The seasonal dynamics of epilithic algae in a third order pristine for
est stream were analyzed over a period of 2 years. Stream water was sl
ightly acidic and nutrient poor. Encrusting, filamentous flocs, and fi
laments were found. Algal standing crop was high (mean concentration o
f Chl a 16-43 mg m(-2)) in spring. Filamentous algae contributed most
to standing crop. Diatoms made up over 85% by number of the epilithon.
Blue-greens were abundant upstream, and chlorophytes downstream. This
shift was ascribed to greater light availability downstrean. The comm
unity was more diverse during spring. Water current was the most impor
tant variable regulating epilithon structure. Total phosphorus (TP), o
rthophosphate (O-PO43-), silica (Si4+), nitrate (NO3-) and conductance
correlated negatively with flow rate. Green algae showed a positive c
orrelation to phosphorus during low and stable flow. During rapid runo
ff, diatoms were the most resistant forms. Seasonal change in the epil
ithic community was mainly regulated by fluctuations in flow rate.