Till about 1965 a rich characean community occurred in the shallow peat lake Botshol with six species of which the rare Nitellopsis obtusa and Chara major dominated at many sites. In the period 1980-1988 characean biomass strongly decreased and only two species, Chara globularis and C. connivens, remained in small populations at a few localities. Of the macrophyte Najas marina also some small populations remained, while the aquatic moss Fontinalis antipyretica and the filamentous alga Vaucheria dichotoma predominated at many sites. These phenomena may have been due to eutrophication by the input of polluted water. This process of impoverishment was stopped by restoration measures in 1989, resulting in a lower phosphorus concentration (ca 25 ig 1-') and a higher water transparency. Immediately after these measures the Characeae community increased in abundance and number of species. During the summer of 1990, and especially 1991, a spectacular growth occurred of Chara connivens. C. connivens was often accompanied by C. major. Other species with scattered occurrence were C. aculeolata, C. aspera, C. contraria and C. globularis. The reasons for the shift in dominance from Nitellopsis obtusa to Chara connivens are discussed. From growth experiments evidence was obtained that neither the recent higher chloride level, nor the lowered phosphate concentration were the main factors for the domination of Chara connivens.
The formerly rich characean community in Botshol with six species, of which the rare Nitellopsis obtusa and Chara hispida dominated at many sites, decreased to only two species, Chara globularis and C. connivens, in the period 1980-1988. The macrophyte Najas marina also remained at some sites, and the aquatic moss Fontinalis antipyretica and the filamentous alga Vaucheria dichotoma predominated at many sites. These phenomena may have been due to eutrophication by the inlet of polluted water. This process of eutrophication was stopped by restoration measures in 1989, resulting in a lower phosphorus concentration (ca. 0.024 mg I-~) and a higher water transparency. Immediately after these measures the Characeae community increased strongly in abundance and number of species. During the summer of 1990, and especially of 1991, a spectacular growth occurred of Chara connivens. Chara connivens was often accompanied by Chara hispida. Other species with scattered occurrence were Chara aculeolata, C. aspera, C. contraria and C. globularis. The reasons for the shift in dominance from Nitellopsis obtusa to Chara connivens are discussed. One of the reasons may be the recent higher chloride content which is one of the consequences of the restoration measures.
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