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input of large rivers that influence the nutrient regimes and their differences.
In the Southern Ocean, the influence of freshwater, mostly from glaciers, is
much smaller.
The Arctic is currently experiencing some of the most rapid and
severe climate changes on Earth. Some effects of increasing temperature
on marine ecosystems are already evident ( Rosenzweig et al., 2008 ). Over
the next 100 years, warming is expected to accelerate, contributing to
major physical, ecological, social and economic changes. In fact, all models
forecast reduction of cold areas and expansion of the warmer ones, with
consequent threat for cold-adapted organisms. However, there is still little
understanding regarding how the loss of a species or groups of species will
affect ecosystem services.
The most important Arctic characters include seasonality in light, cold
temperatures with winter extremes, and extensive shelf seas around a deep
central ocean basin. The Arctic comprises a vast ocean surrounded by the
northern coasts of three continents, open to influx of warm water from
the Atlantic and, to a lesser extent, from the Pacific. The “permanent
cap” of ice, composed of multi- and first-year ice that forms annually and
extends and retreats seasonally, is probably the most important feature of
Arctic marine systems ( Polyak et al., 2010 ). The major decline in sea ice that
began to take place in the Arctic since 2000 is the most important climate-
change signal ( Comiso, Parkinson, Gersten, & Stock, 2008 ). Expectations
are that summer sea ice will continue to decline in the future. Climate
models have indicated that the Arctic Ocean might essentially be ice-free
during summer by the later half of the twenty-first century ( Overpeck
et al., 2006; Wang & Overland, 2009 ), with dramatic and potentially dev-
astating effects on a number of species associated with the sea ice ( Moline
et al., 2008 ) and significant biological consequences ( Clarke et al., 2007 ).
In terms of constantly low temperatures, the southern polar environment
is considered the most extreme on our planet. Antarctica has the capacity to
influence the Earth's climate and ocean-ecosystem function, and from this
standpoint, it is the world's most important continent. As such, its palaeo-
and current geological and climatic history, physical and biological
oceanography, as well as marine and terrestrial ecosystems have been—
and are—the target of a wealth of studies. As a frozen deep mass of ice, most
of Antarctica reflects the sun's radiation, buffering global warming trends.
The current lack of warming is also due to the shielding effect produced
by the stratospheric winds driven by the human-induced Ozone Hole.
These winds generate a Polar Vortex extending to the surface that acts as
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