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

Abstract

An upward-looking 150kHz narrow-band Acoustic Doppler Current profiler was operated in Terra Nova Bay (Ross Sea, Antarctica) from 5 February 2000 to 16 January 2001 to monitor marine currents. The instrument sampled the upper 160m of the water column with a time resolution of 1h. 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 vertical migration of these acoustic targets. A time frequency analysis of the mean backscatter strength time series was conducted using a 240h-wide window with a 1day step. Assuming that the 24h period peak is associated with zooplankton diel vertical migration, the amplitude of the power spectral energy on this band was extracted from each spectrum and the time series of amplitudes was analysed. The migration signal was very weak during summer, December to January, but was 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 the moon 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 the method to extract relevant information even when 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 perform continuous biological monitoring but where a long time series of Acoustic Doppler Current profiler data is available.