Past and present distribution of stoneworts (Characeae) in The Netherlands

Abstract

In The Netherlands 21 Characeae species occur. Chara vulgaris, C. globularis, and Nitella flexilis are common and widespread, occurring in at least 225 of the total of 1677 atlasblocks (5 × 5 km²). Chara aspera, C. contraria, C major, Nitella mucronata and Tolypella prolifera, occurring in at least 30 atlasblocks, are denoted as ‘not uncommon’. Thirteen species are rare and recorded in less than 30 atlasblocks. Regarding the common species, the number of records significantly increased in recent time, presumably thanks to the recently increased flora inventory activities. The other species remained nevertheless rare, with a tendency of decrease. In the first half of this century mass occurrence of Characeae was a rather common phenomenon, especially in shallow lakes in the central western part of the country. Rich occurrence of Characeae is now restricted to localities with clear water which is low in nutrients. Important habitats are dune waters, polder ditches, shallow lakes and moorland pools. Physico-chemical factors in water and sediment, such as nutrients, salinity, CaCO₃, alkalinity, and in the sediment also the redox-value and organic matter, are suggested as important parameters for species composition. In recent years, at several sites where water quality has improved by restoration measures, Characeae reappeared or increased in species and biomass.