Simultaneous in situ Nitrate and Orthophosphate Measurement Using a Dual Chemistry Microfluidic Sensor

Anthropogenic nitrogen and phosphorus loading can have significant impact on land and marine based ecosystems with global impact. The use of nitrogen- and phosphorus-based fertilizers has continued to increase over recent years to stimulate agriculture production, and nutrient runoff from land can impact the stability of marine environments. As it stands, mitigation efforts are limited by a lack of comprehensive visibility over a wide spatial and temporal range, and severely impacted zones are often identified after significant ecological harm. Here, we present a submersible microfluidic sensor designed to simultaneously measure two prevalent forms of nitrogen and phosphorus, nitrate and orthophosphate, in situ within the same instrument. The device consists of two parallel analysis chambers, used to interrogate the same sample fluid with a different colourimetric assay for each species. To maintain small fluid volumes, light absorption spectrophotometry measurements are obtained using integral optical cells based on “inlaid microfluidics The performance of the dual-species nitrate and phosphate “NP Sensor” was first investigated in a controlled laboratory setting using prepared nutrient standards. A comprehensive temperature study followed to calibrate the sensor over a wide range of deployment conditions. Finally, the NP Sensor was deployed to acquire measurements in situ at two sites: a 60m vertical profile, and a long-term stationary shallow water study.