Abstract A total of 129 relevés with Potamogeton alpinus Balbis from sites in Lower Saxony were classified by table work into 18 structurally and floristically defined vegetation types. Additionally, 52 relevés from sites in Schleswig-Holstein were classified into 14 vegetation types with 5 singular cases left. Potamogeton alpinus was found to be present in a large number of species groups. The separate classification of the two data sets resulted in a different vegetation typification even though the study areas are geographically adjacent and similar with respect to ecological characteristics of the waters colonized. For the observation of vegetation dynamics, 65 of the sites were surveyed repeatedly, at least both in 1981 and 1984. Some of the sites were studied more intensively for up to 9 years. The occurrence in time of P. alpinus is very constant in some places, but unstable in others. The behaviour of P. alpinus was compared with the behaviour of the community as a whole. A high degree of independence between population changes in P. alpinus and changes in the composition and structure of the community was observed. The species behave individualistically.