Lake habitat suitability for the rare European macrophyte Najas flexilis (Willd.) Rostk. & Schmidt

1. Najas flexilis (Willd.) Rostk. & Schmidt is a submerged annual macrophyte, rare in Europe, which is protected under the EC Habitats Directive. 2. N. flexilis grows in deep, often coloured or turbid water in mesotrophic lakes. Because of this habitat preference it is difficult to locate and assess the ecological state of populations of the species for conservation monitoring purposes. 3. A method is described based on plant community information that can be used to determine the baseline probability that conditions in a lake are suitable for supporting N. flexilis growth. This can be applied to conservation management decisions, such as whether a detailed underwater survey is justified for monitoring the integrity of existing populations of N. flexilis, or whether the lake may be a suitable site for introduction, or reintroduction, of populations of the plant. 4. Two methods of plant community description are compared: a quantitative micro-habitat scale approach and a whole-lake-scale qualitative approach. Plant community data collected using each method were grouped using TWINSPAN, and environmental descriptors of the sites comprising each plant community group were compared statistically. 5. Micro-habitat-scale community groups differed significantly only in the light extinction coefficient, indicating the zone within the lake in which N. flexilis occurred. 6. The whole-lake-scale community groups differed in a number of environmental variables indicative of eutrophication and acidification, two major environmental threats to N. flexilis survival. 7. This study suggests that a lake-scale qualitative plant community description would be a better indicator of site suitability for N. flexilis growth than a quantitative micro-habitat plant community description. This is because the whole-lake-scale approach could detect a difference in the environmental factors that affect N. flexilis growth, which the micro-habitat scale approach could not.