Long zooplankton time series with high temporal and spatial resolution

Long time series of acoustic sounding data present the opportunity to examine vertical distribution and behavior of zooplankton and other organisms. In this paper, we describe a small, inexpensive, single frequency (200 kHz), upward-looking water column profiler that is mounted as part of the VENUS cabled observatory in Saanich Inlet, British Columbia, Canada, and introduce the concept of depth-time `cubes' for thinking of long time series of acoustic data. Instrument calibration is discussed, and data are calibrated to absolute volume backscattering strength. A two-year long time series of daytime scattering measurements is presented to illustrate seasonal changes at this location. We also show that with simple algorithms and uncalibrated data at a single frequency, we can track seasonal changes in the timing of zooplankton diel vertical migration with high temporal resolution. The diel vertical migration is shown to relate very closely to sun declination as expected, but the simple algorithm used to find the timing was often confused by fish and physical water movements. A more intelligent algorithm taking advantage of the high temporal resolution of the data is required.