The effects of impoundment on the downstream macroinvertebrate riffle fauna of the River Elan, mid-Wales

Abstract

River water quality and hydrological regime in the River Elan, downstream of an impoundment in an upland catchment in mid-Wales, were considerably different from a nearby naturally flowing river. Concentrations of total iron (mean, 0·26 and 0·61 mg litre −1 in May and September) and manganese (mean, 0·12 and 0·16 mg litre −1) were elevated in the River Elan compared with the nearby River Wye (iron ⩽ 0·16 mg litre −1, manganese ⩽ 0·01 mg litre −1). pH in the River Elan (5·3–5·5) was considerably lower than the naturally flowing River Wye (6·9–7·2). There were differences in riffle macroinvertebrates, generally collected with a cylinder sampler on two occasions, in the Elan and Wye. A number of taxa were not recorded or were found at relatively low densities in the River Elan; principally these included representatives of the Coleoptera, Ephemeroptera and Trichoptera, Additionally, total densities in the River Wye (mean, 4900 m −2 in May) were significantly ( p<0·05) higher than the Elan (mean, 900 m −2 in May). There was no evidence that effluents discharging to the Elan affected the distribution of macroinvertebrates. Differences between the fauna of the impounded River Elan and the naturally flowing River Wye probably resulted from the presence of iron- and manganese-rich deposits in the River Elan— probably derived from metal-rich water from the reservoir—although reduced pH and water temperature downstream of the impoundment could have contributed to the paucity of the fauna.