
Consideration of the Instrumental Interferences for in situ Sensors Baseline and sensitivity drift will be estimated during the analytical performance assessment using blanks and Quality Control (QC) standards as it is performed for the reference laboratory method, and this applies to the three main in situ techniques
We hope that this article will help to build consensus on the quality control procedures necessary for in situ nutrient sensor data to be accepted into international databases
Summary
Nutrients are identified by the GOOS expert panels as one of the EOVs1. The measurement of nutrient concentrations in seawater provides a number of challenges, which are specific to the marine system. Wet chemical analyzers have been deployed as components of benthic incubation chambers (NH4-Digiscan, Plant et al, 2009), and seabed landers, in order to detect nutrient concentration at the water/sediment interface which can vary on hourly to seasonal time scales (Gevaert et al, 2011) Despite their high bulk density, in situ nutrient chemical analyzers have been adapted to moving platforms such as AUVs, to process water column depth profiles (LoC, Vincent et al, 2018), and on ROV’s to characterize the chemical gradients in the patchy distribution of hydrothermal fauna (Alchimist, Le Bris et al, 2000). UV sensors are characterized by a wide concentration range, rapid response times, small size and low power consumption, enabling their deployment across both fixed and profiling observing platforms (Table 2) Their sensitivity and accuracy are poorer than those of wet chemical analyzers due to a range of optical interferences (dissolved and particulate organic matter).
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