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.
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.
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:
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.
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.