Water mass circulation, sea ice and zooplankton abundance

Aims

The Lazarev Sea is characterised by marked bottom topography and a complex pattern of ocean circulation, which determines the inflow of water masses into the Weddell Sea and modifies the sea ice cover. Subproject 3 investigates how these physical conditions control the distribution of krill and zooplankton, spatially and temporally.

 
In detail, SP 3 is aimed at answering the following questions:

• Which spatial distribution patterns of krill and zooplankton are revealed by measurements of acoustic backscatter?
• Do these distribution patterns correlate with the general circulation of the Lazarev Sea, or is the mesoscale dynamics dominating?
• How does the diurnal vertical migration of zooplankton change meridionally and during the course of the year?
• Which impact does the seasonal sea ice cover have on the horizontal and vertical distribution of krill?
• Is the hypothesis of a dominant control of the population dynamics of krill by the advection of organisms with water masses in the Lazarev Sea confirmed by data?

Methods

Three different lines of work are followed in order to reach our aims. In each, instruments known as Acoustic Doppler Current Profilers (ADCPs) play a role, because these not only measure profiles of currents but also the strength of the backscattered echo, which can be used as an indicator of the abundance of krill and zooplankton.

1. Analysis of historical data collected with the VM-ADCP aboard RV Polarstern

During previous cruises of the ice-breaking research vessel Polarstern in the Lazarev Sea a data set of acoustic indicators of zooplankton abundance has been collected by use of the vessel-mounted (VM) ADCP, which covers a time span in excess of the last decade. The oceanographic data set collected in parallel comprises always ADCP current profiles in the top 250-300 m of the water column, measurements of surface temperature and salinity and, dependent on the main purpose of the individual cruise, temperature sections, deep CTD profiles of temperature salinity and density, and observations of the sea ice cover.

2. Time series records by moored instruments

Time series of the abundance of krill and zooplankton, which cover with a temporal resolution of a few hours at least one year including the polar winter, can be obtained by moored instruments such as SC(self-contained)-ADCPs. For this purpose we are using so-called Long-Ranger ADCPs with profiling ranges of 600 m, in an attempt to cover also the larger amplitudes of the zooplankton vertical migration. In order to reveal by the time series records also regional differences of krill abundance and behaviour, which may be caused by the circulation pattern and different lengths of the seasonal sea ice cover, we have deployed Long-Ranger-ADCPs at three different latitudes, 64° S, 66°30' S and 69° S, along the 0° Meridian. In long-term moorings maintained by the Alfred Wegener Institute other instruments are used too in order to collect information on thermohaline properties in different layers, for instance, as well as upward looking sonars for the measurement of sea ice coverage and thickness.

3. Mapping regional distribution patters in different seasons

Dedicated to the LAKRIS-Project, several expedition cruises have been conducted with Polarstern during the passed few years in Antarctic summer, autumn and winter. To reveal also aspects of inter-annual variability, another summer cruise is scheduled for the International Polar Year 2007/2008. During these surveys, the Lazarev Sea is covered by a regular grid of stations, at which a multitude of physical an biological data is collected.

All research activities are performed in close cooperation with the other sub-projects of LAKRIS. For instance, biological data gathered in sub-projects SP 1 and SP 2 by the deployment of nets are used for the calibration of the ADCP backscatter measurements. Vice versa, SP 3 delivers the physical-oceanographic expertise needed by the other sub-projects for their data interpretation.