Acoustic insights into the zooplankton dynamics of the eastern Weddell Sea

Abstract

The success of any efforts to determine the effects of climate change on marine ecosystems depends on understanding in the first instance the natural variations, which contemporarily occur on the interannual and shorter time scales. Here we present results on the environmental controls of zooplankton distribution patterns and behaviour in the eastern Weddell Sea, Southern Ocean. Zooplankton abundance and vertical migration are derived from the mean volume backscattering strength (MVBS) and the vertical velocity measured by moored acoustic Doppler current profilers (ADCPs), which were deployed simultaneously at 64°S, 66.5°S and 69°S along the Greenwich Meridian from February, 2005, until March, 2008 While these time series span a period of full three years they resolve hourly changes.A highly persistent behavioural pattern found at all three mooring locations is the synchronous diel vertical migration (DVM) of two distinct groups of zooplankton that migrate between a deep residence depth during daytime and a shallow depth during nighttime. The DVM was closely coupled to the astronomical daylight cycles. However, while the DVM was symmetric around local noon, the annual modulation of the DVM was clearly asymmetric around winter solstice or summer solstice, respectively, at all three mooring sites. DVM at our observation sites persisted throughout winter, even at the highest latitude exposed to the polar night. Since the magnitude as well as the relative rate of change of illumination is minimal at this time, we propose that the ultimate causes of DVM separated from the light-mediated proximal cue that coordinates it. In all three years, a marked change in the migration behaviour occurred in late spring (late October/early November), when DVM ceased. The complete suspension of DVM after early November is possibly caused by the combination of two factors: (1) increased availability of food in the surface mixed layer provided by the phytoplankton spring bloom, and (2) vanishing diurnal enhancement of the threat from visually oriented predators when the illumination is quasi-continuous during the polar and subpolar summer. 3Zooplankton abundance in the water column, estimated as the mean MVBS in the depth range 50 -300 m, was highest end of summer and lowest mid to end winter on the average annual cycle. However, zooplankton abundance varied several-fold between years and between locations. Based on satellite and in situ data of chlorophyll and sea ice as well as on hydrographic measurements, the interannual and spatial variations of zooplankton mean abundance can be explained by differences in the magnitude of the phytoplankton spring bloom, which develops during the seasonal sea ice retreat. Whereas the vernal ice melt appears necessary to stimulate the blooming of phytoplankton, it is not the determinator of the blooms magnitude, its areal extent and duration. A possible explanation for the limitation of the phytoplankton bloom in some years is top-down control. We hypothesize that the phytoplankton spring ...

The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

“…For a more complete understanding of phytoplankton productivity and distribution, we suggest the following approaches: Extend the sampling to include fall and winter and to under‐ice environments with year‐round mooring or ice‐tethered buoy deployments, even in permanently ice‐covered areas. In order to disentangle the relative importance of bottom‐up and top‐down controls and to reveal possible climatic changes, time series measurements are needed (Cisewski & Strass, ). Sediment traps and other autonomous water samplers can collect ice algae and phytoplankton as sinking cells, marine snow and zooplankton fecal pellets (Turner, ). These platforms could potentially also be equipped with water sampling devices, camera systems, fluorescence, and nutrient sensors, as well as other probes which would provide a better understanding of mixed layer and upper water column processes across seasons.…”

“…For example, the backscatter signal recorded by moored acoustic Doppler current profilers enables the estimation of zooplankton abundance. Using this technique, Cisewski and Strass () reported highest zooplankton abundance in the WG at the end of summer and lowest abundances during winter, following a seasonal pattern similar to that of phytoplankton. Furthermore, these authors found that DVM, characteristic of zooplankton, persisted throughout the austral night, with annual modulation centered at local noon in winter and summer solstices.…”

Under the sea ice: Exploring the relationship between sea ice and the foraging behaviour of southern elephant seals in East Antarctica

“…Shallow dives observed in high sea ice concentration close to the Antarctic coast (10 ± 6% of the total dives for males) could correspond to specific foraging activity associated with the rich under-ice community of fish and invertebrates (Ainley et al, 1991). Moreover, these dives were mostly performed at night, where the diurnal vertical migration of adult krill (Euphausia crystallorophias), more pronounced in winter than summer (Siegel 2012;Flores et al, 2012b;Cisewski and Strass 2016) might attract various preys, such as Pleuragramma antarcticum (Fuiman et al, 2002).…”

Impact of tidal dynamics on diel vertical migration of zooplankton in Hudson Bay

“…(1) predator avoidance by staying away from the illuminated surface layer during the day and thus reducing the lightdependent mortality risk (Hays, 2003;Ringelberg, 2010;Torgersen, 2003) and (2) optimization of feeding, with the assumption that algal biomass is greater in the surface layer during evening hours and zooplankton rise to feed on it in the evening (Lampert, 1989). There are three general DVM patterns: (1) the most common one is nocturnal when zooplankton ascend around sunset and remain at upper depths during the night, descending around sunrise and remaining at depth during the day (Cisewski et al, 2010;Cohen and Forward, 2002). (2) Then there is a reverse pattern when zooplankton ascend at dawn and descend at dusk (Heywood, 1996;Pascual et al, 2017).…”