EFFECTS OF FLOW AND REFUGIA ON DRIFT LOSS OF BENTHIC MACROINVERTEBRATES - IMPLICATIONS FOR HABITAT RESTORATION IN LOWLAND STREAMS

1. Current stream restoration practices are rarely based on sufficient knowledge of the physical-habitat requirements of the biota. In this study the drift loss of two lowland stream benthic macroinvertebrates, Gammarus pulex (L.) (Amphipoda, Crustacea) and Ephemerella ignita (Poda) (Ephemeroptera, Insecta), was investigated over gradients of flow forces and abundance of woody debris in laboratory flume experiments. 2. The losses by drift of E. ignita and G. pulex increased significantly at median flume shear stresses of approximately 11 and 31 dyn cm-2 , respectively. 3. Above these critical shear-stress values the population losses of both species significantly decreased with increasing abundance of stationary woody debris. 4. Ephemerella ignita exhibited high population loss in the first period of hydraulic disturbance. Gammarus pulex was affected in a different way, showing an almost constant population loss over time. In contrast to E. ignita, G. pulex used the refugium 'woody debris' actively and more efficiently. 5. Restoration concepts of lowland running waters have to consider hydraulic disturbance by flow as a key element for potential benthic community recovery. 6. Woody debris in the baseflow channel of lowland streams appears to mitigate the impact of hydraulic disturbance to benthic macroinvertebrates caused by rising discharge.