Uptake of NH4 -N by streambed biota of mountain brooks was studied in the alpine zone of the High Tatra Mountains . Experiments were performed involving in situ dosing of ammonium directly to the acidified stream and incubations of ammonium and streambed bryophytes in enclosures within a range of pH from 4 .45 to 8 .10 .NH4 -N uptake length decreased with decreasing stream discharge, while comparable values of dischargenormalized uptake lengths were found during two in situ experiments .Maximum uptake rates of NH4 -N obtained during the incubation of bryophytes (6 to 11 mg m -2 h -1 ) were comparable with results of two in situ experiments (8 and 12 mg m -2 h -1 ) . The average NH4 -N uptake rates observed during incubations lasting 3 to 5 hours (4 .3 mg m -2 h-1 ) were not related to the pH of stream water. Nitrification of about 50% of the NH4 -N added was observed in non-acidified streams, but was negligible in acidified streams . Significant photoinhibition of nitrification was observed in non-acidified streams during enclosure experiments . 157