Composition

The copepods constitute the bulk of the Antarctic mesoplankton (middle-sized zooplankton), accounting for an average of about 70% of its total biomass (Fig. 8.14) The three most prevalent species are Calanoides acutus, Calanus propinquus, and Rhincalanus gigas. Hopkins (1971) contended that the principal contributors to zooplankton biomass in samples taken in the Pacific Sector of the Southern Ocean are the copepods, chaetognaths and euphausiids, in that order. He found that the copepods constitute 67.7 and 59.6% of the zooplankton standing crop in the Antarctic and sub-Antarctic regions, respectively. Russian investigators found similar percentages (72.8 and 61.5%) for the Antarctic and sub-Antarctic copepods in the Indian Sector (Voronina, 1966). The paucity of euphausiids in Hopkins' plankton samples can be attributed to the fact that his collections were made further north in the West Wind Drift, in areas not frequented by the krill. Russian investigators found similarly low numbers of krill because they used a type of net (Juday net) which is poorly suited to capturing adult krill.

The chaetognaths (arrow worms) are an important ecological group because of their abundance among the zooplankton, second only to copepods, and because of their role as carnivorous predators. In both abundance of individuals and frequency of occurrence, Eukrohnia hamata is the most common of the 18 species of chaetognaths which are found in both the Antarctic and sub-Antarctic regions.

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