Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
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CTD PROTOCOLS AND SALINITY
SUMMARY: A commercially-available instrument package is lowered into the ocean on a conducting cable to obtain real- time, high-resolution profiles of temperature, conductivity and pressure from which salinity and depth are calculated. Additional sensors also detect dissolved oxygen concentration and phytoplankton fluorescence. Water samples are collected in bottles attached to the rosette sampler which can be activated by a surface control unit.
High vertical resolution environmental data are collected with a SeaBird CTD having external temperature (T), conductivity (C) and dissolved oxygen (DO) sensors and an internal pressure sensor. There is also a provision for adding fluorescence (F) and other sensors. A General Oceanics 24-place pylon and an aluminum rosette containing 24 12-liter Niskin bottles is used to obtain water samples from predetermined depths. The CTD and rosette are deployed on a 3-conductor cable allowing for the real-time display of data and acquisition, and for tripping the bottles in areas of interest in the water column. The CTD system takes 24 samples per second of pressure (P), T, C, DO and F. The raw data are stored both on the computer and, for redundancy, on VHS-format video tapes.
2. CTD Data Collection
3. CTD Sensors and Calibrations
4. Salinity Determinations
Salinity samples are collected directly from the Niskin bottles into 250 ml polyethylene bottles and stored at room temperature, in the dark, for subsequent analysis at our shore-based laboratory. The time between sample collection and analysis is generally less than 1 week. Prior to analysis, the samples are equilibrated to laboratory temperature and the salinity measured using an AGE model 2100 Minisal salinometer which is calibrated against IAPSO standard (Wormley) seawater. Typical precision of replicate analyses from the same water sample is 0.0003 ‰; for triplicate sample bottles the precision is less than 0.001 ‰. The effects of sample storage in polyethylene bottles was systematically evaluated during year 1 of the HOT program. The data, summarized in the 1988-89 HOT Data Report, indicate that S ‰ changes were negligible.
5. CTD Post-processing
Once the CTD data are collected and laboratory-determined pressure and temperature calibrations are applied, they are subjected to screening and quality control; they are checked for spikes or missing data caused by electrical interference in the hydrowire. Spikes are removed with a 9-point median filter, and missing data are replaced with interpolated values. After this initial screening, the data are averaged to 1/2-second values. Pressure and conductivity are then corrected for thermal hysterisis effects. This processing in the time-domain is required to allow for correction of lags between the C- and T- sensor responses. After the data have been reduced to 1/2-second values, corrections derived from the calibration methods described above are applied to the conductivity and dissolved oxygen. Finally, the data are pressure-sorted to remove effects of shiproll (i.e., only data taken when the CTD is moving downwards are kept), and averaged into 2 decibar values.