Over 100 marl lakes from the British Isles are described and compared. About 75 % of the lakes are extant with the remainder dating from the Late Glacial or Holocene and now completely filled with sediment. Most of the lakes are, or were, Chara-dominated and of glacial origin and the majority are shallow and less than 4 m deep. These lakes contain important plant and animal communities and their sediments provide a valuable fossil and geochemical record of changes since the last glaciation. Most are associated with Carboniferous Limestone, Chalk and Old Red Sandstone formations, and they are widely distributed. Their waters contain a mean dissolved calcium content of 2.79 meq L-1 (140 mg L-1 CaCO3), typical of temperate limestone groundwaters. Marl formation is closely associated with the growth of Chara but these plants are sensitive to disturbance and water pollution. Being shallow, the lakes possess rich emergent and submersed macrophyte floras, providing shelter for diverse invertebrate faunas, with the Crustacea and Mollusca well represented. They range from oligotrophic to eutrophic and several of the deeper lakes stratify during summer.
The future for many of the marl lakes in the British Isles is bleak. Being small and shallow, they are prone to siltation, particularly along the coast, and they are also threatened by sea-level rise, extreme weather events, drainage/abstraction, alien plants and animals. At least half of the lakes appear to have been affected by nutrient enrichment originating from agriculture within their catchments or human sewage and industrial effluents. Irreversible changes will almost certainly occur if nutrient enrichment is prolonged but there are no simple solutions. The review includes an annotated list of the lakes providing locations, geological setting and key references to their biology, chemistry and palaeolimnology where relevant. The lakes are also compared with those of continental Europe and the United States, and a revised definition of marl lakes is provided.