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SUMMARY. 1. The insecticide methoxychlor was applied seasonally to one of three small headwater streams in the southern Appalachian Mountains in North Carolina, U.S.A. The initial application caused massive invertebrate drift (>l,000,000 organisms/week) and resulted in a community with few shredders and reduced abundances of most insecl taxa.2. Bacterial densities and microbial respiration rates were not affected by treatment.3. Disruption of the invertebrate community resulted in significant reductions in leaf litter processing rates (50-74% reduction depending on leaf species) and in the amount of leaf litter processed annually (reduction of 25-28%).4. Reductions in leaf litter processing rates resulted in significant reductions in fine particulate organic matter (FPOM) export. Declines in both concentration and total export were detectable within 1 week of treatment. Annual FPOM export was reduced to 33% of pretreatment levels. Alteration to the invertebrate community had a much greater effect on FPOM export than a severe (50-200 year) drought.6. Course particulate organic matter (CPOM) export was not significantly influenced by treatment but was influenced by hydrologie differences among years.

Export of fine particulate organic matter (FPOM) was measured from streams draining Catchments 53, 54, and 55 (C53, C54, and C55) at the Coweeta Hydrologic Laboratory for 5 yr, encompassing the driest and wettest years of the past 55-yr record. Macroinvertebrate populations in C54 were reduced for 3 yr by seasonal treatments with insecticide. During pesticide treatment, FPOM concentrations decreased abruptly in C54, remained well below those of reference streams for the 3-yr treatment period, and then increased during the 1st yr of recovery. Macroinvertebrate reduction resulted in an estimated 170-200 kg loss in FPOM export during the 3 yr of treatment and 1 st yr of recovery. Annual export of FPOM was strongly related to annual discharge, but the impact of biotic manipulation on FPOM export in C54 was at least as great as that produced by extreme high and low discharges. FPOM export during sampling intervals was directly related to maximum discharge during the intervals. Treatment of CS4 significantly reduced FPOM export per unit maximum discharge. Seasonal relationships between FPOM export and maximum discharge indicated more export per unit maximum discharge in summer in reference streams and C54 in untreated years. In contrast, during treatment years of C54, export per unit maximum discharge was lowest in summer. FPOM concentrations measured during storms showed increasing concentrations with increasing discharge in all streams, however, much more FPOM was suspended in the reference streams than in C54. Thus, macroinvertebrate reduction altered the magnitude of FPOM export during storms, the seasonal pattern of export, and the annual export of FPOM.

A small first-order, Appalachian Mountain stream received successive seasonal treatments with the insecticide, methoxychlor. Despite an application rate of 10 mg/1 methoxychlor for 4 hours, based on stream discharge, only a small fraction (1.6%) of the insecticide was exported to downstream reaches for a 31 h period during and following treatment. Most of the insecticide was incorporated into sediments of the streambed, which had residues ranging from 0.038 to 11.7 #g methoxychlor/g dry wt of sediments in June 1986 following treatments in December 1985 and March 1986. Despite low concentrations of methoxychlor measured in stream water (maximum = 128 #g/l) during the initial treatment, massive drift (> 950 000 organisms, and 70 g AFDM biomass) occurred from a stream area of about 144 m 2 . Numerically, collector-gatherer taxa (primarily Chironomidae) dominated drift (63 %) followed by shredders and predators; however, biomass of drift was dominated by shredders (48.9%), followed by predators and collector-gatherers. Compared with pre-treatment benthic abundances, insects were reduced by 75% following the initial treatment in December 1985, and 85% following an additional treatment in March 1986. Benthic abundances of non-insect taxa showed no significant changes. Benthic abundances of shredder, collector-filterer, and scraper functional groups exhibited significant decreases in the first month following treatment. Although benthic abundances of collector-gatherer and predator taxa were reduced by 48.6 and 40.5%, respectively, the reduction was not statistically significant because of high-sample variance. Comparisons of drift composition during the initial treatment with successive quarterly treatments (March 1986 to January 1988) reflected of ongoing pesticide disturbance of the biota as the community structure shifted from one consisting of a diverse insect and non-insect fauna toward one dominated by copepods, oligochaetes, Collembola, and chironomids.