Pioneering cruises into the pack ice of the Southern Ocean reveal secrets of its fertile ecosystem
At the extreme end of the Earth, Antarctica is a vast, rocky continent, mostly ice-covered and barren. Surrounding Antarctica, the Southern Ocean is equally vast, cold, and ice-covered. But unlike the land, it teems with life, ranging from microscopic plankton to top predators: whales, seals, penguins, fish, and sea birds.
The region's fecundity is fueled by 24hour-a-day sunlight in summer, combined with ocean currents that bring essential nutrients. These provide the ingredients for rich blooms of microscopic marine plants and animals at the base of the food chain-phytoplankton and zooplanktonthat are similar to those in many productive regions of the world's oceans.
But there is one big difference in the Antarctic ecosystem. The food moves swiftly to the very top of the chain through a crucial link: a shrimp-like crustacean called krill, which swarm in great pink oceanic patches that range from tens of square meters to tens of square kilometers. The krill connect the microscopic primary producers, which they eat, to the top predators, which eat them.
This unique and unusually short oceanic food chain is both strong and vulnerable. It efficiently supports large populations of big animals. But a small disruption in the chain could drastically affect the entire ecosystem.
Adding urgency are recent indications of changing conditions around Antarctica-particularly more frequent calving of massive icebergs from the continental ice shelf. To manage and protect this unique environment, we need a more thorough understanding of the intricacies of the ecosystem and the potential effects of climate change on it.
Krill are the glue that binds the Antarctic food web, and 20th-century expeditions learned a great deal about their life stages, distribution, and abundancebut only during the warmer, sunlit, icetree periods of the year. How do adult and larval krill survive the frigid, sunless winter-when photosynthesis diminishes essentially to zero and much of the ocean is covered with pack ice-to become an abundant food source for large animals the next spring? To pull back the veil on this critical and previously shrouded part of the ecosystem, we undertook 11 cruises to the Southern Ocean, including four unprecedented voyages into the Antarctic winter ice pack.
Destination: Marguerite Bay
The cruises were part of the Global Ocean Ecosystem Dynamics Program (GLOBEC), a multiyear, multination, multidisciplinary series of investigations of several touchstone regions throughout the world's oceans where marine life and fisheries historically thrive. Marshalling scientists across several disciplines, GLOBEC sought to define and measure the many factors-oceanic currents, climatic conditions, seafloor topography, biological processes, and others-that converge to create and maintain productive ecosystems. GLOBEC also seeks to provide information on the vulnerability of ocean ecosystems to climate changes.
Fieldwork for the Southern Ocean GLOBEC program, conducted between 2001 and early 2003, focused on a broad and relatively deep (300 to 400 meters) continental shelf region off the western Antarctic Peninsula, due south of the tip of South America, from Adelaide Island to Charcot Island. In between lies Marguerite Bay, which supports a large, persistent stock of krill and large populations of top predators that depend on it for food. We suspect that this area may act as a reservoir for maintaining krill stocks hundreds of miles away in the Scotia Sea, as far as South Georgia Island.
Marguerite Bay is surrounded landward by high, snow-covered mountains and seaward by huge ice shelves. It is dotted by numerous small islands and persistently covered by sea ice in winter. Below the sea surface, the bay is gouged by a trough that cuts diagonally across the continental shelf and ends in fjord-like features up to 1,600 meters deep in the interior of the bay. …