Comparison of Data Logger Operational Parameters Aboard Ship and Towed on the Seafloor

ABSTRACT A number of new, submersible water quality data loggers (probes) have recently been introduced to the market place which have interesting applications to developing Environmental Impact Statements (EIS) and future surveys in the Exclusive Economic Zone (EEZ). The probe chosen for this study is typical of the units found in the market place today. This probe has the capability for multi-parameter measurement of temperature (°C), dissolved oxygen (mg/l), conductivity (mmhos/cm), salinity (ppt), pH (units), and depth (meters). It can be preprogrammed for remote data logging, tethered by underwater cable to a surface readout, or operated aboard ship by a flow cell connected to a submersible pump. A new operational parameter was tested using the probe mounted inside a towed seafloor sled. A pump mounted inside the sled allowed a sediment-water slurry from the seafloor to be pumped aboard ship. A second probe was mounted on the survey ship with a flow cell connected to the submersible pump. With one probe on the surface and the other on the seafloor, a simultaneous comparison was made utilizing the data collected on the same sediment water sample. Advantages gained from this study can be used to cross calibrate in situ seafloor analyses with those made aboard ship. It also demonstrates the probe's durability for seafloor measurements either in the static or towed mode of operation. INTRODUCTION Current marine environmental monitoring usually involves individually sampling preselected stations. Each station must be visited by a survey team and the results recorded. This method takes considerable amounts of time and allows for variations in the data due to the differences in time and weather which may sometimes require days or weeks between the start and finish of a monitoring project. The ability to rapidly monitor a large region involving numerous stations is certainly a prerequisite for future marine survey programs. By utilizing compiled data files, maps can be generated allowing the biologist to readily locate and study areas of possible environmental concern. The advantages of deployment of a water quality data logger for rapid in situ seafloor measurements of the sediment-water interface zone could be a very exciting environmental survey tool. This is especially true considering that this zone is the primary region where most organic and inorganic pollutants eventually reside. Since application of the probes have not been tested in the towed mode, this study was conducted to determine whether the probes would operate best on the seafloor or aboard ship. DEVELOPMENT AND FIELD TEST The basic requirement for an environmental probe for this experiment was the ability to collect rapid, accurate data that could be transmitted to a portable computer. The probe needed to have the capability to identify frequent changes in the sediment-water interface and to withstand the punishment of riding on the seafloor. In addition, the basic environmental parameters, i.e. temperature, conductivity, pH, salinity, dissolved oxygen, and water depth, needed to be part of the package.