The use of chironomid pupal exuviae for characterizing streams.

Samples of chironomid pupal exuviae were collected by drift netting in the River Chew, Somerset, over a period of 4 years. The samples were analysed for percentage composition of types, and in some cases for numbers of exuviae per unit volume of water sampled. Surface samples were shown to contain many more exuviae per unit volume sampled than subsurface samples in the same place, and the catch taken by adjacent nets varied by up to four times. On the other hand, the percentage composition of exuvial types remained very consistent under all sampling conditions. Hourly samples from midday to midnight were made at four seasons in the year, showing that the variation in numbers and proportion of types remained approximately constant throughout the afternoon, but altered at the time of the evening emergence of adults. Samples taken from other rivers show both similarities and differences between themselves and the River Chew in percentage composition of types. In the River Biss, Wiltshire, a marked change in the exuvial types collected downstream from a sewage effluent outfall was shown after improvements had been made to the sewage works. Spring samples from the River Chew showed a change from Tanytarsini- to Orthocladiinae-dominated populations between 1970 and 1972. In the River Chew, exuviae suspended in the stream disintegrated after approximately 1–2 weeks. Coloured polystyrene balls were used to investigate the downstream movement of floating material in the River Chew. In August, balls released 400 m upstream of the sampling nets failed to reach them in 3 h sampling. The average stream flow rate was 0.36 m/s. Some evidence is presented to show that the exuviae float for only about 2 h in the River Chew before sinking. It is suggested that the area from which most of the sampled exuviae are derived is limited on the River Chew to, at most, 400 m upstream. Provisional identification to genus is given for those exuvial types mentioned in the paper. The possibility of using chironomid exuvial analysis as a method of characterizing streams is discussed. [ABSTRACT FROM AUTHOR]