Long-term changes in macroinvertebrate communities of a heavy metal polluted stream : The river nent (Cumbria, UK) after 28 years

The macroinvertebrate fauna of the river Nent, a heavy metal polluted upland stream in northern England, first surveyed in 1976, was re-examined in 2004 to assess long-term changes in relation to activities in the catchment which have included, a river restoration project, works associated with the development of a mine heritage site and the removal of a chicken farm and more effective organic pollution control measures. The degree of change between years in total abundance, numbers of taxa and community composition varied considerably between sites. Two tributary sites showed marked changes between years due to a reduction in acidity in one and changed substratum in the other but in the main river no significant difference in total taxa and total abundance was observed between years, although seasonal differences were significant. Multivariate community analyses grouped 1976 samples with those taken in 2004 but revealed a clear separation between upper and lower Nent sites. This observed difference may be related to increased algal cover in the lower sites in conjunction with geomorphological features of the lower Nent. Environmental assessment methodology River Invertebrate Prediction and Classification System (RIVPACS) identified 'sensitive' faunal groups and indicated that the extensive impact of zinc pollution in the main river has remained practically the same between 1976 and 2004. This persistence of community structure despite the heavily disturbed nature of the river is attributed to relative constancy in instream habitat conditions. The recent activities in the catchment have had little effect on the overall controllers of faunal communities in this system, zinc concentrations and geomorphological characteristics of the stream. However, further disturbances involving movement of spoil heaps and channel alterations in the upper catchment may increase both sediment and heavy metal loadings to the river.