From their introduction in the beginning of the 19th century in the Lez river at Montpellier, the American Ludwigia have, progressively, colonized the whole of France. In spite of their real nuisance for natural habitats, these ornamental hydrophytes are still for sale. The management of these invasive species requires their precise identification, but this appears to be extremely complex. A morphological and cytogenetic study allowed to confirm the presence of two different taxa in France, and the absence of hybrids in the mixed zones that were monitored. The diploids (2n = 16) correspond to L. peploides subsp. montevidensis (Spreng.) Raven, and the decaploids (2n = 80) to L. grandiflora subsp. hexapetala (Hook. & Arn.) Nesom & Kartesz. The diploids colonize especially the Mediterranean region (except the Southeastern part), while the polyploids predominate throughout the other regions. Both taxa exhibit an intense vegetative growth, however they have a distinct breeding system: the self-compatible diploids were always found very fructiferous, whereas the self-incompatible polyploids were often observed as sterile. The breeding system difference could explain the substitution of L. grandiflora (first wave of invasion in the 19th century) by L. peploides, in the 20th century, in the southern part of France.

The paper presents a new index for assessing water trophy and organic pollution. It is based on only true aquatic macrophytes -being calculated on species score, coefficient of ecological amplitude and degree of cover. The method was tested in an acidic lowland river and an alkaline mountain river, and is shown to be validated by bio-indication scales based on macrophyte communities. The practical interest is discussed regarding the Water Framework Directive.

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