The response of benthic macroinvertebrates to artificial disturbance: drift or vertical movement in the gravel bed of two Sub-Alpine streams?

In most streams, invertebrate recovery following hydrological disturbance i s rapid. For three decades, biologists have assumed that recovery after flo oding is facilitated by behavioural migration from refugia, particularly fr om the hyporheic zone. The relevance of this hypothesis was experimentally tested in two medium-gradient streams with porous beds using a new techniqu e that allows the increase of discharge in a 0.20 m(2) enclosed area. Verti cal distribution of benthic macroinvertebrates and drift intensity were com pared between disturbed and undisturbed areas. The pre diction that after a rtificial disturbances invertebrate density should decrease on the surface and increase in the hyporheic zone was not supported by the data. Total inv ertebrate density decreased significantly in the surface layer (0-8 cm) but was not significantly different between disturbed and undisturbed areas in the deep layers (8-60 cm). The same observations were made for most numero us invertebrates such as Chironomidae, Gammaridae and Heptageniidae. The al ternative prediction that disturbance should increase drift intensity was s upported by field observations. Artificial disturbance enhanced drift inten sity and modified drift composition, causing an increase in the relative ab undance of Chironomidae pupae and Gammaridae. This study indicates that dow nward migration of invertebrates within the hyporheic zone during a flood i s not always evident and that recolonization may primarily occur by drift f rom other refugia.