Time-frequency analysis of migrating zooplankton in the Terra Nova Bay polynya (Ross Sea, Antarctica) Paola Picco a, ⁎, M. Elisabetta Schiano b , Sara Pensieri c , Roberto Bozzano c a Istituto Idrografico della Marina, Passo dell'Osservatorio 4, 16134 Genova, Italy b National Research Council of Italy, Institute of Marine Science, Via de Marini 6, 16149 Genoa, Italy c National Research Council of Italy, Institute of Intelligent Systems for Automation, Via de Marini 6, 16149 Genoa, Italy abstract article info Article history: Received 29 October 2015 Received in revised form 14 July 2016 Accepted 15 July 2016 Available online xxxx An upward-looking 150 kHz 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 sam- pled the upper 160 m of the water column with 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 vertical migration of these acoustic targets. A time frequency analysis of the mean backscatter strength time series was conducted using a 240 h-wide win- dow with a 1 day step. Assuming that the 24 h 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 ev- ident 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 un- correlated 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 quantita- tive; 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. © 2016 Published by Elsevier B.V. Keywords: ADCP Polynya Spectral analysis Zooplankton migration Antarctica Ross Sea Terra Nova Bay 1. Introduction Terra Nova Bay is located on the western side of the Ross Sea, bounded on the south by the Drygalski Ice Tongue and characterized by the presence of a recurrent, latent heat polynya with a mean size of approximately 6000 km 2 (Kurtz and Bromwich, 1985; Van Woert, 1999) that persists during winter. This area is of particular interest for climatic studies because dense water formed during winter, the High Salinity Shelf Water (HSSW), contributes to the Antarctic Bottom Water (ABW) that is part of the global thermohaline circulation (Assmann and Timmermann, 2005; Jacobs, 2004; Jacobs et al., 1985). It is also host to an important nursery of the Antarctic silverfish (Pleuragramma antarcticum)(Vacchi et al., 2012), a colony of Adélie Penguins (Pygoscelis adeliae) in Adélie Cove, and the large Emperor Penguin (Aptenodytes forsteri) reserve at Cape Washington (Kooyman et al., 1990). Due to its high ecological value, Terra Nova Bay is an Antarctic Special Protected Area (Antarctic Treaty Secretariat, 2003). For all of these reasons, the area has been the object of scientific inves- tigations since the beginning of the Italian Antarctic Program and was selected as the location for the Antarctic scientific base Mario Zucchelli Station. Polynyas are special areas for polar marine life because they are al- most entirely free from ice and, at the end of the polar night, solar radi- ation immediately penetrates the water, producing early warming and irradiance that can stimulate relatively early seasonal phytoplankton production (Tremblay et al., 2007). This high primary productivity sus- tains a food-rich area for higher trophic levels (Hopkins, 1987; Karnowsky et al., 2007), attracting animals as large as marine mammals, which also take advantage of these ice-free areas for breathing. Zooplankton can be regarded as the trophic link between primary producers and higher trophic levels. Despite their importance, studies on this fundamental component of the Antarctic ecosystem are still lim- ited, largely due to the lack of a long time series with continuous data. In fact, for most of the year, the sea-ice coverage does not allow for in-situ sampling. In the Ross Sea and in Terra Nova Bay, several experimental Journal of Marine Systems xxx (2016) xxx–xxx ⁎ Corresponding author. E-mail address: paola.picco@persociv.difesa.it (P. Picco). MARSYS-02855; No of Pages 12 http://dx.doi.org/10.1016/j.jmarsys.2016.07.010 0924-7963/© 2016 Published by Elsevier B.V. Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys Please cite this article as: Picco, P., et al., Time-frequency analysis of migrating zooplankton in the Terra Nova Bay polynya (Ross Sea, Antarctica), J. Mar. Syst. (2016), http://dx.doi.org/10.1016/j.jmarsys.2016.07.010