A nine-year study of the macroinvertebrate fauna of a chalk stream

Abstract

The macroinvertebrate fauna of a 50 m site on a partially shaded perennial section of the River Lambourn in Berkshire was studied intensively between 1971 and 1979. This period included a minor drought in 1973 and a major drought in 1976. A quantitative sampling programme for macroinvertebrates was undertaken on each of five distinct biotopes (Berula, Callitriche, Ranunculus, gravel and silt). Each one was sampled in March, June. September and December of 1971 and thereafter in June and December from 1972 to 1979. Five replicate sampling units from each biotope were processed separately prior to determination of density estimates, except in 1972 and 1973, when time constraints necessitated bulking and subsampling. Family richness per sampling unit and per sample was higher in December than June, and higher on the three macrophytes than on gravel and silt. Nevertheless, over nine years, a high proportion of the 59 families recorded on the site were collected on each biotope. During the extreme drought of 1976, there was no evidence of loss of family richness on the site, but some biotopes supported unusually high densities of macroinvertebrates from a limited number of families. By combining information on the density of invertebrates per biotope and the area of each biotope, the weighted mean density of invertebrates on the site was calculated for each sampling occasion. Seasonal and between-year changes in the densities of numerically important families were observed and a variety of responses to discharge regime were noted. Overall, the macroinvertebrate fauna appeared to be capable of rapid recovery after an extreme drought event on this unpolluted perennial site.