Drift and benthic population structure of the mayfly Rhithrogena semicolorata (Heptageniidae) under natural and hydropeaking conditions

Are stream insects able to control their entry into the drift under all flow conditions? Do changes in streamflow patterns modify the typical drift patterns? Which larval instars are most affected by fluctuating flows? In order to document these questions, we studied the diel patterns in the drift density and size-structure of the heptagenid mayfly Rhithrogena semicolorata (Curtis, 1834) under natural and intermittent hydropeaking conditions in a mountain stream, i.e., up- and downstream of a hydroelectric power plant with hypolimnetic releases. Under natural conditions, drift exhibited a diel periodicity either during low flow periods (1 m(3) s(-1)) and snowmelt floods (5 m(3) s(-1)), nocturnal drift density being clearly higher. Larger individuals were found to drift at night. On the other hand, peak flows flushed many larvae downstream during the day. Smaller individuals were more affected by passive dislodgement, which resulted in a shift towards larger individuals in the size-composition of the benthic populations. When flow raised from 1 to 11 m(3) s(-1), the catastrophic drift occurring during the day depleted the benthos, and the nocturnal ( active) drift was suppressed. However, the nocturnal drift was preserved when flow only raised from 5 to 10 m(3) s(-1).