The Southern Ocean consists of deep basins separated from each other by three large undersea mountain ridges:
- “Macquarie Ridge” south of New Zealand and Tasmania;
- Kerguelen–Gaussberg Ridge positioned at approximately 80°E; and
- Scotia Ridge which stretches from the coastal shelf at South Patagonia arching eastward toward the South Shetland Islands and islands of the Antarctic Ocean.
The Antarctic Circumpolar Current (ACC) is an ocean current that transports 90-140 Sverdrup (Sv) of polar water from west to east around Antarctica through the Drake Passage. One Sv equals 1 million m3/sec of water being transported, and corresponds to the volume of water being transported by all the world’s rivers combined. Among its other dynamic functions, the ACC serves as a powerful transport mechanism (“conveyor belt”) for krill.
The Front occurs where cold Antarctic waters meet warmer water masses from the North. As it encircles the entire Antarctic continent, the Southern Ocean is clearly distinguishable from adjacent sea areas on the Polar Front.
The Polar Front functions as an effective biological barrier. Accordingly, the Southern Ocean is a relatively closed ecosystem. The Southern Ocean can be divided into three ecological zones:
- An ice-free spawning zone situated between the Polar Front and the northern extent of pack-ice during Winter;
- A middle-zone between the limits of pack-ice during Winter and Summer/Fall; and
- An inner zone with permanent ice encircling the Antarctic continent.
Most productive is the middle zone, which is preferred habitat for the ecologically all-important krill. Within the relatively simple food chain of the Southern Ocean, krill is a key species and an important food source for many species of fish, squid, bird, penguin, seal, and whale.