A revision of the Belgian material of Amblystegium fluviatile and A. tenax
was undertaken. A determination key and distribution maps are provided. A.
fluviatile was observed in 158 IFBL squares, A. tenax in 87. The most impor
tant distribution area of A. fluviatile are the phytogeographical districts
"Ardennais" and "Mosan", that of A. tenax is the "Mosan". Both are rare in
the other districts and absent from the "Maritime". 3/4 of the observation
s of these two species were made after 1980. A. fluviatile grows in waters
less rich in dissolved minerals (median of the observations of total hardne
ss of about 30 degrees F in A. tenax and 10 degrees F in A. fluviatile), of
lower trophic level (median of the observations of N-NH4+ concentrations o
f ca 0.25 mg/l in A. tenax and of about 0.09 mg/l in A. fluviatile) and les
s loaded in heavy metals (median of the observations of Zn concentrations o
f about 100 mu g/l in A. tenax and of about 25 mu g/l in A. fluviatile). Bo
th species include populations exhibiting a large tolerance to water pollut
ion. A. tenax and A. fluviatile were found in rivers with maxima of N-NH4up to 9 and 6 mg/l, respectively. The polluo-tolerance of these species and
their spread in Belgium are discussed.