Charophyte communities in cut-off channels: the role of river connectivity.
Résumé
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 Rhone, 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 x 10(-3) mg l(-1) and [PO4] = 0.05 mg l(-1) in average in the Ain River to [NH4] = 0.1 mg 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 major and Chara globularis occurred in cut-off channels characterized by low levels of flood disturbance, whereas some other species (e.g. Chara vulgaris and 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. (C) 2002 Elsevier Science B.V. All rights reserved.