The drifting of invertebrates and particulate organic matter in an Austrian mountain brook

1. The drift of organisms and particulate organic matter in a calcareous mountain brook near Lunz, Lower Austria, was investigated for 1 year. Five sets of three drift nets (sampling at different water depths) were distributed along a cross-section of the brook. 2. From February to March 1989, samples were taken at intervals of 2–7 days over 24-h periods, at the end of which the drift nets were emptied. From April 1989 to March 1990, sampling was carried out at monthly intervals and the nets emptied every 3h for 1 full day. 3. Of 71 810 organisms caught, the most abundant taxa were Diptera (46.6%; mainly Chironomidae and Simuliidae), Ephemeroptera (24.0%; dominated by Baetis spp.) and Plecoptera (16.1%). 4. The drift density (mean ± 95% CL) was significantly higher (P<0.05) in spring and summer (2.50 ± 0.32 specimens m^?3) than in autumn and winter (2.01 ± 0,22 specimens m^?3). In terms of biomass (wet weight), drift density was also significantly higher (P < 0.001) in spring and summer (2.50 ± 0.48 mgm^?3) than during the rest of the year (1.04±0.12mgm ^?3). 5. Total drift rates through a cross-section of the brook ranged from 17366 organisms per day at low discharge (water depth = 10cm) to 955152 individuals per day at a water depth of 50cm in autumn and winter; in spring and summer the corresponding values were 21600 and 1188000 specimens per day. 6. Ephemeroptera, Plecoptera, Coleoptera and Simuliidae were most abundant in night samples, whereas Hydracarina, Trichoptera and Chironomidae drifted mainly during daylight hours. 7. Drift density (dry weight) of particulate organic matter was positively correlated with discharge and ranged from 2.16mgm^?3 at a water depth of 10cm to 17.39mgm^?3 at 50cm.