The annual retreat and melting of sea ice in
the austral spring stratifies the upper ocean, triggering large phytoplankton blooms.
The magnitude of the blooms is proportional to the winter extent of ice cover,
which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant
metazoan populations on Earth, consume phytoplankton blooms dominated by large
diatoms. Krill, in turn, support a large biomass of predators, including penguins,
seals, and whales. Human activity has altered even these remote ecosystems. The
western Antarctic Peninsula region has warmed by 7°C over the past 50 years, and
sea ice duration has declined by almost 100 days since 1978, causing a decrease in
phytoplankton productivity in the northern peninsula region. Besides climate change,
Antarctic marine systems have been greatly altered by harvesting of the great whales
and now krill. It is unclear to what extent the ecosystems we observe today differ
from the pristine state.