This association includes a single monospecific stand of Isoëtes lacustris on the bottom of Černé Lake at an altitude of 1008 m in the Šumava Mountains. It occurs mainly in depths of 1–4 m. The lake is situated on mica schist bedrock. Lake water is naturally oligotrophic and acidic, but its pH decreased below 5 due to acid rain in the 1960s–1990s. Although water chemistry has begun to slowly improve, concentrations of toxic aluminium remain high and the Isoëtes population does not reproduce in this lake.