title: Recovery by riffle macroinvertebrates in a river after a major accidental spillage of chlorpyrifos.
abstract: The impact of a major spillage of the insecticide 'Dursban' on the riffle macroinvertebrates along 19 km of the River Roding, Essex, is described. Five affected riffles and an upstream control were kick-sampled at approximately 10-week intervals for two years following the spill, and the results are compared with species composition and relative abundance data collected from the same sites during the previous six years. Initial concentrations of the active ingredient, chlorpyrifos, in river water (up to 2.5 mg litre(-1)) exceeded the level lethal to all the aquatic arthropods present by at least 10-fold, and this group of macroinvertebrates was eliminated. Molluscs and annelids, which are relatively tolerant of chlorpyrifos, survived. Since these groups already dominated the lowermost urban reaches, the impact of the spill was greatest further upstream, where reaches with a better water quality previously supported a more diverse and abundant arthropod fauna. Chlorpyrifos residues in water declined below 1 microg litre(-1) within 11 weeks, but sediment within 5 km of the spillage site remained highly contaminated for considerably longer. Of ten arthropod taxa previously common to all sites, chironomid larvae were first to recolonise affected reaches, 13 weeks after the spill. The isopod Asellus aquaticus, was also quick to recover. Although other arthropods had recolonised most sites within 79 weeks, the coleopteran Oulimnius tuberculatus and the epheremeropteran Caenis moesta had failed to return to the lowermost reaches after 108 weeks. Downstream drift was probably the principal factor influencing the pattern of arthropod recolonisation, with the location and aquatic fauna of tributary streams entering the affected reaches being particularly important.
authors.affiliation: Applied Ecology Research Group, Polytechnic of Central London, 115 New Cavendish Street, London W1M 8JS, UK.
authors.forename: P J
authors.initials: PJ
authors.lastname: Raven
authors.forename: J J
authors.initials: JJ
authors.lastname: George