RESTORATION OF BOTSHOL (THE NETHERLANDS) BY REDUCTION OF EXTERNAL NUTRIENT LOAD - RECOVERY OF A CHARACEAN COMMUNITY, DOMINATED BY CHARA-CONNIVENS

Till about 1965 a rich characean community occurred in the shallow pea t lake Botshol with six species of which the rare Nitellopsis obtusa a nd Chara major dominated at many sites. In the period 1980-1988 charac ean 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, whi le the aquatic moss Fontinalis antipyretica and the filamentous alga V aucheria dichotoma predominated at many sites. These phenomena may hav e been due to eutrophication by the input of polluted water. This proc ess of impoverishment was stopped by restoration measures in 1989, res ulting in a lower phosphorus concentration (ca 25 mug l-1) and a highe r water transparency. Immediately after these measures the Characeae c ommunity increased in abundance and number of species. During the summ er of 1990, and especially 1991, a spectacular growth occurred of Char a connivens. C. connivens was often accompanied by C. major. Other spe cies with scattered occurrence were C. aculeolata, C. aspera, C. contr aria and C. globularis. The reasons for the shift in dominance from Ni tellopsis obtusa to Chara connivens are discussed. From growth experim ents evidence was obtained that neither the recent higher chloride lev el, nor the lowered phosphate concentration were the main factors for the domination of Chara connivens.