Correlative and experimental investigations on the segregation of aquatic bryophytes as a function of water chemistry in the Walloon hydrographic network

The aquatic bryophyte assemblages of the Walloon hydrographic network [Belgium] are significantly correlated to the gradients of water pollution. Experimental cultures showed that Fe and benzopyrene, which were among the most significantly correlated factors with the changes in the aquatic bryophyte assemblages, did not inhibit the growth of the investigated taxa, at concentrations much higher than those measured in rivers. The correlation of these factors with the floristic assemblages is thus an artefact due to the inter-correlation between the variables. The concentrations of N-NH4+ inducing a growth inhibition varied between 10 mg/litre in a population of Amblystegium tenax and 500 mg/litre in A. riparium. The lethal concentrations are similar to the current annual mean concentrations of this factor in the polluted rivers of this network. This factor is thus directly implied in the segregation of these species along a gradient of water pollution. The populations of A. tenax coming from a heavily polluted area exhibited a tolerance to N-NH4+ five to ten times higher than the populations coming from unpolluted rivers. These result suggest that ecotypes may exist in this species