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

Abstract

The relationships between water quality and aquatic bryophyte assemblages were investigated in sixteen waterfalls in the Vosges Mountains and in the Black Forest. Species such as Marsupella emarginata and Scapania undulata tolerate neutral pH in oligomineral waters whereas species such as Chiloscyphus polyanthos can tolerate waters with low cation concentrations if the concentrations of protons is also low. The aquatic bryoflora of the waterfalls of the Black Forest and of the Vosges thus includes a combination of species characteristic for waters with low concentrations of dissolved minerals on the one hand and for waters with low concentrations of protons on the other. This combination relies on a fragile physico-chemical balance because the buffering capacity of the waters is very low. At such low concentrations, the chemical factors are limiting for the bryoflora, which may react rapidly to slight changes in ion concentration. Sewage effluent discharges coming from villages or trout hatcheries lead to a simultaneous increase of dissolved mineral content (Mg + +, K +) and trophic level (N03-), which is reflected in a decrease of the species characteristic of waters with low concentrations of dissolved minerals, i.e. Brachythecium plumosum, Hygrohypnum duriusculum, Hyocomium armoricum and Marsupella emarginata. When disturbance 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 capacities of the waters. In this case, the absence of the acid-sensitive species (Chiloscyphus polyanthos, Rhynchostegium riparioides and Thamnobryum alopecurum) probably indicates chronic increases of protons even if pH may remain most of the time close to neutrality.