The expansion and decline of charophyte communities in lakes within the Sejny Lake District (north-eastern Poland) and changes in water chemistry

Over the last 25 years, considerable changes have been observed in the littoral vegetation of 11 lakes within the Sejny Lake District. In eight lakes (Dowcień, Jurkowo, Kunis, Mia?kie, Pilwie, P?askie, Wi?kokuk, Zelwa) where Chara species dominated, their communities declined or disappeared completely. In four of these lakes (Dowcień, Jurkowo, Kunis and Mia?kie), charophytes were replaced by communities of the class Potametea (mainly Nupharo-Nymphaeetum albae and Ceratophylletum demersi). In three other lakes (Pilwie, P?askie and Wi?kokuk), the area covered by charophyte communities decreased. In Lake Zelwa, Charetum rudis and Charetum jubatae disappeared while Charetum tomentosae and Charetum fragilis expanded. A total area of charophyte communities remained approximately the same. A considerable increase in the water total hardness and concentration was noted in each of the eight lakes. In most of them increased levels of dissolved organic matter (measured as COD-KMnO₄) and pH were detected as well. However, the increase in the concentration was smaller in Lakes Wi?kokuk and Zelwa in which only slight changes in the vegetation were observed. In the above two lakes, the concentration of dissolved organic matter decreased and low water colour values are now noted. In three lakes (D?ugie, Dmitrowo and Gajlik), the charophyte communities tended to expand under the condition of moderate or high (Lake D?ugie) concentration, while dissolved organic matter concentrations were low. These lakes are also characterized by low values of colour. Charophyte communities may persist in lakes for a long period of time even when there is a moderate increase in concentration until the colour of water distinctly increases (>20 mg Pt L^?1).