MBARI UNDERWAY MAPPING SYSTEM
Carole Sakamoto, Gernot Friederich, L. A. Codispoti, R. Michisaki

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Figure 1. Underway system results from the Indian Ocean cruise.  Enhancement in the Southern Ocean on Leg 8  in mid-September 1995.   Nitrate levels reached a high of 18ÁM.

Often the traditional methods of discrete sampling do not give us enough information by which we may understand how spatial and temporal variabilities affect oceanic processes. Underway mapping of these areas provide an inexpensive and easy method of obtaining continuous measurements of the biological and physical parameters. Our underway mapping system is designed to work primarily without attendance. In addition, real time measurements offer a glimpse of the current conditions.

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Figure 2. Underway mapping system (101k)

Our underway mapping system measures nitrate, fluorescence, PAR, absorption, and transmittance on a continual basis. A schematic of the system is illustrated in Figure 2. An interface box contains all the connections between the computerized data acquisition system and the instruments, including a GPS system and power supplies. The data acquisition computer is an MS-DOS based Hewlett Packard Vectra. The data (except GPS position) is acquired through a Keithley MetraByte DAS-8PGA analog/digital board installed on the computer. The data acquisition program is written in MS Quick Basic v7.0. The control software is a time based program. The program loops once every five seconds and control commands can be issued directly from the computer. Nitrate concentration, fluorescence, and PAR are displayed in real time. These parameters are plotted against time with a scrolling display of the last ten hours. Several quality control features have been built into the system. In addition to the above mentioned parameters, GPS positions, the latest numerical values, range checking, and system warnings can also be displayed.

 

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Figure 3. Left panel: low resolution nitrate analyzer, right panel: high resolution analyzer.

Nitrate concentration is ascertained by reducing nitrate to nitrite and measuring the nitrite as an azo dye (Grasshoff et al., 1983). The nitrate analyzer, Figure 3, is based upon a Kloehn syringe pump that has been equipped with a colorimeter which measures the color development in the syringe. The pump motor and 5 way rotary selection valve are controlled via a 9600 baud RS232 serial port connect to COM1 on the computer. The colorimeter light source is a high output 565 nm green LED (HP HLMP-3950) and the detector is an EG&G HUV-1100BQ broadband photodiode/amplifier combination. Nitrate measurements are initiated on a twenty minute interval and a 10ÁM seawater standard is analyzed every four hours. A pinch valve attached to the front of the system is used to switch between the seawater intake and the standard. The reagents and standard are kept in a thermoelectric cooler, Koolatron #P9, in order to improve stability. Following is a brief description of a nitrate analyzer cycle:

  1. The system is rinsed by aspirating and expelling 1.1ml of sample through the cadmium reactor four times. On the fourth rinse, the transmittance is recorded and used as a blank value.
  2. In sequence, the following measurements are drawn into the syringe: 0.6ml of sample, 0.2ml of buffer, 0.6ml of sample. The 1.4ml of buffered sample is expelled into the cadmium reactor and ,over a ten minute interval, is reduced to nitrite.
  3. 0.15ml of sulfanilamide is aspirated into the syringe. A 0.9ml of the reacted sample is aspirated back into the syringe. The mixture is allowed to react for one minute.
  4. To ensure mixing before the final color development reaction, 0.5ml of the sample is placed in the waste line, then 0.15ml of NED is drawn into the syringe. The sample is then aspirated back into the syringe. The reactants remain in the syringe.
  5. Light transmission is measured over a 15 second period in the syringe. The values are averaged and written to disk. Finally the sample is expelled to waste.

The following parameters are measured in ten minute intervals. Fluorescence is measured by a WET Labs WETStar miniature fluorometer. PAR is measured by two instruments: a Licor cosine sensor and a Biospherical 2 sensor. Absorption is measured using a WET Labs AC-9. The GPS position is sampled by a Magellan single board GPS.

DISCRETE MEASUREMENTS

During the first 15 seconds of each 10 minute interval, the fluorometer takes a reading. At the beginning of the 20 minute nitrate sampling interval the nitrate analyzer cycles through and collects a new sample. At least twice daily, during these sampling intervals, a 280ml sample of water is collected from the underway system outflow. The sample is filtered immediately through a Whatman GF/F filter. The filtered sample is extracted in 90% acetone bath for 24-48 hours in a freezer. The sample is then analyzed for chlorophyll using a Turner fluorometer set up for discrete samples. The sample collection time (GMT), GPS position, nitrate, sea surface temperature, fluorescence, and chlorophyll values are recorded to an underway sampling log. These discrete chlorophyll samples are used to derive an estimate of the underway chlorophyll from the Wet Labs fluorometer readings.