Long‐term faunal changes in a regulated and an unregulated stream—Cow Green thirty years on

Abstract Cow Green dam is situated in Northern England on the River Tees in a region with high average wind speeds (24 km h −1 ), low average annual temperatures (circa 5°C) and average rainfall of 1800 mm. The River Tees was impounded by the Cow Green dam in 1970 and early studies revealed significant changes arising from flow regulation. This study compares macroinvertebrate communities in 2004 with those recorded in the first 5 years after impoundment in the Tees and in the unregulated Maize Beck. Nineteen of the 31 common taxa in the regulated sites declined in abundance by a factor of 5 or more. These included Hydra sp., Ancylus fluviatilis , Naididae, Heptageniidae, Leuctridae and Brachycentrus subnubilus . Some taxa, Lymnaea peregra , Ephemerella ignita , Hydroptila sp. increased in numbers, and others Hydropsychidae and Gammarus pulex declined at sites nearest the dam but increased downstream. In Maize Beck there were fewer changes. The changes in faunal communities in the Tees were evident from multivariate analyses where the Tees sites sampled in 2004 were separated from those sampled in the period 1972–1975, as a result of abundance changes in common taxa and the appearance of taxa not previously recorded. Maize Beck in contrast was characterized by few changes in abundance and no new taxa and samples from all years grouped together. The results suggest that the Tees communities have changed since 1975 and are still possibly undergoing change, although without evidence from intervening years this cannot be proved. A narrower range of environmental conditions and increased flow stability have led to a dynamically fragile community (indicated by observed changes in community diversity and abundance) which is very susceptible to perturbations because it has developed in their absence. Periphyton and reservoir plankton play an important role in structuring the faunal composition by creating an environment where biotic interactions are more likely. Increased interaction between components of the faunal community may account for the observed changes since 1975 in the regulated sites in contrast to the situation in the unregulated Maize Beck where there has been little change in faunal community between the original study and the 2004 survey. An unregulated natural flow regime continues to dominate the Maize Beck environment and the variable and unpredictable conditions have resulted in a dynamically robust faunal community. Copyright © 2006 John Wiley & Sons, Ltd.