Dynamics of Inorganic Particles in Headwater Streams: Linkages with Invertebrates

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Export of particulate inorganic matter (PIM) from three headwater streams was studied from 1985 to 1991 at Coweeta Hydrologic Laboratory. The study encompassed years of extreme high and low precipitation. During three years of the study, the stream draining Catchment 54 (C 54) received seasonal treatments of an insecticide. This treatment greatly reduced abundance biomass and secondary production of most invertebrate taxa. Streams draining Catchments 53 and 55 (C 53 and C 55) served as reference streams. During treatment, concentration of PIM in C 54 decreased significantly from pretreatment levels for both instantaneous (grab) and continuous samples, while those of reference streams either increased or did not exhibit significant change. Maximum PIM concentrations, measured during rising hydrographs of storms, were over 2× greater in the reference streams than in C 54. During treatment of C 54, export of PIM per unit maximum discharge (L/s) during sampling intervals (∼2 wks) decreased significantly compared with that of reference streams. Annual PIM export in reference streams displayed a strong (p < 0.002) exponential relationship with annual discharge, whereas that of the treated stream did not (p > 0.05). High discharges during post-treatment years (1989-1991) resulted in increased concentration and export of PIM in all streams. However, PIM concentrations in C 54 remained lower than those of the two reference streams. Invertebrate manipulation apparently reduced PIM export from C 54 by at least two mechanisms. First, the rate of particle generation by feeding activities was reduced. Second, invertebrate manipulation reduced rates of leaf litter processing, which resulted in increased storage of leaf litter and enhanced retention of particles. Comparisons of predicted versus measured export suggest that invertebrate manipulation in C 54 reduced PIM export by ∼76%, or 550 (relative to C 53R) to >1000 kg (relative to C 55) over the 6-yr treatment and post-treatment period. These comparisons exceed those made previously for the impact of invertebrate manipulation on FPOM export and provide another example of the link between animal communities and ecosystem processes. This study also provides evidence that stored leaf material, like that of woody debris, augments retention of inorganic particles in small headwater streams.