Nitrate sensors measure the nitrate concentrations in the ocean. Nitrogen is an essential element for plant growth in marine environments, and the dominant labile inorganic form is nitrate. Nutrients typically have higher concentrations in the deep ocean, while concentrations near surface are often very low due to uptake by phytoplankton during photosynthesis. The nitrate vertical gradient may be very dynamic, responding to both physical and biological processes.
The OOI Nitrate sensors optically measure the amount of nitrate (NO3-) dissolved in seawater by examining its absorption of ultraviolet (UV) light. The Sea-Bird SUNA V2 (Submersible Ultraviolet Nitrate Analyzer) is a chemical-free UV nitrate sensor based on the ISUS (In Situ Ultraviolet Spectroscopy) UV nitrate measurement technology developed at the Monterey Bay Aquarium Research Institute (MBARI). These optical sensors provide fast response measurements, allowing for the sensor to be deployed on profilers, gliders, and AUVs.
Nitrate sensors on the Cabled Array are installed on the Science Pods on the Shallow Profiler Moorings at Axial Base (SF03A), Slope Base (SF01A), and at the Endurance Array offshore site (SF01B). Since 2015, the Science Pods have made >30,000 profiles of the coastal and far offshore waters from ~200 m water depth to ~ 10 m water depth.