By Karen Haberman

Antarctic krill (Euphausia superba) are much more than mere whale food. These small, shrimp-like animals are incredible survivors. They may live up to six years, a remarkable old age for such a small animal. Also, they look delicate, but are strong for their size. They can swim against fairly strong currents, using five pairs of small legs (called pleopods) on the underside of their tail-like abdomen to propel themselves through the water, and can even flip their tail for a quick direction change.

Krill are efficient at feeding on phytoplankton (small plant-like cells). The krill sweep the water with bristled feeding legs which form a meshed basket around their mouth. The phytoplankton get caught in the mesh when the water is strained out. This is much the same method as baleen whales use to feed on krill! When there is lots of phytoplankton in the water, the krills’ packed guts turn very dark green.

Phytoplankton is scarce during the winter but, luckily, the adults can survive without eating for many months. Unlike the adults, young krill must eat during the winter or they will die. We aren’t sure where they get their winter food, but divers sometimes see them on the underside of ice, scraping off the ice algae which grows there. This may be the key to their winter survival.

In the waters surrounding Antarctica, krill are abundant and are food for many creatures. Krill often school, and their schools can include thousands, or even millions, of individuals! Most species of birds and mammals in the Southern Ocean, including humpback and minke whales, feed on krill. Some of them, like Adélie penguins and crabeater seals, dine almost exclusively on krill for at least part of the year. Clearly, krill are important for the survival of many Southern Ocean animals.

Because of their importance, Antarctic krill have been the focus of many studies. My graduate advisors, Dr. Robin Ross and Dr. Langdon Quetin, have studied krill for over ten years, focusing on their survival, growth and reproduction. Recently, our research group joined forces with several other biologists and oceanographers in a project titled “The Antarctic Ecosystem: An Ice-Dominated Environment,” one of the National Science Foundation’s eighteen Long-Term Ecological Research (LTER) projects.

The key study organisms are phytoplankton, ice algae, krill, Antarctic silverfish, Adélie penguins and south polar skuas. (Skuas are hawk-like or falcon-like sea birds who take food from gulls and terns or from the water.) They were chosen because of their abundance and importance in the Antarctic food web. While much has been learned by studying individual parts of this ecosystem, we hope this joint venture will allow us to understand how these pieces fit together.

As the title of our project suggests, we believe that ice is important in the life cycles of Antarctic marine organisms. Sea ice extends outward from the Antarctic continent, covering a large part of its surrounding ocean. During winter in the southern hemisphere, the ice cover is fairly thick and continuous. The ice melts and recedes during the spring and summer, shrinking to about half of its winter extent. As the ice melts and the day length increases during the spring, conditions at the ice edge become ideal for the growth of phytoplankton. This event is important to the krill, which feed upon phytoplankton, as well as to other creatures who feed either on the phytoplankton or on the krill.

Not only does the ice cover change from winter to summer but the amount of ice formed can vary greatly between years. We hypothesize that this year-to-year difference in ice cover is an important cause of year to-year differences we see in survival and reproduction of krill and other organisms. For example, previous experiments in the lab suggest that the krill’s food is linked to the amount of ice present. Thus, the health and even survival of krill may well depend on the extent of ice cover. We need much more data to see whether this is true.

This year, I spent my first season in Antarctica. Our group spent a great deal of time surveying the region near Palmer Station in zodiacs (small boats). We took physical measurements such as temperature, salt concentration and light level. We also towed a mechanical “fish” which uses sound pulses to detect schools of krill. Thirdly, we towed nets through the water, systematically searching for krill, fish and zooplankton.

While trawling, we saw much wildlife. A couple of times, loud blows of humpback whales surprised us as the whales surfaced a few feet from our zodiac. They, too, were fishing for krill. Adélie penguins were constantly popping up around us, and I enjoyed watching them fly underwater.

We brought krill back to the lab for a variety of experiments to help us answer questions such as “How old are they?” and “Are they reproducing right now?” All of these are relative measures of the health of krill. We will compare the results of our krill experiments with studies of the physical environment and phytoplankton to help us understand the conditions krill need to do well, and tell us when conditions are good for them.

After we conduct this research project for several years, we will have a much clearer idea of why there sometimes are “good years” and “bad years” for krill, penguins and skuas. We hope to develop the ability to predict when a population will do well, and when it will do poorly.

With this knowledge, we can more clearly detect an abnormal situation, and perhaps determine whether it is a result of a human-caused condition such as over-fishing, ozone depletion or global warming. It is believed that, like the ozone hole, the effects of global-scale changes will first be seen at the north and south poles. Thus, it makes sense to study the lives of critters such as Antarctic krill who live literally at the end of the earth.