Export of fine organic particles from headwater streams: Effects of season, extreme discharges, and invertebrate manipulation

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 C54 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.