Huge though bergs may be, they are nonetheless usually on the move. But come the winter, sea ice forms around them and locks them solid. As winter progresses, so more and more of the sea freezes, spreading out from the margins of the land like an immense skirt so that, in effect, the continent doubles in size. When the ice reaches its farthest extent, you have to travel hundreds of miles from the edge of the continent before you reach open water.

The annual formation of the sea ice is the greatest seasonal change that takes place on this planet and it completely dominates the lives of Antarctic animals. Practically all of them are directly dependent upon the sea for their food, so year-round access to it is essential for their survival. In the summer, when the sea ice melts, they can reach the islands that were trapped in the ice and eventually the continent itself. But when the ice re-forms, they have to retreat north. So now, in winter, with the sea ice at its fullest extent, it's in the sea that we must look for life.

The southern ocean is extremely rich in food. Millions of penguins and seals and thousands of whales feed here. The majority of them rely on just one source of food-krill. Krill are small shrimp-like creatures about 6 centimeters long. In winter, they are dispersed widely, mostly under the ice, but in summer they assemble in vast swarms, some of which may contain a billion individuals. They are the most numerous animals on earth. Their total weight far exceeds that of the total human population.

words and expressions

year-round : (a.) Existing, active, or continuous throughout the year全年的

krill: (n.) The collection of small marine crustaceans of the order Euphausiacea that are the principal food of baleen whales. 磷蝦磷