DISTRIBUTION, ABUNDANCE AND DEVELOPMENT OF TRICHOPTERA LARVAE IN THE RIVER NORTH TYNE FOLLOWING THE COMMENCEMENT OF HYDROELECTRIC POWER-GENERATION

Following turbine installation at the Kielder dam, the River North Tyn e downstream is now characterized by large daily fluctuations in flow. A survey of trichopteran populations (Hydropsychidae, Rhyacophilidae and Polycentropodidae) was carried out to assess the impact of hydroel ectric power generation (Phase III), and to compare the results with s imilar pre-impoundment (Phase I) and post-impoundment (Phase II) surve ys. Benthic samples were collected quarterly upstream from the reservo ir and at four stations downstream. Although the same three species (H ydropsyche siltalai, Hydropsyche pellucidula and Rhyacophila dorsalis) continue to dominate, there were some notable differences found betwe en sampling sites. Species diversity was drastically reduced at the st ation nearest the dam, where 99% of all larvae collected were R. dorsa lis. At the next station 5 km downstream, where previously H. siltalai had been dominant, R. dorsalis now constitutes 65% of collections. At the two stations farthest away from the dam the former prevalence of H. siltalai was re-established. It is suggested that the unstable, hig h flows at the reservoir outlet are likely to favour the predatory R. dorsalis rather than the filter feeding hydropsychids whose nets may b e susceptible to damage. Upstream from Kielder Water, R. dorsalis was prominent in benthic samples, whereas numbers of H. siltalai were 10 t imes lower than in Phase II and 60 times lower than in Phase I. The ex tent to which such upstream changes may be related to altered patterns of community structure below the dam is not known, but these observat ions may warrant a more cautious approach when considering upstream ar eas as unaffected 'reference sites'. Instar analyses suggested that wi nter-warm, summer-cool discharges were continuing to modify larval gro wth rates, as noted in Phase II. The results of this study are discuss ed in relation to present trends in water resource development in Brit ain, especially with regard to future needs in monitoring and research .