National Geographic : 2009 Dec
A squadron of gentoo penguins (above) wings through the waters of Drygalski Fjord. Unlike true Antarctic penguins, the gentoo prefers more temperate climes. As oceans warm, this species is prospering, expanding its range southward. But there's a catch: Swarms of krill (left), the gentoo's primary food and a cornerstone of the entire Antarctic food chain, feed on phytoplankton that grows on the underside of sea ice. In recent years ice cover has been shrinking in waters west of the Antarctic Peninsula---source of South Georgia's krill.