Pesticide Manipulation of a Headwater Stream: Invertebrate Responses and Their Significance for Ecosystem Processes

The influence of macroinvertebrates on detrital processing was evaluated by excluding them from one of two small southern Appalachian streams. Exclusion in the treated stream was accomplished by periodic applications of 10 ppm of the insecticide methoxychlor. This caused massive invertebrate drift (>12,000 organisms/m3 of discharge) during the initial treatment and reduced aquatic insect densities and biomass to <10% of the levels within the adjacent untreated reference stream. Community structure in the treated stream shifted from a system dominated by small numbers of large shredding insects (e.g., Peltoperla, Pycnopsyche, Tipula) with comparatively low reproductive rates, to one dominated by large numbers of small collector-gatherers and predators (e.g., Oligochaeta, Chironomidae, Turbellaria) with high reproductive rates. Non-insect invertebrate biomass and density became significantly higher in the treated stream than in the reference stream following initial methoxychlor treatment. We interpreted this response as a consequence of increased survivorship and growth of non-insect taxa associated with both insect predator removal and a potential increase in the food quality of fine particulate organic matter (FPOM). Total invertebrate density in the treated stream increased to that of the reference stream 117 days after the initial treatment, but total invertebrate biomass in the treated stream remained significantly lower throughout the study. Counts of fungal hyphae, respiration rates of conditioned leaf discs, and ATP content of coarse particulate organic matter (CPOM) were not affected by methoxychlor. However, ATP levels of FPOM were significantly higher in the treated stream than in the reference stream. Methoxychlor treatment reduced the concentration, the amount exported, and the median particle size of transported particulate organic matter (TPOM). Our study indicates that insects in forested headwater streams play a major role in both the generation and subsequent transport of FPOM to downstream reaches. Biological processes in headwater streams, where there is high physical retention of CPOM, produce and entrain small particles, whereas communities in downstream areas have evolved toward exploitation of these entrained particles.