The history of the occurrence of Cladium mariscus (Cyperaceae) in NE Poland at the north-eastern limit of its distribution in Europe is discussed. The decline of the species may be related to gradual oligotrophication of its habitats, caused by the isolation of calcareous deposits and the decline of calcium carbonate in the substratum. The presence of C. mariscus in Kojle and Perty, lakes in NE Poland, is related to the occurrence of calcareous sediments, which may compensate for the lack of sufficient warmth. The occurrence of calcium carbonate in the substratum allows for the growth of C. mariscus in areas that are influenced significantly by a continental climate. The abundance of Ca² cations in the substratum compensates for that climatic factor.