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 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 M, 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=1267) 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 tenfold 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), dissolved organic carbon concentration (0-10 mg/L) and temperature (5-20C) of the water (0.3-13 M 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.
Highlights
The availability of orthophosphate, the phosphorus species directly available to phytoplankton and autotrophic bacteria in natural waters, exerts a major influence on the productivity, phytoplankton species composition, and community structure of aquatic ecosystems
That the reaction pH and [Mo(VI)]/[H+] ratio reported in Patey et al (2010) are incorrect, as the authors appear to have assumed that both H2SO4 protons fully dissociate in dilute solutions
Long-term, high-frequency, in situ observations are crucial to improve our understanding of the fluxes, temporal dynamics and biogeochemical implications of nutrient distributions in coastal and oceanic waters
Summary
The availability of orthophosphate, the phosphorus species directly available to phytoplankton and autotrophic bacteria in natural waters, exerts a major influence on the productivity, phytoplankton species composition, and community structure of aquatic ecosystems. Much of our current understanding of the spatial patterns and temporal trends in phosphate concentrations stems from discrete sampling efforts performed at weekly to monthly intervals, followed by laboratory analysis using complex instrumentation. An improved understanding of biogeochemical variability is needed to untangle natural vs anthropogenic signals in timeseries records For these reasons, the development of in situ nutrient sensors with a dynamic quantification range suitable for high-frequency, long-term sampling in marine and inland waters is essential to improve our understanding of the fluxes and biogeochemical implications of nutrient distributions in aquatic ecosystems
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