IMPACT ASSESSMENT OF REGULATION AT THE REACH LEVEL USING MACROINVERTEBRATE INFORMATION FROM MESOHABITATS

The environmental effects of stream regulation are often assessed with reference to changes in river benthos, and biotic scores and indices have been used to summarize data on faunal community structure and ric hness. Although they have their use in presenting complicated data in a simplified form, they often do not detect the more subtle effects of regulation. In lowland regions the regulation of rivers is most often due to water abstraction, channelization and the construction of weir s or sluices. The effects of this form of stream regulation may someti mes be particularly difficult to demonstrate with conventional biologi cal assessment techniques. In this study we examine a length of stream regulated with sluice gates using conventional techniques and test th e applicability of the mesohabitat method (categorization of reaches o n the basis of their substratum types) in describing changes brought a bout by regulation. Conventional biological assessment techniques were unable to demonstrate the regulation effects. This was because, altho ugh changes had occurred along the reach, these were not associated wi th a significant change in scores and indices based on family richness . The changes were subtle and were most clearly revealed by clustering techniques and analysis of variance. Most of the change was due to al tered physical habitat and this was reflected in the proportions of me sohabitats above and below the sluice gates. The association of mesoha bitats with particular faunal assemblages allows an assessment of the overall effects of habitat change on the composition of the faunal com munity.