The influence of current velocity and substrate on the distribution and drift of two species of mayflies (Ephemeroptera)

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Abstract
The effects of substrate type and current velocity on the distribution and drift of Baetis vagans McDunnough and Paraleptophlebia mollis (Eaton) were examined in an artificial stream. Of those B. vagans (swimming nymphs) remaining on the substrates, most were retrieved from branches while few were found in inorganic substrates. In all cases, numbers of B. vagans remaining on the substrates increased as current velocities increased. Paraleptophlebia mollis (crawling nymphs) were most frequently associated with gravel (11.2–16 mm diameter), whereas branches were of less importance.At 50 cm/s and during darkness, the proportion of P. mollis drifting from a 'least preferred' substrate was comparable with that of B. vagans. At lower velocities or in light, B. vagans drifted more frequently than did P. mollis, regardless of substrate type. For both species, drift was a significant mechanism in dispersal of individuals from unfavourable areas.