At present Lychnothamnus barbatus (Meyen) Leonhardi belongs to the rarest species of charophytes in the world. In Europe it is classified as threatened with extinction. The problem of extinction of this species is intriguing, in particular in the context of its widespread occurrence in Europe and Asia till the last decade of the 20th century. Records of L. barbatus from Wielkopolska region (Western Poland) are know from 15 lakes. The most of them was stated in 19th and on beginning of 20th centuries. Now, this species is growing in 6 lakes, from among 2 sites are new. This study was undertaken to a) determine the abundance of L. barbatus and the co-occurring plant species at different sites in lakes, b) determine the most important ecological parameters controlling the structure communities with L. barbatus co-occurring and quantitative responses of this species. In 7 lakes (area 5.5-197 ha, depth max. 7.8– 38 m, trophic state: meso-eutrophic) in western Poland the species composition and coverage of vegetation were studied at the 23 plots with L. barbatus occurrence in relation to the measured variables. Seventeen environmental parameters were measured including: depth of water, pH, conductivity, SO₄²⁻, NH₄⁺, NO₃⁻, PO₄³⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺, chlorophyll a, Secchi disc visibility, colour, O₂ dissolved, saturation, total Fe during the period July–September. The DCA and CCA analyses were used to assess the relation between vegetation parameters and environmental variables. L. barbatus preferred the water rich in Ca⁺, Mg²⁺ and SO₄²⁻ and with high concentrations of nutrients, especially NH₄⁺ and PO₄³⁻, and moderate values of electrolytic conductivity. In the studied lakes, L. barbatus occupied the separate niche. This species formed the communities in very shallow marginal zones of lakes (0.4–1.5 m) with other macrophytes like: Chara vulgaris, C. tomentosa, Potamogeton ´ nitens, and monospecific stands on margins of steep lake slopes (4–6 m) with Chara globularis fo. hedwigii and Nitella mucronata. The process of extinction of this species seems to be related with increasing turbidity related in turn to algal blooming in lakes and with the spatial competition of vascular macrophytes, especially Ceratophyllum demersum.
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