Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa

CTD Nitrate (ISUS)



Satlantic LP

FTP View Data
To assist in the interpretation of the data, it can be displayed using the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS©).

Sampling Procedure

Real-time nitrate concentrations were measured with a Satlantic ISUS V3 (#097). The ISUS is a chemical-free, solid-state sensor that uses ultraviolet absorption spectroscopy to measure continuous nitrate concentrations. Data was collected from as many 1000m casts as possible. Instrument drift was corrected via calibration with bottle inorganic [nitrate+nitrite] analyzed using a six-channel Bran Luebbe Autoanalyzer III. Only samples below 200m were used in the calibration where nitrite values at Station ALOHA are considered negligable. A linear relationship of the form, Vnitrate = b*VISUS + a, was used.

Data was logged internally and also collected using the Sea-Bird CTD system. Internally recorded profiles were latter downloaded after the cruise. CTD values were preferencially reported unless the data was considered bad. A common reason for bad data was a poor seal when connecting the Analog output cable to the CTD. Internally recorded data was then substituted.

As of HOT-302, the lamp-life of the Satlantic ISUS V3 was < 1% of the 1000 hr rating, with output less than 50% of intensity at 240 nm. Over the life of the sensor, we observed a water column average offset of ~1.7 ± 1.5 micromolar when calibrated to bottle inorganic [nitrate+nitrite]. Given that this sensor is no longer serviced by the manufacturer, coupled with the low accuracy of the data, the instrument was retired in 2018.


Stack plots of CTD nitrate and bottle inorganic [nitrate+nitrite] for 2018 are shown in the figures below. The offset is 5 µmol/kg.