Meridional and interannual variations of the seasonally modulated Zooplankton diel vertical migration in the Lazarev Sea and their possible physical-biological controls

Acoustic Doppler current profilers (ADCPs) not only take measurements of the water velocity components but also of the backscatter strength, which can be used to infer distribution patterns of zooplankton and small nekton. Here we present results obtained from ADCPs that were moored at 64°S, 66.5°S and 69°S along the Greenwich Meridian during the three-years period February 2005 until March 2008. A diel vertical migration (DVM) pattern – downward at dawn and upward at dusk - of two distinct groups of migrators persisted during most part of the years at all moorings sites, closely related to the astronomical daylight cycles. While the DVM was symmetric around local noon, the annual modulation of the DVM was asymmetric relative to the summer/winter solstices at the three mooring sites. This annual asymmetry resulted from a change in the migration behaviour that occurred in late spring (October - November), when the DVM ceases for around three months. In contrast to many previous studies in other regions, DVM at our observation sites persisted throughout winter, even at the highest latitude during the polar night. Using in-situ physical and biological data collected during deployment and recovery of the moorings, ice-thickness time series measured by Upward Looking Sonars, and satellite maps of remotely sensed sea ice coverage and surface chlorophyll concentration we can explain part of the seasonal to interannual variations in the inferred zooplankton distribution patterns by environmental cues. For a more complete explanation of the observed organisms’ behaviour, however, we hypothesize controls by internal drivers that need also be taken into account

Acoustic insights into the zooplankton dynamics of the eastern Weddell Sea

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. Zooplankton 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 hypothesise that the phytoplankton spring development can be curbed by grazing when the zooplankton had attained high abundance by growth during the preceding summer