Case studies and reviews. Vegetation zonation and invertebrate fauna in Pant-y-llyn,an unusual seasonal lake in South Wales,UK

• 1. Temporary waters have been relatively neglected in British nature conservation, due in part to paucity of survey and comparative assessment. Such water bodies hold distinctive and specialized biological communities, and the need for conservation measures at key localities is increasingly recognized in Europe.
• 2. An account of the vegetation and invertebrate fauna in Pant-y-llyn, a small temporary lake on Carboniferous Limestone in South Wales, is presented. Water levels in the basin reflect seasonal variation in the height of the local groundwater, and it fills to a depth of ca 3–4 m in winter but is almost completely dry in summer.
• 3. There is a clearly displayed sequence of bryophyte, herbaceous swamp and woody plant communities which occupy different depth zones in the basin.
• 4. The invertebrate community is dominated by microcrustaceans and a range of insect groups; adaptations for survival over the summer dry phase are considered.
• 5. It is concluded that the ecological characteristics and distinctive hydrological features of Pant-y-llyn closely resemble those of the Irish turloughs. Pant-y-llyn thus represents an important site for conservation as the only known example of this form of seasonal water body in Britain.