Hydrocotyle umbellata is a small clonal and perennial macrophyte growing along the shores of freshwater lakes. This species occurs primarily in tropical/ subtropical and temperate climates. In Canada, this species is listed under COSEWIC as endangered with 2 populations in Nova Scotia. Currently its ecology is largely unknown. The objectives are to: 1) describe responses of plants exposed to different environmental conditions; 2) determine the level of genetic variation within and between populations, and 3) develop recovery measures from the data collected during this project. In 1998-2001, the populations were visited and their ecology examined in function of the change in environmental conditions. Most of the populations slightly varied in size (according to the number of ramets) over time. A new patch was established in 2000 and increased in size from 240 to 480 ramets in 2001. To better understand the species ecology, several clones were examined and the morphological measurements done. This analysis shows that rhizomes have continuous growth with mean primary rhizome length longer than the secondary or tertiary rhizome. Traits, also including leaf width and stem height, vary between sites and locations within site. Comparisons between years and locations suggest that resource allocation might influence growth but environmental variation is the main factor for demographic and morphological changes. Allozymic analysis was carried out using cellulose acetate plates and showed that little variation exists within and between the two populations. Only six genotypes were identified in total. These features are highly important for the adaptation of the species to limiting northern-range conditions and should be strongly considered in the development of the management plan for this species.

Key words: conservation, distribution, ecology, environmental change, genetic variation, rare species