The emergence of insects from a British River warmed by power station cooling-water

During a programme of research on the fauna of the River Severn around Ironbridge Power Station, the emergence patterns of resident insect species were studied upstream and downstream of the cooling-water outfalls. This paper describes the traps used to catch emerging adult insects and relates their overall performance, total catches and species selectivity, to the physical characteristics of the river. Upstream temperatures during 1970 and 1971 reached maxima of 22°C and 20.6°C respectively, while downstream maxima were 27.8°C and 23.4°C. The main cause of elevated temperatures downstream was cooling-water from Ironbridge “A” Power Station. Analysis of trap catches showed that the total numbers of Ephemoptera species and of individuals were greater in certain types of trap but that catches of Trichoptera were less related to trap type. The total emergence period varied during 1969, 1970 and 1971 irrespective of the elevated temperatures and both Ephemeroptera and Trichoptera continued to emerge during and after water temperatures as high as 26–27.8°C and as low as 11°C, in June. There was some relationship between emergence rates, river level (flow) and temperatures, but evidence suggests that river level was the most significant factor. The general conclusion from this paper is that the temperature increases in the Severn, caused by normal operations at Ironbridge “A” and by sporadic operations at Ironbridge “B”, did not have a significant effect on the total numbers and overall emergence period of Trichoptera, Ephemeroptera and Megaloptera downstream of the outfalls. Also the species-composition of trap catches was very similar both upstream and downstream of the power stations. The emergence periods of individual species are described in a second paper.