Distances Travelled by Drifting Mayfly Nymphs: Factors Influencing Return to the Substrate

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Abstract
We determined the distance travelled by drifting nymphs of the mayflies Baetis and Cinygmula to add to existing knowledge of factors influencing drift distance, and to investigate the influence of drift-exiting behavior upon drift magnitude and periodicity. Individuals were released into the water column of a trough at 0.5-m intervals above the exit point, and numbers remaining in suspension over each release distance were fit to a previously developed model which describes return to the substrate according to a negative exponential function. Drift distance increased with increasing current velocity (range studied: 5-55 cm/s) and with decreasing body size (range studied: ca. 0.1-1 mg dry wt) according to log-linear functions. At higher velocities (35 and 55 cm/s), nymphs of both species drifted much shorter distances alive than dead, indicating active exiting from the drift. Drift distances of live nymphs scarcely differed between day and night, which contradicts the suggestion that greater drift abundances at night are caused by disorientation in darkness and consequently greater drift distances. Furthermore, comparisons using nymphs collected from the drift vs. the benthos, and subsequently fed or starved, also revealed no effect of treatment on drift distance, counter to expectations if drifting individuals were intrinsically weaker or less healthy than their benthic counterparts. Thus, although behavioral ability to exit the drift clearly exists in a number of invertebrate taxa, neither the occurrence of drift nor its diel periodicity was explainable on the basis of drift exiting behavior.

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