The distribution, abundance and life-histories of stoneflies (Plecoptera) and mayflies (Ephemeroptera) in a British River, warmed by cooling-water from a power station

Abstract

1. The River Severn at Ironbridge is warmed by cooling-water from Ironbridge “A” power station. Water temperatures over the years 1965–1967 were raised by values ranging from 0.5 to 7.2°C above ambient. 2. Collections of Plecoptera and Ephemeroptera were taken from three reaches in the vicinity of the cooling water discharge. “A” was situated 400 m upstream, “C” 800 m downstream and “E”, 2 km downstream of the outfalls. 3. The river at all three stations was fast flowing and normally fairly shallow with a stony substratum. 4. The water was alkaline with B.O.D. values of 0.9—5.0 ppm. There were no significant chemical changes from “A” to “E”. 5. Natural water temperatures varied from 0°C to 20.6°C. Downstream of the warm water outfalls the range was 0°C to 24.8°C. Mean weekly temperatures were raised by between 0.2°C to 4°C. There was no temperature stratification at the three sampling stations. 6. Thirteen species of Plecoptera and twenty-one species of Ephemeroptera were recorded. Ten species of Ephemeroptera (including three Caenis spp) constituted 95% of the total numbers collected. 7. Distribution of nymphs at “A”, “C” and “E” changed with river level and season. Species found commonly in the marginal grasses were those most active and common in invertebrate drift. 8. When compared with the faunas of other rivers and streams, the Severn at Ironbridge is a transitional area, with species common to hill streams and larger rivers. 9. The use of fine nets (24 meshes/cm) facilitated the more accurate determination of life-histories. 10. Life-histories of Taeniopteryx nebulosa, Ephemerella ignita, Heptagenia sulphurea, Baetis pumilus, Baetis bioculatus, Baetis rhodani, Baetis vernus and Baetis buceratus are described for both heated and unheated reaches. 11. Under natural conditions at “A”, three distinct types of life-history were defined among the eight species and the relationships between their life-histories and the natural temperature cycles differ. (a) T. nebulosa is a winter grower, nymphs only develop to maturity after September, though some were found in May and June at “A”. Temperature range for nymphs was approximately 17°C to 0°C. (b) E. ignita and B. bioculatus are both summer growers. Nymphs occur over a range of about 6.5°C to 22°C naturally. (c) The other species had more complex hatching, growth and emergence periods. H. sulphurea, B. rhodani, B. pumilus and B. buceratus nymphs grew quickly during February–May. Numbers were low in the height of summer and the controlling factor is probably competition for food and space with the “summer” species. 12. All species show a range of temperature tolerance during each life-history phase. This tolerance is considered to be a mechanism evolved to give the species some flexibility to withstand natural variations in climate from year to year. 13. E. ignitanymphs were larger and B. bioculatus better grown in May 1965 than in May 1966. This difference was probably related to the date on which 6°C was first reached and not to the time/temperature factor during the whole winter. 14. There was no really obvious “succession” of species in the Severn as found in small streams. 15. Effects of the power station effluent are considered from two aspects. There is some evidence that small, early hatched nymphs of Taeniopteryx were absent from “C” and “E” in June, but not from “A”. Later nymphs grew and hatched simultaneously at all three stations. There is no evidence of any other effects on the distribution and abundance of species. There may have been a small advance, of up to one week in the hatching times of Taeniopteryx and E. ignita in April 1967 but the data are not highly significant. The advances of 3–5 weeks in temperature shown by LANGFORD (1970) are not reflected in the life-histories of Plecoptera or Ephemeroptera. 16. All the evidence indicates that the heated effluent from Ironbridge “A” has no significant effects on the distribution and ecology of any of the eight species. Nymphs of some of the Ephemeroptera were collected over the whole range of river temperature, i.e. 0°C to 24.8°C, while the summer species were recorded over the whole summer range, i.e. 6°C to 24.8°C. T. nebulosa was restricted in its hatching and growth by temperatures over about 17°C both upstream and downstream of the power station. 17. It is suggested that the ability of each species to tolerate wide temperature range in different life-history stages is sufficient to enable the species to withstand and survive unnatural temperature conditions, provided that these temperatures are not sustained, or lethal to all stages, i.e. egg, nymph or adult. 18. Further studies on the long-term variations in life-histories in relation to river conditions are in progress.