Environmental Engineering Reference
In-Depth Information
industrialization and globalization have brought in many
more pressures on arctic ecosystems since 1970.
Photo: Ken Atkinson.
in June, July and August, when climate conditions (light, temperature) are favourable,
and when the retreat of sea ice allows more primary production in open water. The
average primary production is 60 g carbon m
−2
yr
−1
, but there are large variations,
depending on ocean currents and nutrients. A restricted number of amphids, copepods
and bivalves make up the herbivore compartments of the food web. A critical role in the
ecosystem is played by Arctic cod, with 125,000 tonnes being consumed by marine
mammals and 23,000 tonnes by sea birds annually. The fish consume micro-sized
animals and concentrate the energy into larger 'packets' which can be eaten efficiently by
seal, whales and birds. In Figure 24.6 the links in the food web which relate to Arctic cod
are highlighted. It is clear that the stability of the entire functioning of the ecosystem is
dependent upon this one species. The fragility of the Arctic relates to the concentration of
energy flow through a very restricted number of species. Thus variations in the
populations of seals, whales and polar bears from year to year are due to changes in the
abundance of Arctic cod, which in turn will reflect fluctuations in the physical
environment (temperature, ice extent, nutrients).
The trophic-dynamic model shown in Figure 24.6 is incomplete, owing to the
difficulty of measuring the activities of bottom-dwelling organisms, the migration of fish
and marine mammals into and out of Lancaster Sound, and the size of winter populations.
For these reasons energy-flow studies of oceans and seas are approximate at best.
A major control of polar marine ecosystems is the extent of sea ice. Ice-covered water
has low light penetration, a low biomass of primary producers and zoo-plankton, low
productivity and low rates of sedimentation. The cover of sea ice varies from 14 million
km
2
to 7 million km
2
in the Arctic, and from 20 million km
2
to 4 million km
2
in the
Antarctic. Even in the depths of winter, currents, winds and upwellings keep some areas
ice-free either as leads or as large patches of open water (
polynyas
). These areas are
important ecologically, as they allow light to support phytoplankton and hence maintain a
more productive food web of zooplankton, fish, sea mammals and birds. Such marine
oases show productivity rates comparable with those of the North Sea.
Arctic landscapes can also be regarded as unstable from two other viewpoints, in
addition to the concentration of
