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 Tyne 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 hydroelectric power generation (Phase III), and to compare the results with similar pre-impoundment (Phase I) and post-impoundment (Phase II) surveys. Benthic samples were collected quarterly upstream from the reservoir and at four stations downstream. Although the same three species (Hydropsyche siltalai, Hydropsyche pellucidula and Rhyacophila dorsalis) continue to dominate, there were some notable differences found between sampling sites. Species diversity was drastically reduced at the station nearest the dam, where 99% of all larvae collected were R. dorsalis. 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, high flows at the reservoir outlet are likely to favour the predatory R. dorsalis rather than the filter feeding hydropsychids whose nets may be susceptible to damage. Upstream from Kielder Water, R. dorsalis was prominent in benthic samples, whereas numbers of H. siltalai were 10 times lower than in Phase II and 60 times lower than in Phase I. The extent to which such upstream changes may be related to altered patterns of community structure below the dam is not known, but these observations may warrant a more cautious approach when considering upstream areas as unaffected 'reference sites'. Instar analyses suggested that winter-warm, summer-cool discharges were continuing to modify larval growth rates, as noted in Phase II. The results of this study are discussed in relation to present trends in water resource development in Britain, especially with regard to future needs in monitoring and research.