An In Situ Phosphate Analyzer for Marine Environments Using Inlaid Microfluidics

In situ nutrient sensors are essential for furthering our understanding of phosphate flux dynamics in marine environments during short term events such as tidal cycles and algae blooms. Here, we present a fully automated in situ phosphate analyzer based on inlaid microfluidic absorbance cells with embedded beam-steering optics. The microfluidic component employs colorimetric absorbance spectrophotometry, using the phosphomolybdenum blue (PMB) assay modified by the addition of polyvinylpyrrolidone (PVP), to measure phosphate concentrations in seawater. Bench top calibrations and temperature sensitivity studies characterize the sensor’s performance in the laboratory. It achieves a practical limit of detection (LOD) of less than 100 nM and a high precision, with a relative standard deviation of less than 1.5% across five measurements. Two consecutive field deployments serve as validation tests for its intended in situ applications. The sensor is first deployed from a jetty at a depth of 6 m, with simultaneous bottle samples taken for confirmation. It is next deployed on the Stella Maris testbed, a multi-sensor seabed platform (MSSP), 100 m offshore and 9 m deep in the inlet to the Bedford Basin in Nova Scotia, Canada.