Long-Term Water Quality Monitoring Deployments with the Sea-Bird Hydrocat-Ep - Case Study

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The new Sea-Bird Scientific HydroCAT-EP incorporates a suite of physical and biogeochemical sensors into a single, robust instrument designed to maximize water quality parameters and give sustained accuracy. The instrument uses the same high quality Sea-Bird and WET Labs sensors found on their industry    leading    CTDs    and    ocean systems in a robust, compact package designed for extended deployments in coastal environments. The HydroCAT-EP uses a closed, pumped flow path with anti-fouling infusion for Temperature, Conductivity, Dissolved Oxygen, and pH. Optical turbidity and chlorophyll sensors have a fully copper-protected window to keep biofouling virtually nil.

As part of HydroCAT-EP field tests, an extended deployment was carried out at Shilshole Bay, Puget Sound, Washington. In conjunction with the field tests, an SBE 37 MicroCAT with Dissolved Oxygen was deployed and a water sampling program was conducted. Intercomparisons between the data sets show the HydroCAT-EP is an accurate and stable instrument. These results also highlight the lack of maintenance required to sustain high quality data return. The HydroCAT-EP was deployed for 3.5 months from July 1, 2015 - October 15, 2015, during the biologically productive months with moderate to high biofouling conditions.

HydroCAT-EP Dissolved Oxygen Measurements
In the HydroCAT-EP, the optical SBE 63 DO sensor is in the flow path downstream of the temperature and conductivity sensors (Figure 2). Prior to a sample, the flow path is fully flushed and water is pumped through the sensor for the duration of the sample. By default, this is 7 times the 63% sensor time constant, or approximately 40 seconds.

For the dissolved oxygen intercomparison, the HydroCAT-EP was moored alongside an SBE 37SMP MicroCAT with Pressure, Temperature, Conductivity, and Dissolved Oxygen. The SBE 37 mean depth was 2.5m; HydroCAT-EP was 2.8m. Figure 3 shows the 15-minute values of SBE 37 and HydroCAT-EP dissolved oxygen concentration in ml/I. The two optical dissolved oxygen sensors have the same mechanical and electrical construction; different calibration procedures give them slightly different accuracy specifications. The HydroCAT-EP dissolved oxygen sensor is calibrated to 200% saturation, with an initial accuracy of ± 0.14 ml/1; the SBE 37 is calibrated to 120% saturation, with an initial accuracy of ± 0.07 ml/1. The record includes times of significant diurnal photosynthetic activity and consequent dissolved oxygen saturations approaching 200%.

The difference between coincident SBE 37 and HydroCAT-EP values (excluding values >120% saturation) are shown in Figure 4. The mean difference was 0.064 ml/I with a standard deviation of ± 0.062 ml/1. Both values place the data well within the bounds of their combined accuracy specifications. The offset is consistent, having a linear drift of 0.002 ml/1 over 106 day, and suggests a calibration difference rather than sensor drift.

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