The acoustic water column profiler: a tool for long-term monitoring of zooplankton populations

Measuring acoustic backscatter in the water column is a low-cost, reliable method for examining the long-term behaviour and distribution of zooplankton populations. Backscatter at acoustic frequencies above 20 kHz is useful for profiling those quantities, which, when tracked over long periods of time, can provide a valuable contribution to understanding and monitoring the state of marine ecosystems. The Water Column Profiler/sup TM/ is a self-contained echo-sounder, designed for long-term, autonomous operation. The instrument can be used in either downward-looking mode, from a moored surface buoy, or in upward-looking mode from a submerged mooring. The instrument has selectable parameters for pulse length and sampling interval. The data are recorded in digital form, and averaging in both time and range is available. On-board storage of up to 64 Mbytes of non-volatile Flash RAM allows operation for periods up to 6 months in length for 150 m water depth sampled at 1m intervals every minute. Interfacing to a real-time data link is possible for buoy-mounted installations. A 200 kHz Water Column Profiler/sup TM/ has been operating on a moored buoy in Saanich Inlet, British Columbia, since September 1999. In June, 2000, a 50 kHz unit was added. Time series of acoustic backscatter at both frequencies many months long have been collected, showing the evolution of the behaviour and abundance of the dominant species of zooplankton (Euphausiids and Amphipods) in the inlet over time scales from diurnal to seasonal. Examples of these data are presented and discussed, comparing the results at the two frequencies and the implications for acoustically discriminating size with a multi-frequency instrument.