Time-frequency analysis of migrating zooplankton in the Terra Nova Bay polynya (Ross Sea, Antarctica)

An upward-looking 150 kHz narrow-band Acoustic Doppler Current profiler was operated in Terra Nova Bay (Ross Sea, Antarctica) from5 February 2000 to 16 January 2001 to monitormarine currents. The instrument sampled the upper 160mof thewater columnwith a time resolution of 1 h. Although the experimental setup was not specifically designed to assess zooplankton and fish distributions and behaviour, the Acoustic Doppler Current Profiler ancillary data provided useful information regarding the diel verticalmigration of these acoustic targets. A time frequency analysis of the mean backscatter strength time series was conducted using a 240 h-wide windowwith a 1 day step. Assuming that the 24 h period peak is associatedwith zooplankton diel vertical migration, the amplitude of the power spectral energy on this bandwas extracted fromeach spectrumand the timeseries of amplitudes was analysed. Themigration signalwas veryweak during summer, December to January, butwas evident at the beginning and end of the polar night. Interestingly, the results indicated four "migratory blooms," the first at the end of August and the others approximately every three weeks subsequently, ending at the end of October. The daily migration was found to have a good relation with the solar cycle, while it was apparently uncorrelated with themoon phase.Migration patterns in the upper and the lower ocean layers displayed significant differences. Due to the lack of contemporary in-situ net samples, the results are more qualitative than quantitative; nonetheless, they demonstrate the validity of themethod to extract relevant information evenwhen applied to data obtained from a non-devoted low-resolution system. This may be of particular interest in polar areas where it is difficult to performcontinuous biologicalmonitoring but where a long time series of Acoustic Doppler Current profiler data is available.