Seasonal variation of diel vertical migration of zooplankton from ADCP backscatter time series data in the Lazarev Sea, Antarctica

Abstract

a b s t r a c tTen-month time series of mean volume backscattering strength (MVBS) and vertical velocity obtained from three moored acoustic Doppler current profilers (ADCPs) deployed from February until December 2005 at 641S, 66.51S and 691S along the Greenwich Meridian were used to analyse the diel vertical zooplankton migration (DVM) and its seasonality and regional variability in the Lazarev Sea. The estimated MVBS exhibited distinct patterns of DVM at all three mooring sites. Between February and October, the timing of the DVM and the residence time of zooplankton at depth were clearly governed by the day-night rhythm. Mean daily cycles of the ADCP-derived vertical velocity were calculated for successive months and showed maximum ascent and descent velocities of 16 and -15 mm sHowever, a change of the MVBS pattern occurred in late spring/early austral summer (October/ November), when the zooplankton communities ceased their synchronous vertical migration at all three mooring sites. Elevated MVBS values were then concentrated in the uppermost layers ( o 50 m) at 66.51S. This period coincided with the decay of sea ice coverage at 641S and 66.51S between early November and mid-December. Elevated chlorophyll concentrations, which were measured at the end of the deployment, extended from 671S to 651S and indicated a phytoplankton bloom in the upper 50 m. Thus, we propose that the increased food supply associated with an ice edge bloom caused the zooplankton communities to cease their DVM in favour of feeding.

Seasonal changes in the vertical distribution and community structure of Antarctic macrozooplankton and micronekton

“…Similar limitations apply to other studies using acoustic sensors (Brierley et al, , 1998Zhou et al, 1994), as well as net-based investigations integrating the upper 50-300 m in a single density figure (Boysen-Ennen and Piatkowski, 1988;Fisher et al, 2004;Lancraft et al, 2004;Siegel, 2012). Within the range of uncertainty that is associated with the limitations of the acoustic method in resolving the near-surface layer and in discriminating species, the results of this study are however consistent with the DVM annual time series of Cisewski et al (2010). Taking DVM into account, our results provide evidence for 3 strategies of seasonal use of the surface layer:…”

“…Considerable diel differences in the density of many abundant zooplankton species in the surface layer, however, indicated a decrease in amplitude rather than a pause of DVM during summer (Flores et al, 2011(Flores et al, , 2012. Such changes in the upper metres of the water column could not be detected by Cisewski et al (2010), because they did not sample above 20 m water depth. Similar limitations apply to other studies using acoustic sensors (Brierley et al, , 1998Zhou et al, 1994), as well as net-based investigations integrating the upper 50-300 m in a single density figure (Boysen-Ennen and Piatkowski, 1988;Fisher et al, 2004;Lancraft et al, 2004;Siegel, 2012).…”

“…Acoustic backscatter data showed that zooplankton aggregated at discrete horizontal depth horizons rather than being randomly dispersed in the water column (Brierley et al, 1998;Cisewski et al, 2010). Considering that, without a statistical preference for any specific depth layer, mean areal surface densities would range at $ 1% of mean depth-integrated epipelagic densities, the observed values indicate a pronounced concentration of animals in the surface layer.…”

“…This variability largely reflected the seasonal variability of the upper limit of their DVM. Cisewski et al (2010) (Siegel, 2012). Cisewski et al (2010) assumed that in summer DVM ceased largely, because the bulk of backscattering zooplankton remained in the upper 50 m throughout the day/night cycle.…”

“…Cisewski et al (2010) (Siegel, 2012). Cisewski et al (2010) assumed that in summer DVM ceased largely, because the bulk of backscattering zooplankton remained in the upper 50 m throughout the day/night cycle. Considerable diel differences in the density of many abundant zooplankton species in the surface layer, however, indicated a decrease in amplitude rather than a pause of DVM during summer (Flores et al, 2011(Flores et al, , 2012.…”