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The role of river and groundwater connectivity on the diversity, distribution and temporal dynamics of charophytes has been determined in cut-off river channels of the Rh ˆ one, Rhine, Ain and Doubs. Charophyte richness and abundance decrease (from 12 species in cut-off channels of the Ain River to one species in the ones of the Doubs River) when nutrient contents increase (from [NH4] = 9 × 10 −3 mg l −1 and [PO4] = 0.05 mg l −1 in average in the Ain River to [NH4] = 0. 1m g l −1 and [PO4] = 0.34 mg l −1 in the Doubs River), but only species richness appeared to be significantly negatively correlated with water ammonium content. Chara majorand Chara globularis occurred in cut-off channels characterized by low levels of flood disturbance, whereas some other species (e.g. Chara vulgarisand Nitella confervacea) were able to maintain their presence in cut-off channels frequently scoured by flood disturbance. The temporal dynamics of some species demonstrated their ability to survive over several years in habitats characterised either by low disturbance (C. major, Chara hispida) or high disturbance levels (C. vulgaris). In the latter case, the high variability of the species distribution over time suggested that its maintenance involved propagule dispersal, because its shallow anchorage did not prevent the plant being uprooted during floods. These results suggest that some charophyte species should not be considered as pioneer fugitive species (C. major) because they are able to colonise undisturbed cut-off channels over more than 15 years. Others (e.g. Nitella sp.) could be, because of their limited distribution and high variations in abundance over time. © 2002 Elsevier Science B.V. All rights reserved.

The rare aquatic plant Luronium natans (L.) Rafin. is a representative of the Littorelletea, i.e. plant communities growing typically in unproductive habitats with oligo- to meso-trophic, slightly acid to circumneutral soft waters. Apart from morphological and physiological adaptations to these particular habitat conditions, Littorelletea communities are characterised by their narrow ecological amplitude. Despite the fact that L. natans is reported to be limited to habitats with low calcium content, it is abundant in two of the numerous potential yet calcareous habitats in the floodplains of the Upper Rhone, and its tributary, the Ain River (France). The present study examines whether the restriction to such a low number of habitats is linked to particular abiotic conditions therein. It also aims to identify the range of floodplain habitats that belong to the species' fundamental niche, i.e. those where, after arrival, it would be able to establish in the absence of interspecific competition. L. natans was therefore transplanted to sites along a gradient of sediment composition, and allowed to establish during a competition-free period. Extensive physico-chemical analyses were carried out on sediment, surface and interstitial water of the transplantation sites. The species' fundamental niche appeared very large, since establishment was successful in 3/4 of the transplantation sites. The magnitude of establishment success differed greatly, but only redox-potential was significantly correlated with establishment success. Abiotic parameters could not provide a general explanation of complete establishment failure either. The experiment demonstrated that the restriction of L. natans distribution in these floodplains is not caused by a lack of habitats with suitable water and sediment chemistry. (C)2000 Elsevier Science B.V. All rights reserved.

The regeneration (regrowth into viable plants) and colonisation (establishment in the sediment) abilities of vegetative fragments of six aquatic plant species ( Elodea canadensis Michaux, Hippuris vulgaris L., Luronium natans (L.) Rafin., Potamogeton pusillus L., Ranunculus trichophyllus Chaix, Sparganium emersum Rehm.) occurring in habitats frequently disturbed by floods along the Rhone River (France) were compared through a laboratory experiment. Six types of plant fragments were collected from the plants and placed in water over sediment. Their development and/or their rooting into the sediment were recorded over 10 weeks. The species exhibited different survival tactics to survive after fragmentation: (1) fragments from Sparganium and Ranunculus developed roots and rapidly established into the sediment whereas (2) fragments from Luronium , Hippuris and Elodea developed many propagules that could be dispersed but did not establish within the 10 weeks of the experiment. The fragments of P. pusillus never established nor regenerated. The results showed a trade-off between regeneration and colonisation related to the patterns of recolonisation of cleared patches of the species and their ecological requirements. Maintenance of these species in ecosystems frequently disturbed by floods can partly be explained by their high regeneration abilities, and results are discussed in terms of life-history traits and ecological strategies.