INFLUENCE OF HYDROPEAKING ON THE DISTRIBUTION AND LARVAL DEVELOPMENT OF THE PLECOPTERA FROM A MOUNTAIN STREAM

The downstream distribution and larval development of the Plecoptera f rom a Pyrenean stream were studied upstream and downstream of a hydroe lectric power plant with intermittent hypolimnetic releases. During po wer generation, flow and temperature were the two main environmental f actors modified. The downstream distribution of the various taxa refle cted both the impact of hydropeaking and the natural zonation. The low est densities and biomasses were estimated at 700 m downstream from th e plant, underlining the prominent role of hydropeaking. Above the pla nt, Plecoptera larvae showed a constant drift due to accidental dislod gement, and behavioural drift was mainly nocturnal. Below the plant, t he flushing action of peaking flows added to this a catastrophic drift , which was even higher since the amplitude between natural flow and p eak flow was elevated. The life cycle patterns and the growth rates of the five dominating species (Siphonoperla torrentium, Isoperla acicul aris, Perla grandis, Amphinemura sulcicollis, Protonemura beatensis) w ere similar at the various sampling sites, and only slight differences in hatching and/or emergence periods were recorded. The Plecoptera of the Oriege complete their larval development during periods where art ificial thermal fluctuations are low and have a minor effect on the po pulations: from autumn to spring-i.e. when the temperatures remain rat her low and during snow melt flood (e.g. S. torrentium, I. acicularis, A. sulcicollis)-or in summer when hypolimnetic releases increase the daily thermal fluctuations but reduce the mean diel temperatures (e.g. P. beatensis). Therefore, under this kind of river regulation, the pl ecopteran population structure and distribution is mainly governed by hydraulic disturbances. (C) 1998 John Wiley & Sons, Ltd.