Distribution and ecology of charophytes in the Baltic Sea.
Charophytes form a highly evolved, unique group of benthic macroalgae, which has a worldwide distribution in fresh and brackish waters. The aim of the study was to describe the distribution and seasonal growth pattern of charophytes in the Baltic Sea area and determine the importance of the key environmental factors and invertebrate grazing on the development of charophyte communities. This thesis is focused on two species – the charophytes Chara connivens Salzm. ex A. Braun and Chara tomentosa L. Charophytes are found all over the Baltic Sea. Recently (since 1981) 12 species belonging to the genus Chara are recognized in the Baltic Sea. The distribution of C. connivens has been limited in the Baltic Sea. It has disappeared from several localities where it was found previously. Nowadays C. connivens occurs only in the central part of the Baltic Proper – in Öregrund Archipelago, Åland Sea and coastal waters of the West Estonian Archipelago. Chara tomentosa has a wide distribution in the Baltic Sea and occurs most frequently in the northern Baltic Proper. Depth turned out to be the most important structuring factor for charophyte communities while substrate quality and exposure ranked second. Salinity had no influence on the community structure within the observed interval. Depth influences benthic macrophytes indirectly, mainly as a function of light intensity. Intensive growth of charophytes occurs from spring up to midsummer. Charophyte communities are threatened already from the middle of summer, because damaged communities are not able to recover during the same vegetation season. Herbivores Idotea baltica and Gammarus oceanicus reduced significantly the biomass of charophytes in the field. The strongest influence of herbivory on charophytes was observed in autumn, when their photosynthetic activity was very low.