Biological Oceanography

(^) In the eastward outflow from Drake Passage, particularly in the vicinity of South
Georgia, which is probably a local iron source, the diatom production continues right
through the summer, sustaining high secondary productivity, particularly of krill and
large stocks of whales, seals, fish (e.g. the cod-like notothenioids of the southern
hemisphere), and seabirds (e.g. penguins). Diatoms are the principal food of antarctic
krill, predominantly Euphausia superba, particularly over the outer continental
shelves (Bellingshausen Sea, the outflow from Drake Passage around South Georgia
Island, Ross Sea), but there are also stocks in the cyclonic gyres between the ACC
and coastal currents all around the shelf (Nicol 2006). Lesser numbers scatter out to
the PF. Krill group in massive schools (up to kilometers in breadth), which are as
organized, in terms of even spacing and unitary movement, as fish schools.
Paradoxically, this both protects them, at least on average, from predation, and suits
them to the gulp-and-strain feeding modes of baleen whales and crab-eater seals.
Penguins and leopard seals also depend heavily upon krill for nutrition. The whales
move in winter to the tropics to breed and calve, returning to the Antarctic to feed in
summer. Seals and penguins come and go from the ice on the continent.
(^) Krill spawn in summer (their fecundity is discussed in Chapter 8), and it is thought
that females migrate out over deep water to release eggs, possibly to avoid predation
on eggs by their own dense schools that are capable of clearing virtually all particulate
matter from the water. Possibly for the same reason, eggs sink to mesopelagic depths
before hatching, so that hatched nauplii must swim back up to layers with particulate
food by the molt to first-feeding calyptopis larvae. Krill overwinter in the marginal ice
zone (Daly 1990), along and under the ice. Juveniles and adults survive the season of
low food supply by some carnivory, by feeding on ice-associated algae, and by
progressively repackaging their tissue biomass as smaller bodies through consistently