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

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 dro ught 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 samp ling units from each biotope were processed separately prior to determinati on of density estimates, except in 1972 and 1973, when lime constraints nec essitated bulking and subsampling. Family richness per sampling unit and per sample was higher in December tha n June, and higher on the three macrophytes than on gravel and silt. Nevert heless, over nine years, a high proportion of the 59 families recorded on t he 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 info rmation on the density of invertebrates per biotope and the area of each bi otope, the weighted mean density of invertebrates on the site was calculate d 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 rapi d recovery after an extreme drought event on this unpolluted perennial site . Copyright (C) 1999 John Wiley & Sons, Ltd.