CONTINUOUS DRIFT SAMPLES OF MACROINVERTEBRATES IN A LARGE RIVER, THE DANUBE IN AUSTRIA

1. A new method is described for sampling the drift in large rivers continuously and automatically, using equipment based on the marine, continuous plankton sampler of Hardy (1936). 2. Between April and November 1989, about 790h were sampled continuously, equivalent to a water volume of 2937 m3 and the total catch was 527 drifting macroinvertebrates, in forty-nine taxa. Twenty-five of these taxa are recorded in the drift for the first time and twenty-eight of the drifting taxa were present in benthos samples from the Danube. 3. Total drift density varied between 12 and 31 animals per 100 m3 of water sampled throughout the year, with a maximum of 31 animals in May. The composition of major faunal groups showed a significant seasonal pattern, with Oligochaeta and Diptera predominant in spring, Crustacea and Insecta in summer and only Crustacea in autumn. The overall density of the macrozoobenthos from October 1986 to December 1987 was about 19 360 animals m-2 and the proportion of total benthos animals, drifting at any instant in time, ranged from 0.0026 to 0.0064%. 4. The relationship between drift density day-1 and mean daily discharge was described by a power-function. Total mean drift rate of macroinvertebrates in the Danube was estimated to be 13 600 000 animals per 24h and the mean drift distance was estimated to vary between about 4 and 31 m, dependent on the animal group and the water velocity. 5. No obvious consistent diel pattern could be established from the continuous samples, and no marked diel rhythm could be detected for Oligochaeta, Diptera larvae and Crustacea.