Surviving underneath the sea ice in winter
Sea-ice is important for marine life in the Southern Ocean. Besides providing a structure in which small animals can hide, sea ice is also hosting many organisms that provide a rich food source for life in the water underneath, especially in winter. Using IMARES’s ice-net SUIT, the small animals living underneath the sea-ice of the winter Weddell Sea were investigated. This research was recently published by Carmen David (Alfred Wegener Institute) and colleagues in the journal Polar Biology. IMARES and the Alfred Wegener Institute (AWI) are partners in the project Iceflux.
Photo: The amphipod Eusirus microps feels quite at home underneath the ice (taken by Jan Andries van Franeker)
Every year the sea-ice of the Southern Ocean grows and melts. At the end of winter, the ice covers about 20 million km2 of the Southern Ocean. At the end of summer, only 4 million km2 remains.
When the sea-water is freezing and ice is forming, the salt is excluded and accumulates in small channels within the forming sea-ice. Algae and other small organisms also get trapped in the channels when the ice is growing. In time, the salt that was trapped in the channels is released in the sea water underneath the ice. This causes the sea surface water to become salter. The saltier the water, the heavier and therefore it will start to sink. Algae that are not trapped in the sea ice but still remain in the water will sink down with the water and end up in deeper water layers. The sunlight does not penetrate here, also as a result of the ice cover, and the algae cannot grow. Therefore there is almost no algal growth in the sea water in winter, but only in the sea-ice. As algae form the bottom of the food chain, this has consequences for the food chain in ice-covered oceans.
The small animals that live in the Southern Ocean, for example copepods, krill, amphipods (crustaceans) and jelly fish, have to adapt to large seasonal fluctuations in food availability. Some animals are fattening up in the summer months, or lower their metabolism in winter so they do not need a lot of food. Other animals use the algae and other small organisms within and released from the sea ice as a food source.
Earlier investigations of the surface water underneath the ice using the SUIT, already showed that the community directly underneath the ice is different from the one in deeper water layers. This research was done in various seasons in the Lazarev Sea. It showed that some small animals remain associated with the sea-ice year-round, while others only stay in the surface waters in some seasons. The small animals adjusted their seasonal life cycle to the sea-ice in different ways. However, because the Southern Ocean is such a impassable area, there is limited information on life in the winter months.
Carmen David and colleagues recently published their findings with the SUIT in the winter Weddell Sea. The Weddell Sea is regarded as a part of the Southern Ocean that is very rich in marine life. The surface water of the Weddell Sea was found to have a high diversity of species. There were many copepod species, of which some in very high numbers, young Antarctic krill and different species of amphipods. Not many adult krill, squid or fish larvae were found in Weddell Sea, in contrast to the findings in the Lazarev Sea in winter.
When spring comes and temperatures rise, the sea-ice starts to melt. The organisms that inhabited the sea-ice during winter are released into the seawater. Now light conditions are good for the algae to grow and they begin to bloom. Other research found that the increase in algae in the sea water leads to an increase in animals; there is more food for them.
The opposite, however, was found in the Weddell Sea. At the end of the expedition in spring, when the sea ice had already started to melt, the same species remained underneath the ice but they were much less abundant. A possible explanation is that the sea-ice became less important for the food supply for instance, and that the small animals dispersed into deeper water layers, where they are out of reach for the SUIT. This research provides important information on overwintering strategies and the use of the sea ice by small animals in the Southern Ocean. It increases our understanding on what happens when changes occur in sea-ice coverage, for example as a result of climate change, and how these animals and derived food chains will respond.
Carmen David, Fokje Schaafsma, Jan Andries van Franeker, Benjamin Lange, Angelika Brandt and Hauke Flores (2016). Community structure of under-ice fauna in relation to winter sea-ice properties from the Weddell Sea. Polar Biology.
Hauke Flores, Jan Andries van Franeker, Boris Cisewski, Harry Leach, Anton P. van de Putte, Erik H.W.G. Meesters, Ulrich Bathmann, Wimm J. Wolff (2011). Macrofauna under sea ice in the open surface layer of the Lazarev Sea, Southern Ocean. Deep-Sea Research II 58, 1948 – 1961.