THE SIGNIFICANCE OF ICE-COVERS FOR PLANKTONIC ALGAE
Long-term ice-cover occurring on high-mountain and subarctic lakes as well as on brackish-water seas such as the Gulf of Bothnia promotes certain algae species, well adapted to low-light and low-temperature and/ or heterotrophic nutrition. Such algal species often belong to the dinoflagellates. In the northern Bothnian Sea, as in Lake Torneträsk, dinoflagellates are predominant under the ice-cover. In Torneträsk their maximum occurs in March. Diatoms are able to begin growth under the ice-cover when the light has slightly increased by the snow melting from the ice and have their maximum after ice-break in Torneträsk as they do in the northern Bothnian Sea. Greenalgae in both biotopes develop after ice-break. This means that members of each group of algae behave in a similar way towards the ice-cover in the different biotopes. The period of ice-cover offers not only low-light and low-temperature, but also high nutrients and slight water movement, the last of which is favourable for movable flagellates to be able to put themselves into and remain in a suitable position towards the light.