Several species of zooplankton appear to be present literally everywhere in the Southern Ocean. The chaetognaths Sagitta gazellae and Eukrohnia hamata were found in almost all the samples examined by David (1965). The copepods Calanoides acutus, Calanus propinquus, and Rhincalanus gigas (Baker, 1954) as well as the euphausiid Thysanoessa also have a wide distribution.
Most, if not all, Antarctic zooplankton species are circumpolar in distribution, but frequently show centres of concentrations at different depths or in different latitudes. Voronina (1966,1968) reported that zooplankton may concentrate in the oceanic areas of convergence and divergence; furthermore, she found the total zooplankton biomass at the Polar Front to be 3 to 3.5 times greater than in adjacent areas.
Hydrographie conditions play an important role in the distribution of Southern Ocean zooplankton. As mentioned above, the sinking of Antarctic Surface Water at the Polar Front contributed to the Antarctic Intermediate Water which flows northward at around 800-1,000 m. Planktonic organisms retained in this
water thus extend their distribution range northward, but at a deeper level. Examples of species so entrained include Rhincalanus gigas, Calanoides acutus, Sagitta maxima, S. marri, and Eukrohnia hamata. It is not known, however, how far to the north these organisms may be distributed.
The Subtropical Convergence and the Polar Front are effective barriers to the distribution of some organisms. It is well known, however, that several species "cross" these barriers. John (1936) indicated that the distributions of some euphausiid species in the Pacific Sector are limited by either the Polar Front (e.g., Euphausia frígida) or Subtropical Convergence (E. longirostris). Mackintosh (1946) observed that the other euphausiids cross one or another of the convergences (e.g., E. triacantha) or occur apparently without relation to known oceanographic features (e.g., E. crystallorophias and E. similis).
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