INVERTEBRATE DRIFT AND LONGITUDINAL TRANSPORT PROCESSES IN STREAMS

Benthic macroinvertebrates were disturbed from stream substrata and ca ught in drift nets at various distances downstream. Four experiments w ere carried out at several discharges in each of four streams with con trasting hydraulic transport characteristics. The numbers of all inver tebrates and of Plecoptera in the water column at each distance were f it to a negative exponential function. The channel with the highest me an velocity had the lowest return rate (beta). In any one channel, bet a decreased with increasing velocity; this relationship approximated a power function, with stream-specific slope corresponding to channel h ydraulics. The stream with a high fraction of dead zones had high beta values that varied little with velocity; streams with fewer dead zone s had lower beta values that decreased with increased velocity. At any particular velocity, channel-specific depth and turbulence may accoun t for different beta values in streams with similar dead zones. The re turn rates of two stonefly families varied relative to one another, po ssibly owing to species-specific behavioural changes in response to cu rrent. In a low-velocity stream, Leuctridae had higher beta values tha n Nemouridae; the rates were similar in a higher velocity stream. Our results are consistent with the view that the physical habitat influen ces the distance drifted by dislodged invertebrates.