This association is represented by a single monospecific stand of Isoëtes echinospora on the bottom of Plešné Lake at an altitude of 1087 m in the Šumava Mountains. It occurs mainly at depths of 30–50 cm, rarely up to 75 cm. The lake is situated on granite bedrock and its water is mesotrophic. Due to acid rain, lake water has not exceeded pH 5 since the 1960s, which has caused an increase in toxic aluminium concentrations and restricted regeneration of Isoëtes. However, in recent years water chemistry has improved and regeneration of Isoëtes resumed after 2005.