Environmental Complexity and the Drifting Behaviour of a Running Water Amphipod

Abstract

Overall level and type of activity of Gammarus pseudolimnaeus changed with current speed and pattern in laboratory stream tanks. With increasing current speed (15 to 28 cm/s) active and passive downstream transport increased whereas numbers of animals moving upstream decreased. Upstream movement increased in smooth and rough flow above 28 cm/s. Mean swimming speeds of individuals differed between day and night at most current settings. Total distances drifted by single animals were greater in 43-L ("macroenvironments") than in 3. 5-L stream tanks ("microenvironments"), and were greater in both environments at night On average, individuals had 6.8 drift episodes during 12 h darkness in microenvironments (nightly downstream displacement 3.04 m), compared with 37.9 episodes in macroenvironments (displacement 22.4 m). Level of activity was positively related to amphipod benthic density during both day and night in microenvironments; however, in macroenvironments at night the relationship was negative. Cessation of drift in the presence of fish predators and marked nocturnal activity peaks in macroenvironments were absent in microenvironments. The different responses within the two environments may reflect differences in perception of local environmental scale by the amphipods. This process may occur during the periodic contacts that animals make with the substrate during drift; individuals may retain some familiarity with features of their immediate surroundings.