The emergence of insects from a British River, warmed by Power Station Cooling-Water

Abstract

As a result of some field observations and laboratory experiments, a number of authors (see text) have suggested that insects living in rivers, even slightly heated by power station cooling water, may have their life-cycles altered sufficiently to lead to the elimination of species owing to early emergence into a lethally cold air, shorter emergence periods, disrupted mating behaviour, or the elimination of necessary “chill-periods” in winter. If this were so, an important food resource for fishes in some rivers might be lost.

Part I of this paper (Langford and Daffern — in press) described the results of an emergence-trapping programme on the River Severn around the cooling water outfalls of Ironbridge “A” and “B” Power Stations. This paper describes in detail the emergence pattern and periods of eight species of Ephemeroptera, seven species of Trichoptera and one species of Megaloptera, upstream and downstream of the outfalls during the period July 1969–November 1971. The patterns are analysed in relation to river flows and water temperature.

Although t7emperature increases reached 8°C, maximum water temperatures reached 28°C, total degree-hours were up to 24% more during spring, and there was a four-week advance in degree-hour accumulation; there was no evidence of a pattern of early emergence downstream. Emergence of most species occurred over a temperature range of 12°C to 28°C and there was no evidence of the high temperatures in 1970 halting emergence or shortening the emergence period.

Captures of all species of Ephemeroptera were obviously influenced by spates, i.e. increased river levels and flows. Evidence suggests that emergence may be suppressed under such conditions. Catches of most Trichoptera species were not as markedly influenced by river flows. Implications are that future flow changes in the middle reaches of the Severn as a result of water transfers could affect emergence patterns of Ephemeroptera.

There is no positive indication that temperature changes caused by the cooling-water discharge had any influence on onset or progress of emergence of either Ephemeroptera or Trichoptera. It is suggested that where early emergence has been induced experimentally in the laboratory, constant high temperatures, stable flow conditions and food have been provided. In a natural river system, it is unlikely all these conditions would be maintained consistently enough for lethally early emergence to occur.

It is concluded, therefore, that before predictions about the effects of power station cooling water discharges on the emergent insects of rivers are made, based on extrapolation of laboratory data, it is essential that the relevant field conditions and relationships should be considered. Also the natural variability and adaptability of species should be investigated in more detail.