First-year benthic recovery of a headwater stream following a 3-year insecticide-induced disturbance

1. A headwater stream (C54) was treated seasonally (twelve times from December 1985 to October 1988) with the insecticide methoxychlor for 3 years, resulting in 72 and 75% reductions of insect abundances and biomass, respectively. Following treatment, monthly benthic abundances and biomass were measured in the treated stream and a nearby reference stream (C55) in order to assess recovery. Seasonal average abundances and biomass were calculated from samples taken from December 1988—December 1989, the first post-treatment year of C54. 2. Non-insect abundances and biomass were higher in the recovering stream during the winter + spring and summer periods, but were similar to the reference stream by autumn. Abundances and biomass of collector-filterers, scrapers, and shredders were all lower in the recovering stream during winter + spring; however filterers and scrapers were similar to reference stream values by the end of the recovery year. No distinct differences were seen between collector-gatherers and predators throughout the year. 3. Univoltine and multivoltine taxa, along with treatment-resistant taxa, were the most important contributors to recovery of C54. Taxa with longer generation times, life histories which were more vulnerable to riming of treatments, and/or poor dispersal abilities were poorly represented during first-year recovery. 4. Abundance data alone were not sufficient for a complete analysis of recovery. Biomass estimates reflected differences in taxonomic composition and developmental stages between streams after 1 year of recovery.