A series of measurements taken in the Antarctic Ocean has enhanced our understanding of the vertical migrations of zooplankton communities both during the course of a year and within the range of several years. The Antarctic zooplankton is the most important food source for many fish and whales.
For the first time, researchers documented the seasonal migrations of zooplankton communities in the Southern Ocean continuously over three years. Using data from moored Acoustic Doppler Current Profilers (ADCP), Dr Boris Cisewski from the Thünen Institute of Sea and Dr Volker Strass from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research investigated the influence of environmental parameters such as light, sea ice cover, food availability, circulation, temperature and salinity migratory patterns on the seasonal migration.
The vertical migration of zooplankton is triggered by the daily daytime-nighttime cycle. At dawn, they dive down to the ocean depths where it is dark to avoid potential predators. After the sun sets, they ascend to the upper layers of the ocean to feed on the plant plankton. To date, there have been only minor temporal clippings about zooplankton migration in the Southern Ocean. Due to the seasonal sea ice cover, research ships cannot reach many areas during the austral winter, leading to
scant data about the biological network during this time of year.
The current study is based on the data collected during three Polarstern expeditions and deep-sea moorings in the Southern Ocean in 2005 and 2008. ADCP devices were fixed at three different locations along the Greenwich meridian; they were programmed to emit sound waves at fixed time intervals and were able to detect up to 500 metres below the surface. Researchers used the strength of the echoes to determine the concentration of the zooplankton, and the frequency of the Doppler effect to calculate the migration rate.
The researchers examined both the daily and seasonal vertical migration patterns of the zooplankton, and discovered that their migration took place from the austral summer in November to a few weeks into January. During this time, the zooplankton would abandon the safety of the ocean depths to feed on the large amount of food at the water surface.
Further evidence of this can be found in the measurements of the sea ice cover and chlorophyll distribution. As the ice melts, it leads to the formation of algal blooms, the main food source for the zooplankton during this time of the year.
The difference in the zooplankton population over the three years is currently unclear. Hence, according to the researchers, further physical and biological measurements are required, so that the future changes in the ecosystem – those induced by climate change – of the Southern Ocean can be estimated.
Link to study: www.sciencedirect.com/.../S007966111630057X