Several investigators have shown that there is a seasonal change in the vertical distribution of several species (e.g., Sagitta gazellae, Euphausia superba, and Calanoides acutus) in Antarctic waters; specimens occur in the upper levels in summer and descend to deep water in winter. Hopkins (1971) noted that over half of the zooplankton biomass of the upper 1,000 m of sampled waters occurred in the top 250 m in late spring, summer, and early autumn months (November through April); while during the remainder of the year (May through October), most of the biomass was below 250 m. It is suggested that vertically migrating species, even though they move northward (and eastward) during the summer, are retained within the Antarctic circumpolar system when they descend to the deep water and are returned southward during winter, either as adults, or at an earlier stage in life development. Breeding habits appear to be associated with this seasonal migration so that species at various stages of the life cycle may inhabit different environments (water masses).
The seasonal changes of the zooplankton biomass in surface waters are characterized by two peaks. The first peak seems to result from the formation of pre-spawning concentrations of organisms following the spring migration upward in the water column. This change merely represents vertical redistribution of plankton rather than an actual increase in abundance, as can be seen from the fact that the biomass in the upper 1,000 m layer as a whole remains practically unchanged. The second peak (summer) reflects development of the new generation until the annual peak is reached. Later, the downward migration of older organisms marks a more or less sharp decline in the biomass within surface waters to a level which persists throughout the long austral winter. The summer peak of zooplankton first appears in northern latitudes and then develops further to the south. Zooplankton spawning is attuned to the marked seasonality of the primary production so that larvae are able to obtain abundant food prior to winter. During the summer in the northern region of the Southern Ocean, the zooplankton biomass peaks two months later than that of the phytoplankton. In the southern region, on the other hand, one month elapses between the peaks of phyto- and zooplankton (Hart, 1942).
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