Abstract
The subject of this work is a comparative analysis of stonewort populations in 5 lobelia lakes of the Kashubian Lakeland based on literature data from the mid-1970s and the authors’ own studies from 2006. In the 1970s 15 sites of 7 stonewort species: Chara delicatula, C. fragilis, C. tomentosa, C. rudis, Nitella flexilis, N. opaca and N. mucronata were recorded in the lakes under study. The populations of C. delicatula and N. flexilis occupied the area from 0.3 to 6 m deep and were characterized by 20% cover on average. Thirty years later, in 2006, the presence of only 6 out of 15 stands was confirmed: 3 of C. delicatula and 3 of N. flexilis. Thus, a total of 9 stands (60%) and 5 species (71%) had disappeared completely. The majority of the preserved populations survived in a vestigial form. Population areas of C. delicatula reached a depth of 2 m and their cover did not exceed 5%. Populations of N. flexilis occupied the littoral up to a depth of 7 m and had 10% cover. The disappearance of stoneworts was probably caused by both anthropogenic factors and natural processes.