Seasonal changes in composition and biomass of epilithic algal floras of a chalk stream and a soft water stream with estimates of production

Microscopic epilithic algae from pebbles in the Itchen river (rising from chalk springs; pH near 8.2; mean alkalinity of 236 mg bicarbonate per litre; mean flow rate of 0.33 m per second; discharge 0.34-2.46 m3 per second) and the Ober Water (draining sand and gravel; pH 6.9-7.2; mean alkalinity of 50 mg bicarbonate per litre; mean flow rate of 0.27 m per second; discharge 0.08-1.0 m3 per second) were studied during 1984 and 1985. Diatoms peaked in April/May and comprised 70-95 per cent of all algal cells in both streams. Chlorophytes peaked in summer and cyanophytes in autumn. Species composition and population densities of the epilithic flora in the 2 streams was different. Algal cell numbers ranged between 500-7000 cells per mm2 and 8-320 cells per mm2 of stream flow in the Itchen river and Ober Water, respectively. The chlorophyll-a content of epilithic algae in the Itchen river was between 115-415 mg per m2 of stream floor, representing an annual mean biomass of about 8 g per m2. In the Ober Water, these values were 2.2-44 mg per m2, representing an annual mean biomass of about 1 g per m2. Estimates of epilithic algae annual production were 600 and 75 g organic dry weight per m2 in Itchen river and Ober Water, respectively.