A Lab-On-Chip Phosphate Analyzer for Long-term In Situ Monitoring at Fixed Observatories: Optimization and Performance Evaluation in Estuarine and Oligotrophic Coastal Waters

Abstract

The development of phosphate sensors suitable for long-term in situ deployments in natural waters, is essential to improve our understanding of the distribution, fluxes, and biogeochemical role of this key nutrient in a changing ocean. Here, we describe the optimization of the molybdenum blue method for in situ work using a lab-on-chip (LOC) analyzer and evaluate its performance in the laboratory and at two contrasting field sites. The in situ performance of the LOC sensor is evaluated using hourly time-series data from a 56-day trial in Southampton Water (UK), as well as a month-long deployment in the subtropical oligotrophic waters of Kaneohe Bay (Hawaii, USA). In Kaneohe Bay, where phosphate concentrations were characteristic of the dry season (0.13 ± 0.03 µM, n = 704), the in situ sensor accuracy was 16 ± 12% and a potential diurnal cycle in phosphate concentrations was observed. In Southampton Water, the sensor data (1.02 ± 0.40 µM, n = 1,267) were accurate to ±0.10 µM relative to discrete reference samples. Hourly in situ monitoring revealed striking tidal and storm derived fluctuations in phosphate concentrations in Southampton Water that would not have been captured via discrete sampling. We show the impact of storms on phosphate concentrations in Southampton Water is modulated by the spring-neap tidal cycle and that the 10-fold decline in phosphate concentrations observed during the later stages of the deployment was consistent with the timing of a spring phytoplankton bloom in the English Channel. Under controlled laboratory conditions in a 250 L tank, the sensor demonstrated an accuracy and precision better than 10% irrespective of the salinity (0-30), turbidity (0-100 NTU), colored dissolved organic matter (CDOM) concentration (0-10 mg/L), and temperature Grand et al.In situ Lab-On-Chip Phosphate Sensor (5-20 • C) of the water (0.3-13 µM phosphate) being analyzed. This work demonstrates that the LOC technology is mature enough to quantify the influence of stochastic events on nutrient budgets and to elucidate the role of phosphate in regulating phytoplankton productivity and community composition in estuarine and coastal regimes.

Autonomous Optofluidic Chemical Analyzers for Marine Applications: Insights from the Submersible Autonomous Moored Instruments (SAMI) for pH and pCO2

“…In situ pCO 2 measurements are particularly difficult to calibrate because no reliable liquid standard exists. Other optofluidic systems, however, can use calibration standards to correct for various sources of drift (e.g., Beaton et al, 2012;Grand et al, 2017). One way to avoid the additional complexity of pumping is to use fluorescent lifetime based measurements that have reduced sensitivity to the signal intensity.…”

“…Similar corrections have been reported in other optofluidic sensors to achieve better accuracy and reduce drift (Grand et al, 2017). The pCO 2 calculated using blank and reference intensity corrections (named "normal calculation" as mentioned later or in figures) is compared with pCO 2 calculated using absorbances without these corrections [i.e., using a constant I λ0 or setting log(I λref /I λ0ref ) equal to zero in Equation 8].…”

Toward a Harmonization for Using in situ Nutrient Sensors in the Marine Environment

“…Other devices are based on different flow analysis techniques such as µLFR which is used on WIZ (Systea, Azzaro and Galetta, 2006), or RFA as used on ANAIS (Thouron et al, 2003). Miniaturization of the manifold sensors has been a focus over recent years with the development of sensors based on microfluidics such as LoC devices (Beaton et al, 2011Beaton, 2012;Grand et al, 2017). Most wet chemistry systems are recalibrated in situ at regular intervals by replacing the sample by a blank and a standard.…”

“…However, wet chemical analyzers may not always be suitable for long-term deployment due to limited lifetime of reagents and standards and waste disposal concerns. Most wet chemical sensors have been deployed on platforms for the collection of high-frequency timeseries of surface coastal waters at a fixed station (NAS3X, Mills et al, 2005;CHEMINI, Répécaud et al, 2009;WIZ, Vuillemin et al, 2009b;LoC, Beaton et al, 2017;Clinton-Bailey et al, 2017;Grand et al, 2017). FerryBox-systems (Petersen et al, 2011) also provide a compatible observing infrastructure for wet chemical analyzers to collect surface nutrient data on board Ships of Opportunity.…”

First Deployment and Validation of in Situ Silicate Electrochemical Sensor in Seawater

“…Over the past decade, significant progress has been made in the development of in situ nutrient sensors and a few sensors are commercially available to measure nitrate and phosphate (Thouron et al, 2003;Johnson et al, 2013;Legiret et al, 2013;Grand et al, 2017). Silicate is traditionally analyzed by spectrophotometric/colorimetry methods requiring liquid reagents addition (wet chemical techniques) (Thouron et al, 2003;Ma et al, 2014).…”