Abstract
Nitrate, an essential nutrient for primary production in natural waters, is optically detectable in the ultraviolet spectral region of 217–240 nm, with no chemical reagents required. Optical nitrate sensors allow monitoring at high temporal and spatial resolutions that are difficult to achieve with traditional approaches involving collection of discrete water samples followed by wet-chemical laboratory analysis. The optical nitrate measurements are however subject to matrix interferences in seawater, including bromide, at the spectral range of interest. Significant progress has been made over the last 10 years in improving data quality for seawater nitrate analysis using the ISUS and SUNA (Seabird Scientific, United States) optical sensors. Standardization of sensor calibration and data processing procedures are important for ensuring comparability of marine nitrate data reported in different studies. Here, we improved the calibration and data processing of the OPUS sensor (TriOS GmbH, Germany), and tested five OPUS sensors simultaneously deployed under identical conditions in the laboratory in terms of inter-sensor similarities and differences. We also improved the sampling interval of the OPUS to 3 s in a continuous mode by a custom-built controller, which facilitates the integration of the sensor into autonomous profiling systems. Real-time, high-resolution, in situ measurements were conducted through (1) underway surface measurements in the southeastern North Sea and (2) depth profiles on a conductivity–temperature–depth frame in the tropical Atlantic Ocean. The nitrate data computed from the optical measurements of the sensor agreed with data from discrete water samples analyzed via conventional wet-chemical methods. This work demonstrates that the OPUS sensor, with improved calibration and data processing procedures, allows in situ quantification of nitrate concentrations in dynamic coastal waters and the open ocean, with an accuracy better than ∼2 μM and short-term precision of 0.4 μM NO3–. The OPUS has a unique depth rating of 6,000 m and is a good and cost-effective nitrate sensor for the research community.
Highlights
Nitrogen is a crucial nutrient for the functioning of all living organisms
This study presents a new application of the temperature-corrected salinity subtracted (TCSS) approach for the OPUS sensor
Deployment in coastal surface waters and the deep ocean demonstrated that the OPUS sensor can capture spatial variations across short spatial scales with results that were in excellent agreement with discrete water samples analyzed in the laboratory
Summary
The principal form of fixed dissolved inorganic nitrogen in marine waters is nitrate (NO3−), which is identified as one of the Essential Ocean Variables by the Global Ocean Observing System community (IOCCP, 2017). The availability of nitrate leads to direct and indirect effects on marine ecosystem health: it can limit primary productivity when depleted (Kristiansen et al, 2001) and cause eutrophication when supplied at high levels (van Beusekom, 2018). Infrequent sampling intervals result in missing episodic and transient events that lead to important temporal and spatial variations in nitrate concentrations (Prien, 2007; Pidcock et al, 2010). High-frequency in situ observations on autonomous platforms are required to capture the variability in nitrate concentrations, overcome risks of sample contamination and degradation, and reduce high sampling/analysis costs as well as relatively long analysis times
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