A survey of the macro-invertebrate riffle fauna of the rivers Ystwyth and Rheidol, Wales

SUMMARY. A total of 155 taxa was collected from the Ystwyth (111) and Rheidol (134) catchments and total macro-invertebrate densities ranged from 435 to 9105 m⁻² and 155 to 4290 m⁻² in the two catchments, respectively. The fauna in each catchment was dominated by Insecta, principally Plecoptera, Ephemeroptera and Diptera. Trichoptera and Oligochaeta were proportionally more abundant in the Rheidol catchment. There was no evidence that the distribution or relative abundance of the faunas of the catchments were related simply to heavy metal contamination (mean zinc concentration up to 2.0 mg I⁻¹ and 0.34 mg I⁻¹ in the Ystwyth and Rheidol, respectively). Average linkage clusters of Spearman rank correlation coefficients suggested that community affinities were probably related principally to location within the catchment. There was no evidence in the Rheidol catchment that irregular discharges (up to c . 15 m³ s⁻¹) of water for hydroelectric purposes had any substantial effect on the invertebrate fauna although total invertebrate density below the discharge was significantly higher than at an upstream site.