Studies on the macroinvertebrate bottom fauna of the Irwell, a polluted river

The Irwell is polluted along its entire length. The headwaters are contaminated by drainage from a disused colliery and the rest of the river suffers from organic pollution of domestic and industrial origin. The degree of contamination is particularly great in the lower reaches of the river, downstream of the confluence with the River Roch. Despite this, the physical nature of the watercourse tends to ensure the maintainance of relatively high dissolved oxygen concentrations under all but the worst conditions. Contamination of substrate materials by copper, lead and zinc has been demonstrated at all sites studied except one located near the source of the river. Contamination by copper is particularly severe in Radcliffe; lead reaches highest concentrations at Agecroft. Zinc has peakB of concentration at both these sites. Copper is the most toxic of the metals studied to both Brpobdella octoculata and Asellus aquaticus; zinc is the least toxic. Data obtained from analysis of bi-monthly samples of riffle benthos from ten sites on the river show the fauna to be dominated by Oligochaeta, notably Limnodrilus hoffmeisteri. Using simple association analysis the fauna is divisible into three 'ecological assemblages'. One assemblage, association B, dominates the fauna at all sites and is fairly evenly distributed along the river. This association comprises Tubifex tubifex, Limnodrilus hoffmeisteri, Rais elinguis, Asellus aquaticus and Orthocladiinae larvae. Association A is restricted to the upstream reaches of the river, association C to the downstream sites. The lack of success of association C, which includes Limnodrilus udekemianus and Chironominae larvae, may be related to an inability of its members to compete with association A animals which in turn cannot tolerate downstream conditions. A major faotor influencing the nature of the riffle benthos of the Irwell is the modification of substrate by suspended material and sewage fungus. Toxic metals also clearly exert a deleterious effect