Geoscience Reference
In-Depth Information
Immediately offshore of the shelf break lie the highest concentrations of krill
which, not coincidentally, are still within the foraging range of the seals and
penguins that breed on the continent (or on Antarctic or sub-Antarctic islands).
The zone dominated by krill may extend for several hundred kilometres, as in the
southeast Atlantic, or may be closely restricted to the coast, as it is in the southeast
Indian Ocean. There is an intimate relationship between the distribution of krill and
the physical environment
particularly the currents and the distribution of sea ice,
which is itself affected by the currents. The two current systems of the Southern
Ocean
-
the broad Antarctic Circumpolar current (ACC) and the narrower,
westwards
-
flowing, coastal current interact in a series of gyres, whose location is
dictated by the submarine topography and by the shape of the coastline. The
frontal systems that delineate these currents can also form the borders to oceanic
communities. Between the two currents lie the Southern Boundary of the ACC and
the Southern ACC Front, and these features probably mark the northernmost extent
of the krill-based ecosystem in most regions of Antarctica. They often coincide with
the maximum extent of sea ice in winter. Not surprisingly, the majority of krill-
feeding vertebrates are found south of these fronts.
Krill are not the only oceanic herbivore, and representatives of most of the common
worldwide groups of pelagic organisms are to be found in the waters that surround
Antarctica. Krill compete with
copepods, gelatinous
zooplankton, worms, molluscs and microscopic consumers. The diversity of organisms
in the pelagic zone is actually lower than is found in other productive upwelling zones
of the ocean and many species are slow growing and longer lived.
-
and probably consume
-
Birds
The most studied species in the Antarctic are undoubtedly the penguins, of
which four species (emperor, Adélie, chinstrap and gentoo) breed on and around
the continent and its nearby islands, with other species such as the king, macaroni
and royal penguin on the sub-Antarctic islands. The bird populations in Antarctica
number many millions with many of them dependent on krill and other oceanic
prey for their principal food. This makes seabirds a good indicator of changes in
the marine ecosystem, and research programmes exist which monitor variability and
change in seabird populations that can be associated with, for example,
fisheries
activities and environmental change. There is evidence that more temperate species,
such as Gentoo penguins, are breeding further south along the Antarctic Peninsula
than previously, and numbers of the truly cold-adapted penguins
-
Adélie and
emperor
may well be declining in the northern parts of their range. Of the eight
petrel species found, four
-
-
Antarctic petrel, snow petrel, southern giant petrel and
Wilson
'
s storm-petrel
-
breed on the continent whilst the remainder breed in
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