Variations of aquatic bryophyte assemblages in the Rhine Rift related to water quality. 2. The waterfalls of the Vosges and the Black Forest

The relationships between water quality and aquatic bryophyte assemblages w ere investigated in sixteen waterfalls in the Vosges Mountains and in the B lack Forest. Species such as Marsupella emarginata and Scapania undulata to lerate neutral pH in oligomineral waters whereas species such as Chiloscyph us polyanthos can tolerate waters with low cation concentrations if the con centrations of protons is also low. The aquatic bryoflora of the waterfalls of the Black Forest and of the Vosges thus includes a combination of speci es characteristic for waters with low concentrations of dissolved minerals on the one hand and fur waters with low concentrations of protons on the ot her. This combination relies on a fragile physico-chemical balance because the buffering capacity of the waters is very low. At such low concentration s, the chemical factors are limiting for the bryoflora, which may react rap idly to slight changes in ion concentration. Sewage effluent discharges com ing from villages or trout hatcheries lead to a simultaneous increase of di ssolved mineral content (Mg (+ +), K (+)) and trophic level (NO3-), which i s reflected in a decrease of the species characteristic of waters with low concentrations of dissolved minerals, i.e. Brachythecium plumosum, Hygrohyp num duriusculum, Hycomium armoricum and Marsupella emarginata. When disturb ance increases, all these species and Scapania undulata disappear, whereas species such as Cratoneuron filicinum appear. Conversely, acidic deposition can cause pH to decrease quickly because of the very low buffering capacit ies of the waters. In this case, the absence of the acid-sensitive species (Chiloscyphus polyanthos, Rhynchostegium riparioides and Thamnobryum alopec urum) probably indicates chronic increases of protons even if pH may remain most of the time close to neutrality.